My current setup in a house I moved into is main 100A panel with two subpanels, one with a bathroom fan and two 240v line heaters (for a single room not attached to the main heating of the house for who knows what reason), and a second subpanel which has the garbage disposal, sump pump, another bathroom fan, and an outdoor 20A outlet that rarely gets used.

I have some server equipment that sometimes draws >2000w, and I want to run it on a 240v circuit. The advice I've gotten is to run it on a 30A 240V dedicated circuit with a L6-30r receptacle at the end. The run is just about 25ft.

My plan is:

  1. Remove unused 15A GFCI circuit I installed but never used a while back and move the 20A circuit to the right of it over 1 to the left, then I should have two slots side by side to access both bus bars
  2. Install this QO circuit breaker: https://www.amazon.com/Square-Schneider-Electric-QO230CP-Two-Pole/dp/B00002N5IH/
  3. Run 10/2 Mx cable up the wall, along the underside of joists (attached to each), and then down the concrete wall to gang box and receptacle.

Am I missing anything here? Of course, I'll turn off power to the subpanel first and verify nothing is live before messing in the box.

Would love to hear advice and thoughts before I embark-thanks!

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1 Answer 1


Yes, you're missing the elephant in the room! The dangling power strip stapled the ceiling that's powering all the other stuff. Since you're wiring a receptacle anyway, get rid of that too while you're at it.

30A is too much. /2 is too little.

2000W is only 8.3 amps at 240V. The advice you got about running a 30A circuit is misinformed. You cannot plug an 8.3A appliance into a 30A receptacle. The only way you can use a 30A/240V socket is with a PDU. (and if you have a 30A PDU, that's fine, go for it, however...)

However, your choice of /2 cable (having no neutral) paints you into a corner. You can only power 240V devices, even if you had a PDU.

I propose a 20A multi-wire branch circuit (MWBC)

This gives two complete 120V legs on 1 cable, and also supports 240V loads.

Run 12/3 armored cable. Run it to two metal junction boxes. One houses two GFCI receptacles. The other houses a NEMA 6-20 receptacle.

(I was hoping for an easier way to do this, but you're in an unfinished basement, and that requires GFCI protection on 120V outlets).

The GFCI box needs to be a 4-11/16" square box, which is large enough to comfortably fit two GFCI receptacles (don't try to cram them into a 4" square box). The 6-20 box can be 4" square. Connect them with a short rigid conduit "nipple".

The GFCIs have warning tape on them. Do NOT remove it, that will only lead to errors. There's nothing you want behind that tape.

You bring the /3 cable into the GFCI box, and split the neutral to both GFCIs "Line Neutral". The two hot wires (red and black) will each go onto one GFCI's "Line Hot".

So far, this is just a common MWBC. Now, we do the trick.

Make or get 2 red and black "pigtail" wires long enough to go over to the other junction box. Read the GFCI instructions, they have a procedure for double-tapping each screw. On each GFCI, "Line Hot" gets double-tapped with one supply hot wire and one pigtail.

The two hot pigtails go over to the NEMA 6-20 (240V) receptacle and land on its two hot terminals.

The breaker here must be a 2-pole 20A breaker.

We are feeding all this at once. Any 240V loads draw their amps from both legs at once. Things plugged into any given GFCI only draw from their leg. So you need to take care that each leg doesn't over-pull 20A, but for Pete's sake all your stuff is running off a dangling power strip right now, so I'm sure that 11.7A x 2 will be perfectly adequate for your needs.

Also, breakers aren't completely militant about tripping at 20.00 amps. They have an intentional "inverse time" curve, so they can run 23 amps for as much as half an hour, but trip in seconds on 80 amps. That is fine; they are aiming to protect the wire from overheat.

  • Thank you for your response! The power strip is just to power the computer right now, which is pulling <1000w @120v, but yes needs a proper outlet. The server will run anywhere from 2-5000w, so that was the reason to have some overhead and go for 30A. I will plan to have a PDU here, and having 120V may be nice also, but isn't necessary. I believe all the computer equipment I plan to use will be rated for 240 also, but really just need to power the server for this circuit. I think I understand your proposal but also sounds a LOT more complex-want to try to keep it as simple as possible.
    – Azulith
    Aug 18, 2021 at 19:12
  • The other elephant in the room is running 7 circuits with only one ground wire. What's up with that? Aug 18, 2021 at 19:24
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    @RobertChapin Totally normal for conduit wiring methods. Aug 18, 2021 at 19:35
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    @Azulith Well, then, run conduit (note some of your other wiring is conduit) and run any circuits you want with THHN wire. Any conduit can have up to 4 15-30A circuits. The "5000W max" thing needed to be stated in the question. Aug 18, 2021 at 19:37
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    @Azulith Here's the thing. You can't just run one huge circuit for a bunch of smaller things. If you have 6 of those things you couldn't just run a 50A circuit. The max allowed for small loads is a 20A circuit. So a PDU or... yeah... another (or just a bigger) subpanel would be ideal. If it were me, I'd replace that sad 6-space panel with a 24- or 30-space. Only 3 conduit entries, looks about as easy as that job gets. Aug 18, 2021 at 20:48

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