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I'm attempting to connect a relatively small electric water heater in my pole barn. It requires a 60amp breaker, so my plan was to run 6/3 NM from the main panel (only required 6/2 but I decided to just get the 6/3 in case the next owner wanted to do something different) through the attic to the other side of the building (pole barn garage), come down through the ceiling and down the wall a few inches, and splice/convert to MC inside a massive junction box that is already connected to a piece of unistrut up near the ceiling -- primarily so I don't have exposed NM running 4-5 feet down the wall to the disconnect.

Once the splice from the 6/3 NM to the MC cable was made inside the junction, it would go from the junction box as MC to the disconnect switch, where it would then go to the water heater itself. In retrospect, I should have just run 6/3 MC from the main panel, through the attic, and directly to the disconnect. I wanted to save money with 6/3 NM for most of the run and here I am.

The problem I'm encountering is that the 6/3 NM from the electrical supply house came with 10awg solid copper ground, and 6awg stranded conductors. The 6/3 MC cable I ordered from an online supply house (local only had rolls of 500'+) has 6awg stranded conductors and 8awg stranded ground according to the person I talked to. It is my understanding (I could be mistaken) that you should not splice different-sized wires together.

Is it safe/to code to splice 10awg solid copper ground to 8awg stranded copper ground? If not, is there any solution other than running 6/3 MC from the main panel all the way to the disconnect switch? I've already run the 6/3 NM through the attic, secured it at proper intervals, etc. so ideally I would love not to have to take it all down.

EDIT: Wanted to add this is a 11kW water heater, solely to heat water for a reverse osmosis system to get the stated GPD rate. The stated GPD rate assumes 65 PSI inlet, 77F water, no more than 500ppm. The correction factor is .52 if you have 50F inlet water, which I do from my well.

Also, I have completely separate service to my pole barn (different transformer, meter, panels, etc.) than my house, and have done load calcs and have the room for 60amps without a problem.

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    Wow. One of the best written, most clear electrical wiring questions we've had in quite a while. Well done!
    – FreeMan
    Commented Dec 6, 2022 at 15:01

3 Answers 3

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Nothing wrong with splicing different size wires. You'll want to ground the "massive junction box" so I'd just mount a dual ground lug in the box and connect your two ground wires to it.

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It is quite common to splice different wire gauges together, if they are at or above the minimum for the breaker.

You cannot splice 14 gauge to 12 gauge on a 20 amp breaker, but you can on a 15 amp breaker.

You will not have a problem if use the proper size wire nut.

It seems that the two building are attached. NM cannot be used outside in wet conditions(outside is wet).

The only time ground splices give a problem is with the ground wire from the panel to ground rods. You'll need special non removable splices.

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Correcting a few misconceptions

I'm attempting to connect a relatively small electric water heater

A standard full-house 20-50 gallon tanked water heater is 4500W and takes a 25A breaker (typically used with a 30A breaker).

So unless you are confusing a 2-pole 30A for a "60", a 60A heater is actually rather large. Are you sure your pole barn has the feeder ampacity for that? Are you sure your house has the spare service ampacity to add a 60A load? You need to do a NEC Article 220 Load Calculation on both.

It requires a 60amp breaker, so my plan was to run 6/3 NM from the main panel

That's a problem. #6 NM-B is only 55A wire. See for yourself: Copper, left column. #6 NM, UF, TW = 55A.

enter image description here

Note that there is no such thing as 60A wire. Why do manufacturers have trouble receiving this memo?

For why NM is in the 60C column, see NEC 334.80.

come down through the ceiling and down the wall a few inches, and splice/convert to MC inside a massive junction box that is already connected to a piece of unistrut up near the ceiling -- primarily so I don't have exposed NM running 4-5 feet down the wall to the disconnect.

A simpler way of doing that is to use a stick of literally any kind of metal pipe into a junction box. Some have been known to use electrical conduit, which fits neatly onto junction boxes (but then, so does pipe if the box is threaded).

