Every building that has electricity has a point where the utility's power stops being the utility's responsibility and starts being the building owner's. That transition point — and everything involved in bringing power from the utility source into the building safely, with a way to shut it all off — is what NEC Article 230 covers, under the general topic of "service." If you can't clearly explain where a service starts and ends, what a disconnect actually does, and how to size one, you're going to struggle on any exam that touches service entrance work, and you're going to struggle even more on the job, because service work sits at the very front door of almost every electrical installation. This article builds the concept up from first principles, then walks through worked examples so the sizing math becomes second nature.
What "Service Entrance" Actually Means
Start with the basic idea: a building needs power, and that power comes from somewhere — a utility company's distribution system. The "service" is the general term for the whole arrangement of conductors and equipment that brings that utility power onto the property and into the building, ending at the point where it gets divided up into branch circuits and feeders for actual use.
"Service entrance" specifically refers to the conductors and equipment involved in that entry point — think of it as the front door of the electrical system. Everything upstream of the service (the utility's poles, transformers, and distribution wiring) belongs to the utility. Everything at and downstream of the service point generally belongs to the property owner and falls under the property owner's (and their electrician's) responsibility to install and maintain correctly.
Understanding this ownership boundary matters for more than just billing purposes — it defines where NEC requirements for service equipment actually start applying, and it defines the point past which a fault or problem becomes the building owner's liability rather than the utility's.
Overhead vs. Underground Service — Two Ways In
Power gets from the utility's system to a building's service equipment in one of two general ways, and understanding both conceptually (without needing to memorize every construction detail) is foundational.
Overhead Service
In an overhead arrangement, utility conductors run through the air on poles and connect to the building at some point, typically up near the roofline. The conductors that span from the utility's last pole to the building are commonly called the service drop — this is the utility-owned portion of the overhead run. From the point where those utility conductors attach to the building (often at a weatherhead, a fitting designed to keep water out of the conduit where overhead conductors transition to a protected raceway), the conductors continuing down into the building's meter and service equipment are the service entrance conductors — these are typically owned and installed by the property owner's electrician, not the utility.
Overhead service has some practical advantages: it's often cheaper to install since it doesn't require trenching, and repairs to the utility-owned portion are generally more visible and accessible. It also has downsides — exposure to storm damage, tree contact, vehicle strikes on poles, and aesthetic concerns that make many newer developments prefer underground service instead.
Underground Service
In an underground arrangement, the utility conductors run below grade instead of overhead. The underground utility-owned portion running from the utility's transformer or distribution point to the building is commonly called the service lateral. As with the overhead case, once those conductors reach the building and transition into the property owner's system, they become service entrance conductors from that point forward.
Underground service avoids the storm and visual-impact downsides of overhead lines, which is why it's become the standard in a lot of newer residential subdivisions and commercial developments. It generally costs more to install (trenching, conduit, and sometimes concrete encasement depending on local requirements) and can be more involved to repair if a fault develops below grade, since locating and accessing the fault isn't as simple as looking up at a wire.
Service drop = utility-owned overhead conductors from the pole to the building attachment point.
Service lateral = utility-owned underground conductors from the utility source to the building.
Service entrance conductors = the conductors from the point of utility attachment (overhead) or the transition point (underground) to the building's service equipment — generally owned and installed by the property owner's side.
Service Equipment Location — The General Concept
Service equipment — the meter socket, main disconnect, and associated enclosures — has to be installed somewhere accessible and safe. Rather than memorizing exact clearance dimensions (which vary by installation type and should always be pulled from your current code book and any local utility or jurisdictional requirements), it's more useful for exam purposes to understand the underlying principles the location rules are built around:
- Readily accessible. Service equipment needs to be reachable without needing to move obstacles, climb over things, or use a ladder or special tool just to get to the disconnect. The whole point of a disconnect is that someone — a homeowner, an electrician, a firefighter — can get to it quickly in an emergency. Equipment buried behind stored furniture or locked inside an inaccessible closet defeats that purpose even if it's technically "installed."
