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Running 240V water heater on 120V?

Subdood

Subdood

Photon Wrangler
Joined
Mar 27, 2022
Messages
581
Location
NE Kentucky
Hey all,

We have a 3600W 30 gallon water heater which runs on 240VAC. I have a EG4 6500 single phase inverter and would like to run the water heater off of 120VAC during sunny days as a load dump for the solar after it's done charging up the batteries. If it is a sunny day, they're usually recharged by noon or so. So I'd like to use any excess solar to heat the water.

The WH runs on 12awg wires into a 20A DP breaker in the house panel. I have an idea of using a 20A DPDT switch to go between 240V grid and 120V inverter power. The switch would be mounted near my critical loads panel that the inverter powers. I realize that at 120V the tank element would only put out about 900W, but that's not a big deal during the day if there's enough sun.

The circuit would look something like the enclosed pic. The switch would be a 20A DPDT light switch. It would switch the two water tank inputs between the 120V inverter sub panel, and the 240V house panel.

Think this would work? Would it be safe? Code compliant?

Thanks
 

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I put in a Leviton 30A DPDT switch to switch my water heater and clothes dryer from grid to generator (both 240V).

1288-W

leviton.com leviton.com

Note that this 30A switch is rated for continuous 30A current.
Not sure if they have a lesser 20A version.

Tip: 6 wires per switch fills up your box more quickly than expected (thus the extra space on the right to meet code).

1715103583003.png
 
Last edited:
Would it work? Yeah.
Code compliant? Not really. Water heater through what effectively looks like a light switch would at least raise eyebrows.
Would I do it? Probably lol. I used 3 way switches as my method of switching 120V circuits from grid to generator.
 
SparkyJJO said:
Would it work? Yeah.
Code compliant? Not really. Water heater through what effectively looks like a light switch would at least raise eyebrows.
Would I do it? Probably lol. I used 3 way switches as my method of switching 120V circuits from grid to generator.
Click to expand...

Nothing wrong code-wise with a DPDT switch, it is UL listed.
You can't use regular light switches or "three-way" switches, both poles have to be switched for 240V.
 
SparkyJJO said:
Would it work? Yeah.
Code compliant? Not really. Water heater through what effectively looks like a light switch would at least raise eyebrows.
Would I do it? Probably lol. I used 3 way switches as my method of switching 120V circuits from grid to generator.
Click to expand...
Ok thanks. Is that why your name is "Sparky"??

My first ever electrical experiment involved hooking up an old wire plug to a big stereo speaker. Then I plugged it in.

It hummed for about a second then poof..
 
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Subdood said:
Thanks @ricardocello . Yeah Leviton does make a 20A version, model number is 1286. Does yours have a center "off" position, the 1286 does.
Click to expand...
Get the 30A version, because the 1286 spec sheet says 16A max, with 20A rating (not continuous 20A).

Actually, the 16A max is for motors and inductive devices. I just like lots of margin for these things.
 
ricardocello said:
Get the 30A version, because the 1286 spec sheet says 16A max, with 20A rating (not continuous 20A).

Actually, the 16A max is for motors and inductive devices. I just like lots of margin for these things.
Click to expand...
Probably a good idea to go with the 30A. Those suckers are expensive tho, the 1286 runs for about $50.

I considered using a water heater timer, but those are even more expensive, and I don't want to clutter up the wall too much.

I've yet to spring this idea on my wife.. I wonder if she appreciates my mad scientist mindset.
 
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Subdood said:
I considered using a water heater timer, but those are even more expensive, and I don't want to clutter up the wall too much.
Click to expand...
I put my switches below my sub-panel and ran some conduit because all of the wires were convenient at that location.

Subdood said:
Probably a good idea to go with the 30A. Those suckers are expensive tho, the 1286 runs for about $50.
Click to expand...
Same price for the 1288, they come in brown and white.
I think I paid $35 about 3 years ago. There's the inflation rate.
 
ricardocello said:
Same price for the 1288, they come in brown and white.
I think I paid $35 about 3 years ago. There's the inflation rate
Click to expand...
Ok. I was looking for a simple new push mower to replace our 15 year old Craftsman, Lowe's is selling them for a smooth $349?!? I think we paid about $150 for the original one.
 
