ContractorHandbook

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.

It does not stop at a number.

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panelloadone way length of the run, ftvolts at the far end are lower than volts at the panel3 percent is the limit to design to, and length is what breaks it
Copper is not free of resistance, so voltage arrives smaller than it left, and the loss grows with distance and current. Three percent is the design target, and the fix is nearly always one size bigger conductor rather than a shorter run.

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

Drop5.83 V
Voltage at the load234.2 Vfrom 240 V at the panel
Smallest conductor under 3%2 AWG
Longest run at 2 AWG under 3%185 ftone way
Conductors to pull3including the equipment ground

The material list this produces

2 AWG copper conductor, ft495
Conduit or cable, ft165
Breaker, 100 A, each1
Terminations and lugs, each6

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 lengthVoltage dropSmallest conductor under 3%
50 ft0.81% on the run6 AWG
100 ft1.62% on the run4 AWG
150 ft2.43% on the run2 AWG
200 ft3.24% on the run1 AWG
300 ft4.86% on the run2/0

What else matters here

Two AWG copper gets a 100 amp feeder 150 feet at 2.43 percent.

On a feeder this long the aluminum trade is often worth pricing. Where copper needs 2 AWG, aluminum wants 1/0, and 1/0 aluminum lands at 2.51 percent on this run.

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. The 150 foot run pushes 3 AWG copper to 3.06 percent, just past the guideline.

Can I use aluminum instead?

Yes. 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.

Different numbers?

The voltage drop calculator takes any dimensions you like and hands back the same list.

Other common sizes

50 A sub panel, 100 ft6 AWG20 A circuit, 150 ft6.21% on the run

6 more sizes are answered on the voltage drop calculator page

Contractor Handbook · https://contractorhandbook.com/calculators/voltage-drop-calculator/100-amp-detached-garage-150-ft/