- Clearances around the equipment. Working space in front of electrical equipment has to stay clear enough that a person can safely operate, inspect, or work on it without being cramped, off-balance, or forced into an awkward position near energized parts. This is the same working-space logic that applies to panelboards throughout the code — it's not unique to service equipment, but it applies there just as strongly.
- Weather protection. Equipment installed outdoors has to be suitable for the environment it's exposed to — protected from rain intrusion, corrosion-resistant where needed, and rated appropriately for outdoor use. This is why you see specific enclosure types used outdoors versus indoors.
- Not in a location prone to physical damage. Placing service equipment somewhere it's likely to get hit by a vehicle, damaged by falling objects, or exposed to unusual physical hazards works against the whole purpose of having reliable, accessible equipment there in the first place.
Exact clearance dimensions, mounting height ranges, and specific location restrictions differ across code editions and are frequently affected by local utility requirements layered on top of the NEC — always confirm the specific numbers with your current code book and your local utility's service requirements before finalizing a real installation.
The "Up to Six Disconnects" Concept
One of the most important ideas in Article 230 — and a favorite exam topic — is the general concept that a service is allowed to be shut off using up to six disconnects, rather than being limited to just one big main breaker or main switch. Conceptually, think of it this way: instead of installing a single main disconnect that kills everything, an installer can instead install a group of separate disconnects (up to six total), each one controlling a portion of the load, as long as they're grouped together at one location and clearly marked.
Why would anyone want six disconnects instead of one? A few practical reasons come up in real installations. Sometimes it's simply more economical or practical to use several smaller disconnects rather than one large one, especially when a service feeds several distinct sub-loads (for example, a building with separate disconnects for general power, HVAC equipment, and a large motor load). Other times it's about future flexibility — if the building's load is going to be split into logical groups anyway, doing that split right at the service can simplify downstream wiring.
The key conceptual limit to remember is the number six — the code allows this "up to six" arrangement rather than requiring or forcing a single main disconnect, but it doesn't allow an unlimited number of separate service disconnects grouped this way. And practically, the disconnects have to be grouped together and clearly identified — you shouldn't be hunting through a building for six different disconnects scattered in six different rooms if someone needs to kill power to the whole service in an emergency.
Meter Equipment Basics
The meter is the device that measures how much electrical energy the building consumes, which is how the utility bills the customer. Conceptually, the meter sits at (or very near) the boundary between utility-owned conductors and property-owner conductors, giving the utility a measurement point right at the edge of their responsibility.
Meter equipment is typically housed in a meter socket — an enclosure the meter plugs into — which is usually mounted on the outside of the building at an accessible, visible location so utility personnel can read it (on older analog-style setups) or so the equipment is at least reasonably accessible for utility service work, even where meters are now commonly read remotely via modern metering technology. The meter socket is typically installed ahead of (upstream of) the main service disconnect in the flow of power, though the exact arrangement of meter and disconnect equipment can vary by local utility practice and by whether the installation uses a combination meter/main disconnect enclosure or separate equipment.
Utilities typically have their own specific requirements for meter equipment — approved socket types, sealing requirements (many meter sockets have a physical seal that indicates unauthorized tampering), and specific mounting height and clearance rules — layered on top of whatever the NEC requires generally. Always check with the serving utility directly for their specific meter requirements in addition to the NEC and local jurisdiction's electrical code requirements.
Grounding at the Service — The Bonding Jumper Concept
Grounding and bonding at the service is one of the most safety-critical parts of the entire installation, and it's a big enough topic that it deserves its own dedicated treatment — see our companion article Grounding and Bonding Explained (NEC 250) for the full conceptual walkthrough. For the purposes of understanding service entrance work specifically, here's the core idea you need: at the service, the grounded (neutral) conductor and the equipment grounding system are bonded together — connected — at one specific point, using what's generally called the main bonding jumper.
