Wire Size Calculator
Find the minimum wire gauge that meets ampacity and voltage drop limits.
| Recommended Gauge | 10 |
|---|---|
| Ampacity | 35 A |
| Voltage Drop | 2.48 V |
| Voltage Drop | 2.07% % |
· Wire size was increased beyond ampacity to keep voltage drop at or below 3%.
· Result assumes single-phase, 2 conductors, up to 3 conductors in raceway, 75C termination rating. Verify with NEC and a licensed electrician.
How It Works
Wire sizing involves two separate limits that both must be satisfied: 1. Ampacity: the wire must carry the full load current without overheating. NEC Table 310.16 lists the allowed ampacity by gauge, material, and insulation temperature rating. 2. Voltage drop: resistance in the wire causes voltage to fall along the run. The calculation uses: Voltage drop = 2 x I x (ohms per 1000 ft / 1000) x length The factor of 2 accounts for both the outgoing and return conductors. This calculator returns the larger of the two required gauges so both limits are met simultaneously. Most engineers target 3% or less voltage drop on branch circuits. Over 5% is generally considered poor practice, though the NEC does not mandate a specific limit.
Example
20 A at 120 V over 100 ft at 3% max drop. 3% of 120 V = 3.6 V allowed. 12 AWG copper (1.98 ohms/kft) gives 2 x 20 x 0.000198 x 100 = 7.92 V drop (6.6%), so the calculator sizes up to hold the drop within 3%.
Frequently Asked Questions
Does this account for conduit derating?
No. When more than 3 current-carrying conductors share a raceway, NEC Table 310.15(C)(1) requires derating. Multiply the load current by the appropriate correction factor before entering it here.
Can I use aluminum wire on a 20A branch circuit?
Generally no for 14 or 12 AWG branch circuits in most applications. Aluminum is common for feeders and service conductors in larger gauges. Always verify local code.
What does temperature rating mean?
It refers to the conductor insulation's rated maximum temperature. Most modern wiring devices are listed for 75°C terminations, which is the column you should use in most cases.