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NEC Article 450.3(B) & Table 450.3(B)Worked ExampleElectrical

45 kVA 3-Phase Transformer Primary and Secondary Full-Load Amps

Calculate primary (480V) and secondary (208V) full-load current (FLC) and minimum overcurrent protection for a 45 kVA 3-phase dry-type transformer under NEC 450.3.

Direct Answer
Primary FLC: 54.13A (70A Breaker) | Secondary FLC: 124.91A

Calculated strictly in accordance with NEC Article 450.3(B) & Table 450.3(B).

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Given Scenario Parameters

wattage45000
voltage480
continuousContinuous (125%)

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Interactive Electric Load Calculator

Pre-loaded with NEC 220.51 & 424.3(B) parameters

Nameplate wattage of the heating unit or load

Line-to-line or line-to-neutral

NEC 424.3(B) mandates that fixed electric space heating equipment branch circuits must be sized at 125% of the total load current.

Calculated OutputLive Recalculation
Base Running Current93.75 A45000W ÷ 480V
Required Ampacity (125%)117.19 A93.75A × 1.25
Minimum Circuit Breaker:125 Ampere
Recommended Conductor (75°C Cu):1 AWG Copper
Overcurrent Protection Rule:NEC 240.4 & 424.3(B)
Adjust wattage or voltage on the left to recalculate continuous service loads instantly.

Step-by-Step Solution

Formal engineering derivation citing applicable National Electrical Code articles.

Governing Code Equation
FLC = kVA / (Voltage * 1.732)
Citing NEC Article 450.3(B) & Table 450.3(B)
1

Calculate 480V Primary FLC: 45,000 / (480 * 1.732) = 54.13 Amperes.

2

Calculate 208V Secondary FLC: 45,000 / (208 * 1.732) = 124.91 Amperes.

3

Determine primary overcurrent protection per Table 450.3(B) at 125%: 54.13 A * 1.25 = 67.66 A (Next standard size up: 70A breaker).

Conclusion: Primary FLC: 54.13A (70A Breaker) | Secondary FLC: 124.91A
NEC Article 450.3(B) & Table 450.3(B)