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Electric Hot Water Heater trivia


Jeff Remas
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During your morning inspection you read the data plate on a 50gallon electric hot water heater.

The data plate states 240v and 4500W per element (2- 1 upper & 1 lower)

All terminals rated at 75 degrees.

What size circuit is required?

A) 10awg NM with 30A breaker

B) 10awg NM with 25A breaker

C) 12awg MC with 25A breaker

D) 10awg MC with 30A breaker

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I don't have time to calculate right now, but here is what I believe to be the applicable code:

422.13 Storage-Type Water Heaters.

A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load.

210.19 Conductors — Minimum Ampacity and Size.

(A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Should help when checking the chart........

Gotta go.

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Originally posted by randynavarro

Originally posted by Richard Saunders

I would say any of those would be OK, even though I have never seen a hot water heater ;)

You see them everyday.

Tank style water heaters heat a lot of water that's already warm.

Sorry for the thread drift, Jeff.

Sorry. Pet peeve. I guess you can call them whatever you want, whether correct or not.

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I'll take a stab at it...

4500 VA (watts) / 240 volts = 18.75 Amps

18.75 A x 125 % (continuous load factor) = 23.5 Amps

23.5 Amps needs 10 AWG copper minimum.

A 25 Amp breaker should suffice.

NM wire is satistactory provided the wire is not exposed.

MC / Flex metal conduit should used to house any wire that would have been exposed. Or if you feel the need, run the entire circuit in MC /Flex.

The only thing I don't like about my answer is that I always see a 30 Amp breaker on a Water Heater circuit. So my guess is that I missed some thing in the code requirements or no one makes a 25 Amp circuit breaker???.

That's my story and I'm stickin' with it (unless I'm wrong, then I'll change it).

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Originally posted by SWagar

I'll take a stab at it...

4500 VA (watts) / 240 volts = 18.75 Amps

18.75 A x 125 % (continuous load factor) = 23.5 Amps

23.5 Amps needs 10 AWG copper minimum.

A 25 Amp breaker should suffice.

NM wire is satistactory provided the wire is not exposed.

MC / Flex metal conduit should used to house any wire that would have been exposed. Or if you feel the need, run the entire circuit in MC /Flex.

The only thing I don't like about my answer is that I always see a 30 Amp breaker on a Water Heater circuit. So my guess is that I missed some thing in the code requirements or no one makes a 25 Amp circuit breaker???.

I think you did great. The thing you missed was an asterisk in a table. Can you find it?

That's my story and I'm stickin' with it (unless I'm wrong, then I'll change it).

That's the spirit!

- Jim Katen, Oregon

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Originally posted by SWagar

Jim,

Are you talking about the asterisk in table 310.16 (2008 NEC), that points to 240.4 (B)(3)? This seems to allow the next larger breaker size to be used i.e. 25 A upsized to 30 A. Otherwise I can't find it ?!?

-Scott

Sorry to be unclear. Yes, the asterisk is the one in table 310.16 but it points to 240.4(D) which limits the overcurrent protection on #14, #12 & #10 conductors. The point I was trying to make is that people frequently confuse the rules for a conductor's ampacity with the rules for its overcurrent protection. They're two different things.

In terms of ampacity, the water heater needs a conductor that can carry 23.5 amps (18.75x125%). A #12 copper wire will do that.

However, in terms of overcurrent protection, a #12 wire (regardless of its ampacity) has to be protected at nothing greater than 20 amps. So even though the #12 wire has the ability to carry the load, the breaker doesn't.

That forces us to go to the next size up, #10 with plenty of ampacity at 30 amps for NM and 35 amps for MC. The maximum size breaker for it is 30 amps which is also plenty big enough.

So, as Jeff said, A, B & D would each be fine.

As for why you don't see 25-amp breakers (the asterisk has nothing to do with that, sorry for the confusion) is that, while they do exist, they're special items. 30-amp breakers are more common & often cheaper.

- Jim Katen, Oregon

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