How to Calculate Wire Sizing
What is Wire Sizing?
Wire sizing selects conductor diameter to limit voltage drop and prevent overheating. Code requires ampacity matching and voltage drop limits.
Formula
Calculate voltage drop: VD = (2×K×I×L) / (1000×A) for DC or single-phase
- VD
- (2×K×I×L) / (1000×A) for DC or single-phase — (2×K×I×L) / (1000×A) for DC or single-phase
- K
- K value — Variable used in the calculation
- I
- I value — Variable used in the calculation
- L
- L value — Variable used in the calculation
- A
- A value — Variable used in the calculation
- DC
- DC value — Variable used in the calculation
Step-by-Step Guide
- 1Look up ampacity from tables at operating temperature
- 2Calculate voltage drop: VD = (2×K×I×L) / (1000×A) for DC or single-phase
- 3Select wire where VD <3% feeder, <5% combined
Worked Examples
Input
I = 100 A, L = 50 m, copper wire, assume 60°C ampacity table
Result
AWG 6 (106 A) with ~3.1% drop, marginal
AWG 4 better choice
Common Mistakes to Avoid
- ✕Using listed ampacity without derating for temperature or bundling
- ✕Neglecting voltage drop in long feeder runs
Frequently Asked Questions
Why limit voltage drop?
Excessive drop reduces equipment performance, efficiency, and motor starting capability.
How does temperature affect wire sizing?
Higher temperature reduces ampacity; must derate if ambient >30°C.
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