By Murugan Vellaichamy · 2026-01-22 · 7 min read
The most common voltage drop question in the real world is not abstract — it is "what do I bury in the trench to the garage?" Here is the complete decision, in order.
A 60 A subpanel runs lights, openers, a compressor or a welder, and a modest EV trickle. A 100 A subpanel runs a serious shop with simultaneous loads. The trench costs the same either way; conductor cost is the difference, and 100 A future-proofs an EV charger.
The NEC's 5% total recommendation has to cover feeder plus branch circuits. Spend 2% on the feeder so every branch inside the garage still has 3% to work with. At 240 V, 2% is 4.8 V.
At 100 ft: a 60 A feeder at 2% wants 3 AWG copper or 1 AWG aluminum; a 100 A feeder wants 1 AWG copper or 2/0 aluminum. (At 50 ft, ampacity governs instead and the sizes relax — run your exact distance through the solver at a 2% budget.) Aluminum is the overwhelming choice at these sizes: a 2/0 aluminum feeder costs a fraction of 1 AWG copper and is completely standard with AL-rated lugs.
Four conductors (two hots, neutral, ground — separate ground required for a detached structure with a feeder), typically in 1¼″–1½″ conduit buried per local rules. Upsizing the conduit one trade size costs a few dollars and makes the pull humane — check the fill calculator.
The punchline: at outbuilding distances the trench and labor dominate cost. Going one conductor size beyond minimum is the cheapest insurance you will ever buy against doing it twice.