I just in the process of removing my oil fired water heater with an electric heater




MAX.
WATTS 120 V 208 V 240 V
1500 20 A 10 A 10 A
3000 35 A 20 A 20 A
3500 40 A 20 A 20 A
4500 30 A 25 A
5500 35 A 35 A




I read through the manual, said 20a blahblahblah when I test the line going into the tank its saying 120v, wouldn't I need the 35a breaker? Not the 20a? Since the power going in is only 120? I'm assuming if the power entering the tank was 240 the 20a would suffice?


But the plate on my tank says


240v 3000w (upper and lower) 1 phase


I'm just confused


Really appreicate it

You are mixing electrical terms.

Voltage (120 or 240) - This the difference of "Potential" usually from the Neutral point.

For example, your home (U.S.A.) is supplied by a single phase circuit. The phase is split at the "Neutral Point" and then measured between the neutral point and each "Line".. The lines are "Line 1" and "Line 2" in a conventional residential home.

Between the Neutral point and each Line, there is 120 volts. The voltage measured across each line (Line 1 to Line 2) is 240 Volts ac.

Amperage is akin to the volume. And bears a mathematical relationship to voltage.

Voltage is not the main factor, however. For example a 12 AWG cable can handle either a 120 Vac or a 240 Vac connection as long as the amperage (load) does not exceed 20 amps (dedicated circuit or 16 amps on a multi-outlet circuit).

Mathematically speaking Watts / Volts will yield the required amperage.

3000 w / 240 Vac = 12.5 amps. If the circuit is on a 120 Vac source, then 3000 / 120 = 25 amps.

So if your circuit is a 240 Vac circuit, than a 20 amp circuit is perfect, however, if the circuit is being supplied by a 120 Vac source, than a 25 amp circuit would be used. This would actually turn out to a 30 amp circuit and require the use of #10 AWG cable.

In an electric water heater, the heating elements work independently of each other. When the upper element is on, the lower element is off.

First,you must supply 240 volts because the heating elements are engineered for 240 volts.

Next the cable must be sized based on the amperage draw. That is, 3000 watts at 240 volts equals 12.5 amps.

But NEC says:
"Storage-type Water heaters A branch circuit supplying a fixed storage-type water heater having a capacity of 120 gallons of less shall have a rating of 125 percent of the name plate rating of the water heater."

12.5 X 125% = 15.3 amps

Meaning you could use 12/2 with ground cable and 20 amp breaker. The maximum breaker size is determined by the cable size.

However, should you determine that you have a higher than anticipated hot water demand or wish to have a faster recovery you can change out the heating elements to 4500 watts. Standard heating elements are 3000 and 4500 watts. So, 4500 watts at 240 volts is 18.75.

18.75 X 125% = 23.44 amps

That would exceed you 12/2 cable and 20 amp breaker.

So the usual installation is 10/2 with ground cable and 30 amp breaker. Also means should you at some time in the future, replace the water with a larger capacity you won't be changing the wiring again.

Hank,

Thanks for the catch on the 125%. I know better!