I wanted to save money with 6/3 NM for most of the run and here I am.

And here you are. Now you'll need to either figure out the heater is actually 30A, or see if you can derate that heater to 55A (not likely). Another option is to go with a tanked heater which are typically 4500W or 3800W for the mobile home models. They're insulated quite well; just the same, in my shop I use a wind-down timer switch such as an Intermatic SW12HWK.

It seems like you "blindly" overbuild instead of do research, and that's how the #6 NM error snuck in there. I would flip that around: research then wisely overbuild e.g. I'm all for the /3 thing.

Once the splice from the 6/3 NM to the MC cable was made inside the junction, it would go from the junction box as MC to the disconnect switch, where it would then go to the water heater itself.

The disconnect can be avoided by permanently installing a factory approved "locking kit" on the circuit breaker. This clips onto the breaker and provides a hole in the right place to use a lock to prevent handle movement. The purpose of both the lock and the disconnect is maintenance lockout/tagout; either will suffice.

On the other hand, the disconnect is a great place to splice from aluminum to copper wire. Nothing wrong with aluminum at large sizes; it's proven safe because it's on lugs rated for aluminum (typically made of aluminum) and it is torqued to spec with a torque driver (which must be done on ALL terminals by the way).

So what about dissimilar size splices?

It is my understanding (I could be mistaken) that you should not splice different-sized wires together.

There's no problem with that, as long as the connecting device is listed/rated for that.

I've already run the 6/3 NM through the attic, secured it at proper intervals, etc. so ideally I would love not to have to take it all down.

Leave it in the sky for some future application which does not exceed 55A. Such as 50A EV charging (the EV companies didn't get the memo either).

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    How much you want to bet it is a small tankless. In a quick search, I found 14.4kw == 60A x 240V gets you 2.93 GPM. So small relative to a whole house, but fine for a sink or two in a barn. And a crazy waste when you can just get a small tank and use the usual 4500W/25-30A etc. Commented Dec 5, 2022 at 21:29
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    OP added useful details - it is a tankless (I knew it!) at 11kW = 46A @ 240V. Which is interesting. Because without a continuous derate, it should only need 50A breaker. But with a continuous derate it needs 46 x 1.25 = 57.5A. Which makes me think the calculation is 11kW = 44A @ 250V. Which x 1.25 = exactly 55A! Which meets the NM 6 AWG limit and gets a 60A breaker. Mystery solved and cable is OK. Commented Dec 6, 2022 at 2:46
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    Correction based on the spec sheet. They're rating it at 11kW @ 220V yet coming up with 54A. Not sure how...Ohm's law calculator...here we go: 11kW @ 220V means 4.4 Ohms and 50A. Bump it up to 240V and you get 13kW and 54A. So no continuous derate (that would put it over 60A (whether starting at 50A or 54A), so that means they're properly rating it for 6 AWG NM at 55A max. (just squeak by) and the 60A breaker is breaker only not actual usage. And life is good. Commented Dec 6, 2022 at 3:00
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    The way to look at from a user/installer standpoint is: Device needs 'x' Amps. I need 'y' wire to support that. I need a breaker which protects to a level of 'x' Amps to match the wire rating. Oops, 'x' breaker is not available (generally, not talking about supply shortage issues - those do not exempt code, usually) so I can round up to the next size breaker but am on Scout's Honor to stick to the 'x' current. x = 55 (54 and a fraction == 55), y = 6 AWG NM, round breaker to 60. Commented Dec 6, 2022 at 3:03
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    @ShunsukeSengoku "only an 11kW" is huge. All tankless are huge. If the unit is actually 55A (and that sounds about right given the kW) then yes, it probably is a 54A unit, and perhaps the manufacturer did get the memo. If it's UL listed and the instructions (not brochure) say 6 AWG NM, then you are all set. Commented Dec 6, 2022 at 6:06

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