Why does this connection matter so much? It creates a low-impedance path back to the source for fault current, which is what allows overcurrent protection (breakers and fuses) to actually detect a fault and trip quickly. Without a properly bonded connection at the service, a fault on a metal enclosure or equipment case might not create enough current flow to trip a breaker, leaving that metal part energized and dangerous to anyone who touches it. The service is specifically where this bonding connection is made — it's not something that gets repeated at every downstream panel, which is part of why understanding exactly where and how it happens at the service is so important for both exam purposes and real installations.
Multiple Services on One Building
Most residential buildings have exactly one service — one utility connection point feeding one set of service equipment. But larger buildings, especially large commercial and industrial facilities, sometimes have more than one service supplying a single building. This can happen for a number of practical reasons: a building large enough that a single service's capacity isn't practical or economical to provide from one location, distinct parts of a large facility (say, a large industrial process area versus an office wing) that make more sense as separate services, or redundancy and reliability considerations for facilities that can't tolerate losing power to their entire operation from a single point of failure.
Multiple-service arrangements are a more advanced topic generally reserved for larger commercial and industrial work, and the specific conditions under which multiple services are permitted on a single building are governed by the code and typically require coordination with the local utility and jurisdiction. For most residential and light commercial exam preparation, the key takeaway is simply that the "one building, one service" pattern is the default assumption but not an absolute rule — larger and more complex buildings can and do have more than one service under the right conditions.
Emergency Disconnect Concept for One- and Two-Family Dwellings
A newer and increasingly important concept in service design for one- and two-family dwellings is the idea of an emergency disconnecting means — a way to shut off power to the dwelling quickly, ideally without needing to enter the structure. The core safety logic is straightforward: first responders, most notably firefighters, sometimes need to cut power to a structure quickly during an emergency, and requiring them to enter a burning or otherwise compromised building just to reach an indoor panel is a real and serious safety problem.
This concept has continued to develop across recent code cycles. Notably, the 2026 NEC edition adds a newly required outdoor disconnecting means specifically for one- and two-family dwellings — meaning the disconnect has to be reachable from outside the structure, not just present somewhere inside it. This is one of the more significant service-related changes in that edition; if you want the full context on how it fits into the broader set of 2026 changes, see our companion article NEC 2026 Code Changes Explained.
For exam purposes, the conceptual takeaway is this: the trend across recent code cycles has been toward making sure a dwelling's power can be shut off quickly and safely by someone who may not be familiar with the building's interior layout — and the 2026 edition takes that a step further by specifically requiring that capability from outside the structure for one- and two-family dwellings.
Why Service Work Deserves This Much Attention on an Exam
It's worth pausing to explain why licensing exams lean so heavily on service entrance topics, since understanding the "why" makes the material stick better than treating it as an arbitrary list of facts to memorize. Service equipment is the single point in a building's electrical system where a mistake has the broadest possible consequences — everything downstream depends on the service being sized correctly, grounded correctly, and equipped with a disconnect that actually works when someone needs it in an emergency. A wiring mistake in one branch circuit affects one circuit. A service sizing or grounding mistake affects the entire building.
That's also why service work sits at the intersection of so many other topics you've likely already studied: load calculations feed directly into service sizing, grounding and bonding concepts anchor at the service, overcurrent protection principles determine how the service disconnect actually behaves under fault conditions, and conductor sizing and ampacity rules determine what the service entrance conductors themselves need to be rated for. If you understand service entrance concepts well, you're really demonstrating that you can pull together several other areas of code knowledge into one coherent picture — which is exactly what a licensing exam is trying to verify you can do as a working electrician, not just as someone who memorized isolated facts.
