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A tankless water heater I bought arrived with a pig tail with 3 6 AWG wires. Black, Red, and Yellow with Green stripe ground. The black and red go to a 2 pole 60 amp breaker with the yellow/green going to ground/common on a very old Cutler-Hammer breaker box.

I went to the local box store and they didn't have the 3 6 AWG wire but did have a two 6 AWG with a 12 AWG ground. I was told that it would work just fine with the Black 6 AWG, White 6 AWG and 12 AWG ground.

I connected everything and the heater works great. However, I'm concerned that the 12 AWG wire is a huge mistake and it might actually be a fire hazard and I should remove that wire and replace with a 3 6AWG wire.

Should I be concerned or not?

Thanks in advance.

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  • Are you sure it was a 12 AWG ground? 6/2 w/ground usually has #10 ground wire.
    – JACK
    Dec 9, 2020 at 13:55
  • It could very well be #10, I didn't measure it, just eyeballed it. My concern is it being so much smaller than the #6. Both of the #6 are carrying 124V and I'm just concerned about the ground being too small.
    – Earl Davis
    Dec 9, 2020 at 14:17
  • The ground is a non current carrying conductor unless there's a fault and then it's only temporary.That's why it's smaller. #10 is the right size for #6 NM cable.
    – JACK
    Dec 9, 2020 at 14:20
  • Cutler-Hammer CH's are fantastic boxes and very well supported. Stop using the word "old" to describe it and use the word "premium".
    – Harper - Reinstate Monica
    Dec 9, 2020 at 17:22
  • Wait-a-minute -- can you link the thing you bought from the box store?
    – ThreePhaseEel
    Dec 10, 2020 at 0:42

3 Answers 3

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NEC allows safety ground conductors to be smaller than active conductors.

Neutral carries current under normal conditions, however your appliance does not use neutral. Thus, you MUST re-mark both ends of your white wire with black electrical tape to connote that it is a hot wire. This is mandatory; you are no longer allowed to skip this marking "if the usage is obvious".

Neutral is not ground. Ground carries current only under fault conditions, i.e. long enough for the breaker to trip, so it doesn't need to be sized for continuous use.

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  • Thanks for the comment regarding the white wire and the black tape. Will do.
    – Earl Davis
    Dec 9, 2020 at 19:53
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To the best of my knowledge, which has been failing lately, you can't get 6/2 NB w/ #6 ground or with a #12 ground. What you can get is 6/2 NB w/ #10 ground which would be OK. I would verify that your ground wire is really #12. My guess is that it's #10. Take a piece of it into an electrical supply store and they can verify what size you have. There are no safety concerns with a #10 ground being used in this circuit. Enjoy your new water heater.

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  • Or just slip your quality wire stripper onto it as a gauge. It should be apparent.
    – isherwood
    Dec 9, 2020 at 14:21
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    The conductor sizes are frequently printed on the cable jacket too. If all conductors were the same size it might say something like 8/2 NM WG to indicate 8 ga, 2 insulated conductors, with ground. The mixed-gauge cable here might say something like 6 GA 2 CDRS 10 GA 1 CDR for example.
    – Greg Hill
    Dec 9, 2020 at 14:22
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I would double check the wire size a 60 amp breaker will require #10 for the equipment grounding conductor per NEC table 250.122. Although the true limiting factor would be that # 6 wire is limited to the 55 degree table (55 amps) per NEC 110.14.C.2 if all components in the system are listed for use at 75C 110.14.C.1.B.2 allows 75 Column to be used. My jurisdiction has really been hitting this one hard lately the terminal blocks can state 75 but of the equipment listing is for 60 we are stuck at 55 amps on #6. I believe all the #6 I have has #10 grounds some has #6.

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  • Thanks for the feed back. I checked the wire and it is a #10. Per the heater manufacturer, the breaker is a 2 pole 60 amp breaker. I put the #6 white on the common terminal of the breaker and the #6 black to the other. I spliced the #6 black to the black in the heater pig tail and the #6 white to the red in the pig tail and the #10 ground to the yellow with green stripe ground of the pig tail. The black and white both are carrying 124 volts. My concern and question: with such a small ground running to the ground/common bar is there a potential fire hazard due to the size of the ground?
    – Earl Davis
    Dec 9, 2020 at 14:39
  • I reread Jack's comment above about there being no safety concerns. That answered my question and I apologize for missing that statement. Thanks to all for the input.
    – Earl Davis
    Dec 9, 2020 at 15:02
  • The key is that ground wires do not carry any current under normal circumstances. They only carry current during a fault. As a result, they get to (with larger circuits) be sized a step or two smaller than the main conductors. Not a fire hazard because if there is a problem causing full current to go through ground it will almost always result in a fairly quick breaker trip, which would happen long before the ground-abnormally-carrying-current has a chance to overheat to a dangerous level. There could be exceptions (I think more in 120V than 240V) of multiple failures (e.g., neutral
    – manassehkatz-Moving 2 Codidact
    Dec 9, 2020 at 15:12
  • back to breaker panel broken and neutral shorted to ground in the device) but the code takes these things into account.
    – manassehkatz-Moving 2 Codidact
    Dec 9, 2020 at 15:12

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