EV Charger Circuit Calculator

625.41: EVSE is continuous, and that is not a judgement call. On a general branch circuit, whether a load runs for three hours or more is something you have to decide. On a charger it is settled — a car charges for hours, every time. So the 125% factor applies unconditionally, and this calculator does not offer it as a choice. That single rule is where most EVSE installation errors come from: a 40 A charger needs a 50 A circuit, not a 40 A one.

The same factor runs backwards on a plug-in installation. If a circuit may only carry 80% of its rating continuously, then a receptacle only accepts a charger rated at 80% of what is stamped on it — which is why the plug-in option runs out well before the hardwired one does.

625.42: more than one charger is a decision, not a sum. Where several EVSE share a supply, the code permits either sizing for the full connected load of all of them, or installing a listed energy management system that limits what they can draw at once and sizing for that limit. Those are very different installations at very different prices, and which is right depends on the service. This calculator sets out both rather than quietly picking one.

Then the ordinary work happens. The design current goes to the standard-rating search for a breaker, and to the ampacity and voltage-drop checks for a conductor. Run length matters more here than on most branch circuits, because EVSE is often at the far end of a garage or in a detached one, carrying a large current continuously — the conditions under which drop stops being academic.

A 48 A hardwired Level 2 charger — 11.52 kW at 240 V, which is currently the largest common residential unit — on a 50 ft run from the panel.

The continuous factor. 48 × 1.25 =60 A. Both the device and the conductor are sized from this figure, not from the 48 A on the label.

The breaker. 60 A is itself a standard 240.6(A) rating, so it rounds to a 60 A breaker with nothing wasted. That neatness is why 48 A is such a common charger rating — it lands exactly on a stocked breaker size.

The conductor. The ampacity check asks Table 310.16 for the smallest copper conductor carrying 60 A at the 75 °C column, and gets6 AWG at 65 A. The voltage-drop check over 50 ft asks for less than that, so ampacity binds and 6 AWG is the answer — with the finished circuit dropping 1.23%.

Note where that conductor figure came from: the same Table 310.16 data the ampacity and wire size calculators read. It is not a number typed onto this page, and if that table is ever corrected this example moves with it.

What changes the answer. Push the run out to 150 ft and voltage drop overtakes ampacity, calling for a larger conductor on the same 60 A breaker. Add a second identical charger and 625.42 arrives: two units is 120 A of continuous demand, which very few existing dwelling services will absorb without either an upgrade or load management.

The whole Level 2 range in one table, with the 125% figure shown as its own column so the step from nameplate to circuit is visible rather than implied. Use it to check an existing installation, or to work backwards — if the garage already has a 50 A circuit, this tells you a 40 A charger is the largest thing that belongs on it.

Read the last two rows together. A 48 A charger and a 50 A charger are nearly the same product, and they need different breakers and, at longer runs, different conductors. Two amps of extra charging rate is rarely worth the circuit it costs.