Transformer Calculations -2 : NEC Code Questions
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Question 1 of 6
1. Question
1 pointsThe “Next up size” protection rule is not applicable when sizing over current protection device of transformer secondary conductors.
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Refer: NEC 240.4 (B)
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Question 2 of 6
2. Question
1 pointsWhat is the primary over current device rating of a 2 wire 480 V to 120 V transformer rated at 1.8 KVA?
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Primary current = 1800 / 480 = 3.75 A
The primary current is less than 9A So as per NEC Table 450.3[B] primary protection is 167% of primary current.
Primary protection = 3.75 * 1.67 = 6.26 or 6 A fuse
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Question 3 of 6
3. Question
1 pointsWhat is the primary protection required for a 2 wire 480V to 120 V Transformer rated at 900 VA?
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Primary current = 900 / 480 = 1.87 A
Primary current is less than 2 A
Refer NEC Table 450.3[B]. Primary protection is 300% of primary current
Primary protection = 1.67 * 3 = 5 A
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Question 4 of 6
4. Question
1 pointsWhat is the rating of primary over current protection device required for a 2 wire 480V to 120 V transformer rated at 7KVA?
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Primary current = 7000 / 480 = 14.58 A
Primary current is greater than 9 A
Refer NEC Table 450.3[B] primary protection is 125% of primary current
Primary protection = 14.58 * 1.25 = 18.22 => 15 A fuse
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Question 5 of 6
5. Question
1 pointsWhat is the minimum size secondary conductor is required for a 2 wire 480V to 120 V transformer rated 2.2 KVA with 600C terminals?
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Secondary current = 2200 / 120 = 18.33 A
Refer: NEC Table 310.15(B)(16)
Conductor with size 12 AWG rated at 600C is suitable.
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Question 6 of 6
6. Question
1 pointsDry type transformer rated more than …………… are permitted to install in hollow spaces of buildings.
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Refer: NEC 450.13 (A)