Lessons · Electrical · reading a nameplate
The nameplate tells you what to size for
Every appliance and motor carries a plate with its voltage, its watts or VA or horsepower, its amps, and its frequency; the amps line, or watts divided by volts, is the number the circuit is sized from.
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
A customer wants a receptacle for a new kiln, a welder or a dryer. The answer is on the plate, and the plate is the only honest source; guessing from the size of the box is how a 30 A appliance ends up on a 20 A circuit.
How to think about it
Read volts first, because a 240 V plate and a 120 V plate with the same watts draw different amps. Then read amps directly if the plate gives them. If it gives only watts, divide by volts. If it gives VA, use VA for the wire and breaker, because VA is what the wire carries even when the watts are less.
Worked example
Plate: 120 V, 1800 WA portable heater. Volts and watts, no amps line.
I = 1800 / 120 = 15 AThe whole of a 15 A circuit. In practice it wants a 20 A circuit to itself.
Plate: 240 V, 24 AA cooktop. The amps are given; no arithmetic, just read it.
Plate: 120 V, 8 A, 960 VA, 770 WA motor. VA is bigger than W because of power factor; the wire carries the 8 A regardless of the watts.
Your turn
A plate reads 240 V, 4800 W. Write the current.
I = 4800 / 240 = A
Solve one, graded on the server
The trap
Reading the watts and forgetting the volts. 3600 W is 30 A on a 120 V plate and 15 A on a 240 V plate; the same number needs a different circuit.