400 kcmil vs 500 kcmil Copper
Going from 400 kcmil to 500 kcmil adds 45A of capacity and extends maximum wire run distance before hitting the 3% voltage drop limit. The trade-off is cost: 500 kcmil copper costs more per foot and fits in the same conduit (2" EMT).
Specification comparison
Side-by-side specifications per NEC 2023 for THHN copper conductors.
| Property | 400 kcmil | 500 kcmil |
|---|---|---|
| 90°C ampacity | 380A | 430A |
| Usable ampacity | 335A | 380A |
| Standard breaker | 200A | 200A |
| Conduit (EMT) | 2" EMT | 2" EMT |
Voltage drop crossover
At what distance does 400 kcmil fail the 3% recommendation and 500 kcmil still passes? This shows the distance where upsizing is required, not optional.
Load: 335A on 240V single-phase.
| Distance | Verdict | 400 kcmil | 500 kcmil |
|---|---|---|---|
| 25 ft | Either works | 0.2% | 0.2% |
| 50 ft | Either works | 0.5% | 0.4% |
| 75 ft | Either works | 0.7% | 0.5% |
| 100 ft | Either works | 0.9% | 0.7% |
| 125 ft | Either works | 1.1% | 0.9% |
| 150 ft | Either works | 1.3% | 1.1% |
| 200 ft | Either works | 1.8% | 1.4% |
| 250 ft | Either works | 2.2% | 1.8% |
| 300 ft | Either works | 2.7% | 2.2% |
When to upsize
Stay with 400 kcmil when
- Circuit load is within 335A capacity
- Wire run is short (under ~333 ft at 240V)
- Budget is the primary constraint
Upsize to 500 kcmil when
- You need more than 335A (loads up to 380A)
- Wire run is long and voltage drop is a concern
- Future load growth is expected on this circuit
Detailed ampacity: 400 kcmil copper, 500 kcmil copper. Material comparison: 400 kcmil Cu vs Al, 500 kcmil Cu vs Al.
Frequently asked questions
Can I use 400 kcmil instead of 500 kcmil?
Only if your load is 335A or less and your wire run is short enough to stay within 3% voltage drop. 400 kcmil carries 335A maximum. If you need more than 335A, or your run exceeds the distance limit, you must use 500 kcmil or larger.
How much more does 500 kcmil cost than 400 kcmil?
500 kcmil typically costs 40-60% more per foot than 400 kcmil for the same insulation type. The larger wire also requires larger conduit (2" EMT vs 2" EMT). However, the lower voltage drop can save energy on long runs.
What to buy
If you are buying wire for a new installation, consider your total project requirements. For 400 kcmil: buy NM-B cable for in-wall residential or individual THHN conductors for conduit. For 500 kcmil: same insulation types apply but expect approximately 40-60% higher cost per foot. When in doubt about which gauge, upsize: the cost of the wire is a small fraction of the total project cost (labor, conduit, breaker, permits), and upgrading later means repulling the entire run.
Common products for 500 kcmil Copper circuits
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NEC 2023 references verified February 2026