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I moved into a 50 year old home about 2 years ago and we’ve been using a (miraculously working) water heater from 1994. Yesterday, its life finally came to an end when I found it had sprung a leak in our laundry room. Since it’s a rental, we had a technician come to assess it and he’s going to bring us a new one later today. Since the house is pretty old, however, I checked our circuit today and found something (maybe?) troubling.

We’re still using the old fuse boxes from the sixties and while our current water heater has its own 30amp circuit, the wire that runs to the unit is 12 gauge, which I’ve read is a no no. Our current heater is 40 gallons and 3000 watts with a max amp of 18.75, so it’s probably never maxed out the load, but I’m concerned that a new one might.

When I mentioned it to the technician, he waved off my concern and said the new units run at a lower max amp than this and 12 gauge would be totally fine. He’s obviously more experienced than I am so I have no choice but to defer to the professional but I just figured I’d check here to ease my mind a little.

Is the tech right or am I putting myself at risk by having a new unit put in?

Thanks in advance for any help or advice!

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    If its so efficient and uses less power, then why hasn't it been spec'ed to use 12ga wire and a 20A breaker/fuse? I'd want 20A protection on that 12ga wire.
    – JPhi1618
    Jan 7 '20 at 20:19
  • What make and model is the breaker in question, and the water heater for that matter? Jan 8 '20 at 2:12
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The installer's language sounds like the standard platitudes they give people to get them to drop the subject.

You can't put a 30A breaker on a 12 AWG feeder cable, except for certain very arcane motor applications where the data plates from the numbers support it.

  • There is a new type of water heater which is basically an electric motor. This is a heat pump type water heater, which is now mandatory for large tanks above (55?) gallons, and optional at lower sizes.

  • Gas dryers take very little power, and will be happy as a clam on 12 AWG wire; however they are not eligible for the special motor rule, which means the breaker on 12 AWG wire must be 15A or 20A.

But still, the installation must be done in accordance with the unit's instructions, NEC 110.3(b), and that definitely includes exact breaker size and minimum wire size.

By minimum, I mean if the book calls out 14 AWG, you're allowed to use the larger 12 AWG. That's always OK. But if the unit calls out a 15A breaker, then you must use that even if you've upsized the wire to 500 kcmil.

I would share your concerns with the landlord. If the old cable is readily accessible, it's not expensive to change it, and 30A / 10 AWG is the de facto standard for whole house water heaters.

If the landlord is a jerk about it, call the AHJ (Authority Having Jurisdiction) and warn them of the improper installation. However, expect backlash if you do.

The only thing that would give me pause is if this is an old FPE or Zinsco panel. In that case, your option for a new breaker is a Connecticut, and they're even worse than the breakers which got these panels condemned in the first place. If it's a Challenger panel, Challenger breakers are bad news, but the panel is fine, and Eaton BR/C breakers are actually listed for the panel.

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NEC 240.4(D)(5) Limits #12 copper to a maximum circuit breaker size of 20 amps. So if that existing wire is used then the breaker needs to be changed.

NEC 422.10/422.13 Requires storage type water heater circuits be rated at 125%, so that breaker can only accept a water heater rated at a maximum of 16 amps.

Watts are calculated Volts x Amps = Watts, so the maximum power water heater at 240 volts is 240V x 16A = 3840 Watts. You can order one under that maximum like this one from Home Depot, but they don't carry them in stock.

The installation instructions will detail the specific requirements as the above code sections will require, I have never seen a water heater that the instructions deviated from the code requirements.

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You really need to tell us what water heater you have now and what it will be replaced with. We can't really answer your questions without such data.

You indicate you have "3000 watts with a max amp of 18.75". I don't get it.

Most water heaters run on 240V, 3KW would require 12.5A. 120V (if such exists) would require 25A. 18.75 is neither.

Most electric water heaters sold by Home Depot draw 4500W, this would require 18.75A which is a little marginal for a 12 gauge circuit. There are a few rated at 3800W at 15.83A.

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  • Actually it is irrelevant what water heater is used, you can't use #12 for any water heater on a 30A breaker. Jan 7 '20 at 23:21
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    That's true, but if the breaker is 30A, it's usually not hard or expensive to change it to a 20A breaker. Changing the wires is a much bigger deal. Jan 8 '20 at 0:18
  • Yes exactly+, pure resistive water heaters (the vast majority) shouldn't spike to a maximum like that, the watt/amp/volt numbers should jibe perfectly. Maybe it's a heat pump unit with a motor?? Jan 8 '20 at 3:53

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