Water heater wiring

[6768rogues]

I typically see 40 gallon water heaters wired with 10 gauge wire. A house my son bought to flip has a 40 gallon water heater that was wired part of the way with 12 gauge wire and the rest of the way with 10 gauge wire (on a 30 Amp breaker, I know that is not smart). The label on the water heater says it is 240 volts and 4500 Watts. 20 Amps of 240 Volts will supply 4800 Watts, so 12 gauge on a 20 Amp circuit appears to be sufficient.
Am I missing something?
Are most 40 gallon water heaters 4500 Watts or should I upsize the wire and breaker to accommodate a future change of applicances?

 

[The FIB]

I think 40 gallon electric water heaters usually have 2- 4500 watt elements.

 

[The FIB]

Even if we are talking about a single element at 4500 watts, at a full 240 volts you have an actual load of 18.75 amps on #12 wire, this load should be on a #10 wire minimum (assuming you have copper wire). 30 amp breaker feeding a #12 wire is a no go for sure.

 

[zmaxmotorsports]

Both elements generally aren't being loaded at the same time,That being said Ive always wired them with 10g wire on a 30a 2p myself.

 

[Slowgsr]

4500w tank @ 240v requires 10awg & 30amp 2 pole. Should have local disconnect which no one does - fyi. Hot water tank capacity has nothing to do with the element(s) sizes.

 

[black00lightning]

I was planning on having a j box at floor level, guess I can use the disconnect switch as the j box and then run the cable to the heater.

 

[wyliesdiesels]

I typically see 40 gallon water heaters wired with 10 gauge wire. A house my son bought to flip has a 40 gallon water heater that was wired part of the way with 12 gauge wire and the rest of the way with 10 gauge wire (on a 30 Amp breaker, I know that is not smart). The label on the water heater says it is 240 volts and 4500 Watts. 20 Amps of 240 Volts will supply 4800 Watts, so 12 gauge on a 20 Amp circuit appears to be sufficient.
Am I missing something?
Are most 40 gallon water heaters 4500 Watts or should I upsize the wire and breaker to accommodate a future change of applicances?

Circuits for continous loads need to be sized at 125%. 18.75a (4500w) x 125% = ~23a. So either #12 THHN or #10 NM-b needs to be used...

In reality, the nameplate not the size of the element should be consulted for the correct circuit size...

EDIT: due to 240.4d #12 is not allowed to be used for more than 20a even if #12 THHN with an ampacity of 25a is used...

4500w tank @ 240v requires 10awg & 30amp 2 pole. Should have local disconnect which no one does - fyi. Hot water tank capacity has nothing to do with the element(s) sizes.

wait. Wouldnt a larger capacity tank dictate larger elements?

 

[Charles (in GA)]

Indeed, the fact that any heating device, be it water or air, is classified by the NEC as continuous means that the 80% rule applies as Wylies has noted. You can easily have different size elements in the same size water heater, many units have various options as to the wattage size for a given model of heater, so you need to know for sure what the wattage is for each of the elements in the heater. Both elements DO NOT operate at the same time. the top thermostat is the determining device for this. If the top of the heater is hot enough to satisfy the top thermostat, then the bottom element will heat unit the bottom thermostat is satisfied. If the tank is completely emptied of hot water, the top thermostat will CUT OUT the bottom element and switch on the upper element until the top portion of the tank is hot, at that point, the upper thermostat being satisfied, shifts control to the lower thermostat, which then switches on the lower element until it is satisfied.

Common element sizes are 3800 watt and 4500 watt. Marathon for example has both sizes available, the larger one being a different metal than the smaller one, and I'm told it (the 4500 watt) is more problematic on well water around here.

 

[Norcal]

Circuits for continous loads need to be sized at 125%. 18.75a (4500w) x 125% = ~23a. So either #12 THHN or #10 NM-b needs to be used...

In reality, the nameplate not the size of the element should be consulted for the correct circuit size...

[/b]

The use of 12 AWG on a branch circuit would be 20A max, there are other NEC articles that do allow 12 to be used for 25A but not for a appliance (other then A/C) circuits.

I checked to see if there was a exception for a WH, but could not find one, but could be wrong, won't be the 1st time nor will it be the last time.

 

[wyliesdiesels]

The use of 12 AWG on a branch circuit would be 20A max, there are other NEC articles that do allow 12 to be used for 25A but not for a appliance (other then A/C) circuits.

I checked to see if there was a exception for a WH, but could not find one, but could be wrong, won't be the 1st time nor will it be the last time.

Yup! Silly me. I wasnt thinking about that!

240.4(d) bit me in the ***...