Guide
Panel Sizing Methodology: Power, Wire, Protection
Load, wire, protection, then heat.
Size as a system: total the loads with demand, choose wire for ampacity and voltage drop, then select protection per NEC 240 and 430. Check heat last.
Formulas
Three-phase power
\[P = \sqrt{3} \times V \times I \times PF\]
Real power from line voltage, line current, and displacement power factor.
Voltage drop (1-phase)
\[V_d = \frac{2 K I L}{cmil}\]
K is 12.9 for copper and 21.2 for aluminum; L is one-way length.
Worked example
Checking a motor feeder
Inputs: Three 460V motors: 12.2A, 8.0A, 4.5A; 6A other continuous load.
Calculation: Feeder = 1.25 x 12.2 + 8.0 + 4.5 + 1.25 x 6 = 35.25A, about 29.3 kVA at 480V.
Lesson:
Frequently asked questions
Which comes first, wire or breaker?
Load first, then wire for ampacity and drop, then protection. Protection guards the wire; it never substitutes for ampacity.
Do these tools replace an engineered review?
No. They screen sizes against common NEC rules. Derating, short-circuit ratings, and local amendments need site review; the AHJ governs.
Sources and further verification
- NFPA 70, National Electrical Code National Fire Protection Association
- Electrical Safety in the Workplace NFPA
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