Worked Examples: Sizing a Service
Service sizing ultimately comes down to a simple relationship once you have a total calculated load in volt-amps (VA): current equals power divided by voltage. For single-phase systems, that's I = VA ÷ V. For three-phase systems, you divide by voltage multiplied by the square root of three (approximately 1.732), because three-phase power delivery splits the load across three conductors in a way that changes the voltage-to-current relationship compared to single-phase. Let's work through several sizing examples, from small dwellings up to a simplified commercial example.
Example 1 — Small Single-Family Dwelling, 120/240V Single-Phase
A small single-family dwelling has a calculated total load of 24,000 VA after all load calculation steps (general lighting, appliance loads, demand factors, etc. — the full process for getting to this total VA figure is covered in our companion article Load Calculations Explained (NEC 220)). The dwelling is served at 120/240V single-phase, which is the standard voltage arrangement for most U.S. single-family homes.
I = VA ÷ V
I = 24,000 ÷ 240
I = 100 amps
This tells us the service needs to be sized to carry at least 100 amps. In practice, an installer would then select the next standard overcurrent protective device rating at or above this calculated figure (common standard sizes include 100A, 125A, 150A, 200A, and larger) — always confirm standard device sizes and any additional sizing considerations against your current code book.
Example 2 — Mid-Size Single-Family Dwelling, 120/240V Single-Phase
A larger single-family dwelling has a calculated total load of 38,400 VA, also served at 120/240V single-phase.
I = VA ÷ V
I = 38,400 ÷ 240
I = 160 amps
A calculated load of 160 amps would typically be served by the next standard size at or above that figure — commonly a 200-amp service in residential practice, since 200A is one of the most common standard panel and service sizes used for mid-size to larger dwellings. Note that this example shows the math process; always verify the appropriate standard size selection against your current code requirements and equipment availability.
Example 3 — Large Single-Family Dwelling With High Electrical Demand, 120/240V Single-Phase
A large dwelling with significant electrical demand — multiple large appliances, electric heating, and a sizable general load — has a calculated total load of 57,600 VA, still served at 120/240V single-phase.
I = VA ÷ V
I = 57,600 ÷ 240
I = 240 amps
This is a good example of a load that falls between two common standard sizes (200A and larger sizes like 400A, since jumps in standard equipment sizing aren't always small increments). A calculated load of 240 amps would require selecting equipment rated at least that high — in practice this often pushes an installer toward larger standard equipment, or toward evaluating whether load management strategies could bring the calculated demand down. This is exactly the kind of scenario where understanding the underlying math (rather than just memorizing "houses are usually 200A") pays off, because not every dwelling fits the common assumption.
Example 4 — Small Two-Family Dwelling (Duplex), 120/240V Single-Phase, With Outdoor Disconnect Consideration
A duplex has two dwelling units, each with its own calculated load of 20,000 VA, for a combined total of 40,000 VA if served by a single combined service (in practice, duplexes are sometimes served by individually metered separate services instead — this example assumes a single combined service for simplicity).
Combined VA = 20,000 + 20,000 = 40,000 VA
I = VA ÷ V
I = 40,000 ÷ 240
I = 166.7 amps
Rounding up to the next standard size, this would typically be served by 200-amp equipment. And because this is a two-family dwelling, remember the emergency disconnect concept discussed earlier — under the 2026 NEC edition specifically, this occupancy type requires an outdoor disconnecting means as part of the service design, in addition to whatever standard disconnect arrangement is used.
Example 5 — Simplified 3-Phase Commercial Example, 208Y/120V
A small commercial building has a calculated total connected load of 125,000 VA, served at 208Y/120V three-phase — a very common commercial service voltage arrangement.
I = VA ÷ (V × 1.732)
I = 125,000 ÷ (208 × 1.732)
I = 125,000 ÷ 360.3
I ≈ 347 amps
This tells us the service needs to be sized to handle at least 347 amps. An installer would then move up to the next standard equipment size at or above that calculated figure — commonly a 400-amp service in commercial practice, since 400A is a widely available standard rating above the 347A calculated minimum.
