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Lessons · Electrical · a 240 V circuit, end to end

A 240 V circuit end to end: from the nameplate to the receptacle

One appliance, one circuit: read the nameplate, find the amps, apply 125 percent if the load is continuous, choose the next standard breaker at or above that, choose a wire whose ampacity covers the breaker, check the voltage drop for the run, and finish with the receptacle that matches the breaker.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

The dryer is the first circuit most apprentices run alone. Every step of the trade is in it: the arithmetic, the tables, the four-wire cord that carries 240 V for the heater and 120 V for the motor and drum light, and the fact that it all gets checked by an inspector who has seen every mistake.

How to think about it

Work down the list and write each line: nameplate, amps, continuous or not, breaker, wire, voltage drop, receptacle. Two hots, a neutral (the dryer has 120 V parts) and a ground: a four-wire circuit on a two-pole breaker. Before landing anything in the panel: main off, lock, verify dead.

Worked example

Nameplate: 240 V, 5.4 kW
The dryer's plate. 5400 W.
I = 5400 / 240 = 22.5 A
Not a continuous load, so no 125 percent.
Breaker: 30 A, two-pole
The next standard size above 22.5 A, and the standard dryer circuit.
Wire: 10 AWG copper is rated 30 A; 10-3 cable with ground
Two hots, a neutral, a bare ground.
Voltage drop, 40 ft: 2 × 12.9 × 22.5 × 40 / 10380 = 2.24 V = 0.9 %
Well under 3 percent. On a 200 ft run this line is where the wire size changes.
Receptacle: NEMA 14-30, four wire
Matches the 30 A, 240/120 V circuit.

Your turn

A 240 V water heater at 3000 W, treated as continuous. Write the line that gives the required circuit amps.

3000 / 240 ×  = 15.6 A

The trap

Running three wires to a dryer because it is a 240 V appliance. The heater is 240 V; the motor, timer and light are 120 V and need the neutral. Three-wire dryer circuits with the neutral doing the ground's job are the old way, and a new circuit is four wires.

Practise a 240 V circuit, end to end on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.