350 AWG vs 400 AWG Copper
Going from 350 AWG to 400 AWG adds 25A of capacity and extends maximum wire run distance before hitting the 3% voltage drop limit. The trade-off is cost: 400 AWG 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 | 350 AWG | 400 AWG |
|---|---|---|
| 90°C ampacity | 350A | 380A |
| Usable ampacity | 310A | 335A |
| Standard breaker | 200A | 200A |
| Conduit (EMT) | 2" EMT | 2" EMT |
Voltage drop crossover
At what distance does 350 AWG fail the 3% recommendation and 400 AWG still passes? This shows the distance where upsizing is required, not optional.
Load: 310A on 240V single-phase.
| Distance | 350 AWG | 400 AWG | Verdict |
|---|---|---|---|
| 25 ft | 0.2% | 0.2% | Either works |
| 50 ft | 0.5% | 0.4% | Either works |
| 75 ft | 0.7% | 0.6% | Either works |
| 100 ft | 0.9% | 0.8% | Either works |
| 125 ft | 1.2% | 1.0% | Either works |
| 150 ft | 1.4% | 1.2% | Either works |
| 200 ft | 1.9% | 1.7% | Either works |
| 250 ft | 2.4% | 2.1% | Either works |
| 300 ft | 2.8% | 2.5% | Either works |
When to upsize
Stay with 350 AWG when
- Circuit load is within 310A capacity
- Wire run is short (under ~319 ft at 240V)
- Budget is the primary constraint
Upsize to 400 AWG when
- You need more than 310A (loads up to 335A)
- Wire run is long and voltage drop is a concern
- Future load growth is expected on this circuit
Detailed ampacity: 350 kcmil copper, 400 kcmil copper. Material comparison: 350 kcmil Cu vs Al, 400 kcmil Cu vs Al.
Frequently asked questions
Can I use 350 kcmil instead of 400 kcmil?
Only if your load is 310A or less and your wire run is short enough to stay within 3% voltage drop. 350 kcmil carries 310A maximum. If you need more than 310A, or your run exceeds the distance limit, you must use 400 kcmil or larger.
How much more does 400 kcmil cost than 350 kcmil?
400 kcmil typically costs 40-60% more per foot than 350 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 350 kcmil: buy NM-B cable for in-wall residential or individual THHN conductors for conduit. For 400 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 400 AWG Copper circuits
As an Amazon Associate we earn from qualifying purchases.
NEC 2023 references verified April 2026