What Size Wire for a 100 Amp Detached Garage 150 Feet Away?
Smallest conductor under 3%
2 AWG
240 volts, single phase, 100 amps, 150 feet one way, 2 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 current100 A
- One way run length150 ft
- Conductor size2 AWG
- Conductor materialCopper
Voltage drop2.43% on the run
| Drop | 5.83 V |
|---|---|
| Voltage at the load | 234.2 Vfrom 240 V at the panel |
| Smallest conductor under 3% | 2 AWG |
| Longest run at 2 AWG under 3% | 185 ftone way |
| Conductors to pull | 3including the equipment ground |
The material list this produces
| 2 AWG copper conductor, ft | 495 |
|---|---|
| Conduit or cable, ft | 165 |
| Breaker, 100 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 | 0.81% on the run | 6 AWG |
| 100 ft | 1.62% on the run | 4 AWG |
| 150 ft | 2.43% on the run | 2 AWG |
| 200 ft | 3.24% on the run | 1 AWG |
| 300 ft | 4.86% on the run | 2/0 |
What else matters here
Two AWG copper gets a 100 amp feeder 150 feet at 2.43 percent. Ampacity alone would let you use 3 AWG at 100 amps, so this is a case where the drop, not the ampacity, picked the wire.
Two AWG copper for 150 feet is expensive, and this is the classic place to price aluminum instead. The equivalent is 1/0 aluminum, which is normally cheaper per foot at these sizes even after going up two gauges, and direct burial aluminum feeders to outbuildings are ordinary practice.
A detached building takes a separate grounding electrode of its own, a ground rod or two, bonded to the equipment ground bar in the garage panel. The feeder ground from the house does not remove that requirement, and it is the other item that regularly fails inspection on outbuildings.
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 do I need for a 100 amp sub panel 150 feet away?
2 AWG copper, or 1/0 aluminum. Ampacity alone would allow 3 AWG copper, but the 150 foot run pushes the drop past 3 percent on that size.
Does a detached garage need its own ground rod?
Yes. A separate structure fed from the house needs its own grounding electrode system, bonded to the equipment ground in its panel, in addition to the equipment grounding conductor pulled with the feeder.
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.