Subdood said:
Ok. I was looking for a simple new push mower to replace our 15 year old Craftsman, Lowe's is selling them for a smooth $349?!? I think we paid about $150 for the original one.
Click to expand...
I used to laugh at my father when he said newspapers were only 5 cents when he was a kid, complaining that they were $0.75 in the 80s.
Now I'm at his age, and you can't even buy newspapers!

Prices that have shocked me since COVID:
Rigid Conduit ($83 for 2" x 10') wow
Pressure treated 6x6 lumber
Any shipping costs for things on pallets.
 
ricardocello said:
I used to laugh at my father when he said newspapers were only 5 cents when he was a kid, complaining that they were $0.75 in the 80s.
Now I'm at his age, and you can't even buy newspapers!

Prices that have shocked me since COVID:
Rigid Conduit ($83 for 2" x 10') wow
Pressure treated 6x6 lumber
Any shipping costs for things on pallets.
Click to expand...
Seems like the only thing that hasn't gone up lately is solar equipment.

Seems like lumber and conduit is cheaper now than when I bought my stuff 1-2 years ago. We bought a 4x6x8ft PT post at Lowe's and it was about $22, I believe.

Both my Lifepower4 batts were shipped on a pallet, and shipping was free both times. But I ordered during a Sig Solar free shipping promotion, otherwise it would've been about $230.

So you said you had to add a spacer because of all the wires in your switch box? Why is that?
 
Last edited:
Subdood said:
So you said you had to add a spacer because of all the wires in your switch box? Why is that?
Click to expand...
Code 314.16 of the NEC: Box Fill

up.codes

Box Fill Calculations | UpCodes

Explore a searchable database of US construction and building code. Code regulations are consolidated by state and city for easier navigation.
up.codes up.codes

314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies

Use the image to help answer the question. Some feel that there may be an issue with the spray foam behind the box in contact with the NM cable. See NEMA bulletin 95 for additional information.
www.electricallicenserenewal.com
www.bhg.com

How Many Wires Can Safely Go in a Junction Box? Here's How to Find Out

Before doing any electrical work for a home remodel, it's important to know about the National Electrical Code for wiring to avoid exceeding the electrical box volume.
www.bhg.com www.bhg.com

In my case, I had qty 12 AWG 10 current carrying wires coming into/out of the box to go to two DPDT switches.

I'm rusty on my counting, but on this forum, I'm sure someone will quickly correct me if I am wrong.

Each switch takes two 10 awg allowances, ground is one allowance, so the total 10 AWG count for my box is 12+2+2+1 = 17
According to code table in 314.16, this requires the box to be 17 * 2.50 cu in = 42.5 cubic inches.

The standard plastic two-gang box is only 32 cu in, so I got a three-gang box, they come in 44 and 55 cu in sizes.
EDIT: Except I didn't use plastic, they were metal boxes with conduit running THHN.
I route the THHN wires in the empty space to make it all fit neatly.
Code is satisfied.
 
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Ok thanks. I would have seven 12ga wires since it's a 20A circuit. Don't know if I could use a single gang box for those and the switch. I'll check out your links.
 
Subdood said:
Ok thanks. I would have seven 12ga wires since it's a 20A circuit. Don't know if I could use a single gang box for those and the switch. I'll check out your links.
Click to expand...
I would guess: 6 wires + 1 for ground + 2 for switch itself = 9
Each 12 AWG counts for 2.25 cu. in., so you need a minimum box size of 20.25 cu. in.

The cu. in. of the box is usually stamped on it.
Looks like the biggest single gang boxes at Home Depot and Lowes are 20 or 21 cu inches.
Get the biggest one if you want to meet code (it may be a 4" deep one).

Or don't follow code, and answer the old question "How many wires will fit in this box?" with "One more!";)
 
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