What Size Wire for a 50 Amp Sub Panel 100 Feet Away?
Smallest conductor under 3%
6 AWG
240 volts, single phase, 50 amps, 100 feet one way, 6 AWG copper.
When the figures look right, the circuit material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- System voltage240 V
- PhaseSingle phase
- Load current50 A
- One way run length100 ft
- Conductor size6 AWG
- Conductor materialCopper
Voltage drop2.05% on the run
| Drop | 4.92 V |
|---|---|
| Voltage at the load | 235.1 Vfrom 240 V at the panel |
| Smallest conductor under 3% | 6 AWG |
| Longest run at 6 AWG under 3% | 146 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 6 AWG copper conductor, ft | 330 |
|---|---|
| Conduit or cable, ft | 110 |
| Breaker, 50 A, each | 1 |
| Terminations and lugs, each | 6 |
One way run length compared
Drop is a straight line in length. Double the run and you double the drop, which is why the same wire that is fine in a basement is not fine across a yard.
| One way run length | Voltage drop | Smallest conductor under 3% |
|---|---|---|
| 50 ft | 1.02% on the run | 10 AWG |
| 100 ft | 2.05% on the run | 6 AWG |
| 150 ft | 3.07% on the run | 4 AWG |
| 200 ft | 4.1% on the run | 4 AWG |
| 300 ft | 6.15% on the run | 2 AWG |
What else matters here
Six AWG copper, which is also the ampacity answer for a 50 amp feeder at 75 degrees C, so in this case the two requirements agree and there is nothing to argue about.
It comes out at 2.05 percent, comfortably inside the guideline, and 6 AWG holds 3 percent out to 146 feet on this circuit. If the sub panel is further than that, or if you expect to load it harder later, step to 4 AWG.
A sub panel feeder is four wires, not three: two hots, a neutral and a separate equipment ground. The neutral and the ground must be isolated from each other in the sub panel, which means removing the bonding screw and adding a separate ground bar. That single detail is the most commonly failed item on a sub panel inspection.
What this calculator assumes
- The formula is the standard one: drop equals 2 times K times amps times the one way length, divided by the circular mil area of the conductor. Three phase uses 1.732 in place of the 2.
- K is 12.9 for copper and 21.2 for aluminum, the usual figures for a conductor running warm in conduit. Circular mil areas come from NEC Chapter 9, Table 8.
- Enter the one way distance, not the loop. The formula doubles it for you, because current has to get back.
- The 3 percent branch circuit and 5 percent total figures in 210.19 and 215 are informational notes rather than requirements. Nobody fails an inspection on voltage drop alone, but motors, LED drivers and long well pump runs care a great deal.
Questions people ask
What size wire for a 50 amp sub panel 100 feet away?
6 AWG copper. It satisfies both the 50 amp ampacity requirement and the 3 percent voltage drop guideline, landing at 2.05 percent over that run.
How far can 6 AWG carry 50 amps?
About 146 feet at 240 volts before it passes 3 percent drop. Ampacity itself does not limit the distance, only the drop does.
Can I use aluminum instead?
Yes, and on a long feeder it often saves real money. Aluminum carries about 61 percent of copper's conductivity, so you go up roughly two sizes for the same drop. What matters is the terminations: lugs listed AL or CU-AL, antioxidant compound on the strands, and a torque wrench rather than a feel for it. Aluminum fails at the connection, not in the middle of the run.
The voltage drop calculator takes any dimensions you like and hands back the same list.