Example 6 — Larger 3-Phase Commercial Example, 480Y/277V
A larger commercial or light-industrial building has a calculated total connected load of 300,000 VA, served at 480Y/277V three-phase — a common higher-voltage arrangement for larger commercial and industrial facilities, often chosen specifically because higher distribution voltage reduces the current (and therefore the conductor size) needed to deliver the same amount of power.
I = VA ÷ (V × 1.732)
I = 300,000 ÷ (480 × 1.732)
I = 300,000 ÷ 831.4
I ≈ 360.9 amps
Compare this to Example 5: a much larger total load (300,000 VA vs. 125,000 VA — more than double) results in a similar calculated amperage (about 361A vs. about 347A), because the higher 480V distribution voltage in this example spreads the same power over a much lower current. This is exactly why larger facilities often choose higher distribution voltages — it keeps conductor sizes, and therefore cost and physical space requirements, more manageable even as the total power delivered goes up substantially.
Table: Comparing the Examples Above
| Example | Occupancy | Total VA | Voltage/Phase | Calculated Amps | Likely Standard Size |
|---|---|---|---|---|---|
| 1 | Small single-family dwelling | 24,000 VA | 120/240V, 1-phase | 100 A | 100 A |
| 2 | Mid-size single-family dwelling | 38,400 VA | 120/240V, 1-phase | 160 A | 200 A |
| 3 | Large single-family dwelling | 57,600 VA | 120/240V, 1-phase | 240 A | Above 200 A — verify with designer |
| 4 | Two-family dwelling (duplex, combined) | 40,000 VA | 120/240V, 1-phase | 166.7 A | 200 A |
| 5 | Small commercial | 125,000 VA | 208Y/120V, 3-phase | ≈347 A | 400 A |
| 6 | Larger commercial/light industrial | 300,000 VA | 480Y/277V, 3-phase | ≈360.9 A | 400 A |
Common Mistakes
How This Changed: NEC 2020 → 2023 → 2026
Service entrance and disconnect requirements under Article 230 have seen incremental updates across recent code cycles, the way most established articles do — always check the current edition your area enforces for the precise details of any given cycle's changes to this article, since exact subsection-level changes to Article 230 itself aren't part of this article's verified fact sheet. The one specific, verified change worth calling out directly is the newly required outdoor disconnecting means for one- and two-family dwellings, which is added in the 2026 edition. That's a genuine, documented service-related change — for the fuller context of everything else changing in that edition, see NEC 2026 Code Changes Explained. If you're not sure which edition your exam is based on, our NEC 2023 Code Changes Explained article and our NEC code adoption by state reference page are good starting points for figuring that out before you commit study time to edition-specific details.
Frequently Asked Questions
What's the difference between a service drop and service entrance conductors?
The service drop is the utility-owned overhead conductor run from the last utility pole to the building's point of attachment. Service entrance conductors are the conductors from that attachment point (or, for underground service, from the transition point after the service lateral) into the building's service equipment — these are generally owned and installed on the property owner's side, not the utility's.
Can a residential service really have six separate disconnects instead of one main breaker?
Conceptually, yes — the code allows up to six disconnects to be used to shut off a service, grouped together at one location and clearly marked, rather than requiring a single main disconnect. This applies broadly, not just to commercial buildings; the exact conditions and any specific limitations should always be verified against your current code book.
Is the meter part of the utility's equipment or the property owner's?
The meter itself is generally utility-owned equipment, even though it's often mounted on the property owner's building. The meter socket enclosure is typically installed by the property owner's electrician, built to the utility's approved specifications, and the meter is then installed into that socket by the utility. Always confirm the specific ownership and installation split with your serving utility.
Why does three-phase service sizing use 1.732 in the formula?
