Electrical question, running a couple of 120 line off of a 240 circuit

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Forum topic by Bill1974 posted 01-08-2014 05:51 PM 1749 views 0 times favorited 7 replies Add to Favorites Watch
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163 posts in 3990 days

01-08-2014 05:51 PM

Break box is pretty much full and it’s a rats nest from the previous electricians and home owners, so I don’t want to mess with it right now. Eventually I will just replace the panel with something wire more space.

In my garage I have 40 amp 240 line (8-3 wire, 3 conductors and a ground) for an air compressor. The compressor isn’t used too much unless I am spraying something. I was wondering if I could tap off two 120 lines off of this outlet? I know that I can, but was wondering how to do it safely? I don’t know of a 40 amp 120 outlet, that has a common plug. I am concerned with the potential of overloading the wire between the 120 out and the 8-3 wire. I know I can put 10 or 12 gage wire in and probably be fine, but I know it’s not correct. Are there 120 volt outlets with breakers built in? Or is my only option to put in a break box on the end of the 8-3 line and go out from there?

7 replies so far

View BArnold's profile


175 posts in 2837 days

#1 posted 01-08-2014 06:06 PM

As you stated, you know you can take two seperate 120 outlets from a 240 circuit. That’s what a breaker box does all the time. If I understand your post correctly, your compressor is plugged to the 240 outlet. You want to unplug it and plug in a junction box with two 120 circuits, which would be no problem. You can safely use 12 gauge wire on the 120 side.

On the other hand, if I were doing it, I’d mount a second outlet box beside your 240 outlet and tap into the 120 in the 240 box with 12 gauge. That way, your compressor stays plugged in and you still have access to a 120 circuit. I’m not saying that arrangement would be to code, just that it will work in the technical sense.

-- Bill, Thomasville, GA

View Bill1974's profile


163 posts in 3990 days

#2 posted 01-08-2014 06:13 PM

The second part of what you said is what I would like to do, but my concern is nothing is protecting the 12 gauge wire. I know it’s not too common to trip a 20 amp circuit, but the 12 gauge wire could see up to the 40 amps that would trip the breaker. I am pretty sure the 12 gauge wire would be toast at that point. Really was just looking to see if there was a simple way. I kind of figured a second breaker box is what was going to be needed. I was looking online and I see the BORG has a 12 space loadcenter with 5 breaker for $35, hard to pass up and really the right way to do it.

View JayT's profile


6419 posts in 3216 days

#3 posted 01-08-2014 06:40 PM

In order to do it safely, I’d say put in a sub panel and then run a 240 outlet and one or two 120 outlets out of that.

-- - In theory, there is no difference between theory and practice. In practice, there is.

View bigblockyeti's profile


7073 posts in 2725 days

#4 posted 01-08-2014 06:53 PM

To maintain everything to code (assuming it is currently) I would make a pigtail to plug into the outlet and split the two legs in a metal junction box at the opposite end of the plug. In the box you could install two 15 or 20 amp receptacles. To make thing safer you could use 8/3 + ground for the pigtail wire, it’s expensive but it would then make the weak link your power tool cord instead of possibly starting a fire next to your wall outlet. This would be to code only when not plugged in, therefore not messing with your current house wiring. Not sure of the legal ramifications if something were to go terribly wrong as a result of it being plugged in, but a little common sense can go a long way to preventing such problems.

-- "Lack of effort will result in failure with amazing predictability" - Me

View Bill1974's profile


163 posts in 3990 days

#5 posted 01-08-2014 07:04 PM

Turns out it cost a little more and it’s easier to just get a load center and do it correctly. It’s less them $40 for a small load center and few breakers to go in it. Plus it’s readily available parts at HD, Lowes or an electrical supply house. A 4 circuit box is $12 then it’s just the cost of the breakers. All the other parts I would need are what I would need anyway. So it makes sense to do it right (safe and inexpensive).

Now I just kick myself for not putting in a large amp circuit when I put the compressor in. Then again it’s all going to get redone when I get time to turn the convert the room that was part of the garage back to garage space. But I am sure that will be further away then I think.

View BArnold's profile


175 posts in 2837 days

#6 posted 01-08-2014 09:26 PM

Good thinking on going the sub-panel route, Bill1974.

My situation was somewhat different than yours but similar in function. The shop building on the property we bought never had electrical run to it. The previous owner ran an extension cord from the house to operate his tools. Don’t ask!!!

I had an electrician look at the job and got a quote from him. Turned out he could do the entire job for what it would have cost me to buy the materials. To make a long story shorter, he installed a “100A” sub-panel with 30 breaker positions in it. The main breaker for the sub-panel is in the distribution panel on the side of my house. The electrician terminated the run in the sub-panel and I ran all of my shop circuits as I built out my shop. I provisioned for way more than I’ll ever need. I have two 30A 220V, six 20A 220V, and fourteen 110V circuits. Counting up the breaker capacity, it’s a total of 460 amps! If I add up the greatest possible load I would have in a practical situation, it would be around 70 amps. No worries!

My point being that your 40A 220V circuit can be set up to handle a lot more “sockets” than the pure amp capacity would indicate.

-- Bill, Thomasville, GA

View John_'s profile


251 posts in 2711 days

#7 posted 01-08-2014 11:19 PM

Quad Circuit Breaker

If your panel can handle it, you could replace your two pole, 240v breaker with a quad breaker. They come in different configurations. Here is an example – 30 amp 240 in the middle flanked by two separate 20 amps

You panel has to have “notched bus stabs” for these breakers to fit

Another option would be a quad breaker with two 240v circuits and just run a new line for a sub panel from it

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