How to use: Cut out each card, fold along the center line, and read the term on one
side and the definition on the other — or just fold the answer under while you quiz yourself off the sheet.
Q
Ampacity -- definition
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A
The maximum current, in amperes, a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
NEC Art. 100
Q
Table 310.16 -- what it covers
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A
Ampacities for insulated conductors rated up to 2000V, based on ambient temperature of 30 degC (86 degF), with no more than three current-carrying conductors in a raceway/cable/earth.
NEC Table 310.16
Q
Termination temperature rating -- the 60/75 degC trap
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A
Even if a conductor is rated 90 degC (THHN), its usable ampacity is capped by the termination rating of the equipment it connects to -- typically 60 degC for 100A-or-less equipment, 75 degC above that, unless the equipment is listed otherwise.
NEC 110.14(C)
Q
Why the 90 degC column still matters even when terminations cap ampacity
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A
The 90 degC ampacity is used as the starting point for derating calculations (ambient temperature correction, conductor bundling) -- the final derated ampacity is then compared against, and capped by, the termination limit.
NEC 110.14(C), 310.15
Q
Ambient temperature correction factors
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A
Applied when the ambient temperature differs from the table's base temperature (30 degC for Table 310.16) -- higher ambient temperatures reduce the usable ampacity via a multiplying correction factor.
NEC Table 310.15(B)(1)
Q
Adjustment factors for more than 3 current-carrying conductors
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A
Bundling conductors together limits their ability to dissipate heat -- ampacity is reduced by a percentage that gets steeper as more current-carrying conductors share a raceway (e.g. 80% for 4-6, 70% for 7-9, and so on).
NEC 310.15(C)(1)
Q
What counts as a "current-carrying conductor" for adjustment purposes
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A
Ungrounded (hot) conductors always count; the neutral counts only if it carries unbalanced current from other conductors on the same circuit (e.g. it does NOT count as current-carrying on a balanced 3-phase 4-wire system feeding mostly linear loads).
NEC 310.15(E)
Q
THHN insulation rating
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A
Thermoplastic, High Heat-resistant, Nylon-coated -- rated 90 degC in dry locations. THWN adds Water resistance for wet locations, typically rated 75 degC wet / 90 degC dry.
Q
XHHW insulation
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A
Cross-linked polyethylene, High Heat-resistant, Water-resistant -- rated for both wet and dry locations, commonly 90 degC dry / 75 degC wet (XHHW-2 is 90 degC in both).
Q
Conductor material factor -- copper vs. aluminum ampacity
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A
Aluminum conductors have lower ampacity than copper of the same AWG size at the same insulation/temperature rating -- an aluminum conductor typically needs to be about two sizes larger to match a copper conductor's ampacity.
NEC Table 310.16
Q
Neutral conductor sizing on a 3-phase, 4-wire system with significant harmonics
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A
Non-linear loads (electronic ballasts, VFDs, computers) can cause the neutral to carry more current than any single phase due to triplen harmonics -- may require an oversized (not undersized) neutral in these cases.
NEC 310.15(E), Informational Note
Q
Conductor ampacity in free air vs. in a raceway
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A
Table 310.17 (single conductors in free air) generally allows higher ampacity than Table 310.16 (conductors in a raceway/cable) for the same size and insulation, since free air dissipates heat more effectively.
NEC Table 310.17
Q
Smallest conductor size generally permitted for branch circuits
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A
14 AWG copper is the smallest generally permitted for general branch-circuit wiring, protected at 15A -- smaller sizes are reserved for specific limited applications (e.g. fixture wires, Class 2 circuits).
NEC 240.4(D)
Q
Conductor color coding -- grounded (neutral) conductor
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A
Must be white or gray (or three continuous white/gray stripes on other than green insulation) for sizes 6 AWG and smaller; larger sizes may be reidentified at each termination.
NEC 200.6
Q
Conductor color coding -- equipment grounding conductor
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A
Must be green, green with one or more yellow stripes, or bare -- reserved exclusively for equipment grounding, never used for an ungrounded or grounded conductor.
NEC 250.119