300 kcmil vs 350 kcmil Copper
Going from 300 kcmil to 350 kcmil adds 25A of capacity and extends maximum wire run distance before hitting the 3% voltage drop limit. The trade-off is cost: 350 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 | 300 kcmil | 350 kcmil |
|---|---|---|
| 90°C ampacity | 320A | 350A |
| Usable ampacity | 285A | 310A |
| Standard breaker | 200A | 200A |
| Conduit (EMT) | 2" EMT | 2" EMT |
Voltage drop crossover
At what distance does 300 kcmil fail the 3% recommendation and 350 kcmil still passes? This shows the distance where upsizing is required, not optional.
Load: 285A on 240V single-phase.
| Distance | Verdict | 300 kcmil | 350 kcmil |
|---|---|---|---|
| 25 ft | Either works | 0.2% | 0.2% |
| 50 ft | Either works | 0.5% | 0.4% |
| 75 ft | Either works | 0.8% | 0.7% |
| 100 ft | Either works | 1.0% | 0.9% |
| 125 ft | Either works | 1.3% | 1.1% |
| 150 ft | Either works | 1.5% | 1.3% |
| 200 ft | Either works | 2.0% | 1.7% |
| 250 ft | Either works | 2.5% | 2.2% |
| 300 ft | Upsize to 350 kcmil | 3.1% | 2.6% |
When to upsize
Stay with 300 kcmil when
- Circuit load is within 285A capacity
- Wire run is short (under ~294 ft at 240V)
- Budget is the primary constraint
Upsize to 350 kcmil when
- You need more than 285A (loads up to 310A)
- Wire run is long and voltage drop is a concern
- Future load growth is expected on this circuit
Detailed ampacity: 300 kcmil copper, 350 kcmil copper. Material comparison: 300 kcmil Cu vs Al, 350 kcmil Cu vs Al.
Frequently asked questions
Can I use 300 kcmil instead of 350 kcmil?
Only if your load is 285A or less and your wire run is short enough to stay within 3% voltage drop. 300 kcmil carries 285A maximum. If you need more than 285A, or your run exceeds the distance limit, you must use 350 kcmil or larger.
How much more does 350 kcmil cost than 300 kcmil?
350 kcmil typically costs 40-60% more per foot than 300 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 300 kcmil: buy NM-B cable for in-wall residential or individual THHN conductors for conduit. For 350 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 350 kcmil Copper circuits
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NEC 2023 references verified February 2026