How big a subpanel do I need?

Add up your connected loads, apply the NEC 220.42 demand factor to the general lighting portion, add continuous loads at 125% and the larger of heat or cooling, then divide by the feeder voltage to get the current. Round up to the next standard breaker, typically 60, 100, 125, or 200 A.

What is the NEC 220.42 demand factor?

It lets you reduce the general lighting and receptacle load because not everything runs at once. The first 3000 VA counts at 100% and the remainder up to 120000 VA counts at only 35%, which significantly lowers the calculated feeder load.

Why does the feeder need a voltage-drop check?

Subpanels are often far from the main panel, and a long feeder loses voltage along the way. The tool computes round-trip resistance for the chosen copper conductor and flags any feeder whose drop exceeds the recommended 3% so you can upsize it.

Does the conductor size match the breaker exactly?

The conductor is selected to carry the calculated feeder current using the copper 75°C ampacity column, and it must also be protected by the chosen breaker. For most subpanels the conductor and breaker pair up directly, but always verify ampacity after derating.

Do I size the feeder for the subpanel's bus rating or the actual load?

You size the feeder and its breaker for the calculated demand load, not the panel's bus rating. A 200 A bus panel fed by a 100 A calculated load only needs a 100 A feeder, though the panel must still be rated for at least the feeder breaker.

Subpanel Sizing Calculator

A subpanel feeder is sized from the loads it actually serves, after the NEC allows realistic demand reductions. This calculator sums your connected loads, applies the NEC 220.42 demand factor, and returns the minimum subpanel ampacity, the copper feeder conductor, the main breaker rating, and a feeder voltage-drop check for garages, workshops, ADUs, and tenant build-outs.

How it works

Loads are entered in volt-amperes (watts) so the code demand factors apply directly:

general demand  = first 3000 VA × 100% + remainder × 35%   (NEC 220.42)
total demand    = general demand + appliances + continuous × 1.25 + larger of heat/AC
feeder amps     = total demand / feeder voltage
breaker (OCPD)  = next standard breaker ≥ feeder amps
conductor       = smallest 75°C copper wire with ampacity ≥ feeder amps
voltage drop    = feeder amps × 2 × R_per_1000ft × length / 1000

The 35% demand factor on general lighting reflects that not every receptacle and light runs simultaneously, while continuous loads are kept at 125% because they run for hours at a time.

Example and tips

A workshop subpanel with 8000 VA of general lighting, 6000 VA of fixed appliances, a 3000 VA continuous load, and a 5000 VA heater works out to roughly (3000 + 5000 × 0.35) + 6000 + 3750 + 5000 = 19,500 VA, or about 81 A at 240 V, calling for a 100 A subpanel on a 3 AWG copper feeder. Tips: enter the larger of heat or cooling only, never both, since they rarely run together; keep the feeder voltage drop under 3% by upsizing on long runs; and remember the panel bus rating must be at least the feeder breaker, even if the load is smaller.