Nearly every North American home is split-phase — two 120 V legs, 180° apart, from a center-tapped transformer. Most calculators make you guess how to enter it. Here is the calculator with the three cases spelled out.
1. A pure 240 V load (both legs). Dryers, EV chargers, well pumps: current flows out one hot and back the other — the neutral carries nothing. Select 240 V · both legs; the ×2 factor covers the two hots. This is the calculation that matches a "single-phase 240 V" entry on other tools.
2. A 120 V load (one leg). Receptacles, lights: out the hot, back the neutral. Select 120 V · one leg. Same ×2 physics — but the identical volts lost are now measured against half the voltage, so the percentage doubles. Worked pair: 16 A over 80 ft of 12 AWG copper drops 4.94 V either way — that's 2.1% at 240 V but 4.1% at 120 V. Same wire, same load current, pass vs fail.
3. A balanced multiwire branch circuit (MWBC). Two 120 V circuits on opposite legs sharing one neutral: when both legs carry equal current, the neutral currents cancel and each load's return trip is free. Effective drop per 120 V load is then half the one-leg calculation — model it by selecting 240 V with one leg's current, or treat it as the 120 V case for a conservative answer. This cancellation is the entire reason MWBCs exist, and why an unbalanced or broken neutral on one is dangerous: the cancellation becomes a voltage divider between the two legs.
Whenever a load comes in both flavors, the 240 V version buys you 4× the distance for the same conductor and percentage budget (half the current × double the budget). For outbuildings, never run long 120 V circuits — run 240 V out and split locally. The free-lunch guide runs the economics.
Both terms describe the same residential service: it is technically single-phase power, delivered as two 180°-opposed 120 V legs ('split'). If your panel has two hot busbars and 240 V appliances, it's split-phase.
For a pure 240 V load, no — it carries no current. For a 120 V load, yes — it's the return conductor, already covered by the ×2 factor. For a balanced multiwire circuit, the neutral currents cancel and effective drop halves.
Their 'single phase' at 240 V equals our '240 V · both legs'; at 120 V it equals '120 V · one leg'. The math is identical — the labels here just remove the guessing.