The 1.732 figure is the square root of three, and it shows up because three-phase power delivery splits a load across three conductors in a way that changes the relationship between line-to-line voltage and current compared to a simple single-phase circuit. For exam purposes, the practical takeaway is simpler than the underlying trigonometric explanation: for three-phase amperage, divide VA by (voltage times 1.732) instead of dividing by voltage alone.
Does every building need its own separate service, or can one service feed multiple buildings?
This article focuses on the more common pattern of one service per building and multiple services on a single large building, both of which are directly addressed by Article 230 concepts. Arrangements involving one service feeding multiple separate buildings on a property involve a different, more specialized set of code considerations. If you're working with multi-building properties, treat that as a distinct topic to research specifically rather than assuming it follows the same rules as the single-building examples in this article.
What's the practical purpose of the main bonding jumper at the service?
It connects the grounded (neutral) conductor system to the equipment grounding system at the service, creating the low-impedance fault-current path back to the source that allows breakers and fuses to detect and clear a fault quickly. Without it, a fault on a metal enclosure might not generate enough current to trip protection, leaving that metal part energized and dangerous.
Why would a building have more than one service?
Larger buildings sometimes use multiple services for capacity reasons (a single service isn't practical or economical for the total load), for logical separation of very different parts of a large facility, or for reliability and redundancy so the entire building doesn't lose power from a single point of failure. This is mostly a large commercial/industrial consideration rather than something typical residential or light commercial work involves.
Is the new outdoor disconnect requirement the same thing as the "six disconnects" concept?
No — they're related but distinct ideas. The "up to six disconnects" concept is about how many separate disconnects can be used to shut off a service in general. The newly required outdoor disconnecting means for one- and two-family dwellings (added in the 2026 edition) is a specific requirement that at least one accessible disconnect location be reachable from outside the structure. A service could satisfy the outdoor disconnect requirement using just one of its (up to six) disconnects, as long as that one is appropriately located outdoors and accessible.
How do I know if my state already requires an outdoor disconnect for dwellings?
That depends entirely on which NEC edition your state has adopted and whether any local amendments address it separately — some jurisdictions may have added similar requirements locally even before the 2026 NEC edition made it a national baseline concept. Check directly with your local jurisdiction and our NEC code adoption by state reference page rather than assuming either way.
Key Terms
- Service drop — the utility-owned overhead conductors running from the last utility pole to the building's point of attachment.
- Service lateral — the utility-owned underground conductors running from the utility's source to the building.
- Service entrance conductors — the conductors from the point of utility attachment (or underground transition point) into the building's service equipment, generally owned and installed by the property side.
- Service equipment — the meter socket, main disconnect(s), and associated enclosures that make up the entry point of a building's electrical system.
- Main bonding jumper — the connection made at the service between the grounded (neutral) conductor system and the equipment grounding system, creating the fault-current path that allows overcurrent protection to work correctly.
- Readily accessible — reachable without needing to move obstacles, use a ladder, or use special tools — a core principle behind service equipment location requirements.
Keep practicing service sizing and disconnect concepts with the Service Entrance, Branch Circuits and Feeder Conductors practice test and the Commercial Load Calculations quiz. For the grounding and bonding concepts referenced throughout this article, see Grounding and Bonding Explained (NEC 250) and try the Grounding and Bonding practice test. For the full load calculation process that produces the total VA figures used in the worked examples above, see Load Calculations Explained (NEC 220). And if you want the details on the new 2026 outdoor disconnect requirement in the context of everything else changing that cycle, read NEC 2026 Code Changes Explained. You can also run your own numbers with our voltage drop and ampacity calculators.
Study aid disclaimer: This article is a study aid intended to build conceptual understanding of service entrance and disconnect requirements for exam preparation. It is not a substitute for the official NEC text, your state's adopted edition, local amendments, or your serving utility's specific requirements. Always verify exact requirements, clearances, and sizing rules against your current code book and local authority having jurisdiction before performing real installation work.