I want to power a rack of commodity servers with 110v power supplies from single NEMA 14-50 outlet. I plan to distribute servers between L1 and L2 such that even at full tilt current on either side doesn't exceed 50A. However load is likely to be uneven most or all of the time.

Is this a problem?

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    The context of the question indicates a location that is subject to the US National Electrical Code or Canadien electrical code. The rules for branch circuit taps is rather complex. Also you must also not connect equipment in a way that it is not intended to be connected by the manufacturer as described in their literature. If the equipment is furnished with a 15 amp plug, that implies it is intended for connection to a 15 or 20 amp circuit. You need someone who is qualified and perhaps licensed to evaluate the installation. – Charles Cowie Aug 14 '19 at 18:21

Drawing Uneven Current Is Fine

On a NEMA 14-50 plug, it's typically the case that the load on the two hot legs are uneven, and the neutral is used to return the difference current. If the load on both hot legs was exactly the same, neutral wouldn't be needed at all, and you could use a NEMA 6 series plug instead.

Connecting 15/20A Receptacles to a 50A Circuit Is Not

You don't mention how you plan to break out from your 2x50A feed into individual 120V circuits to send to your servers, but if you're intending to directly wire from the 14-50 plug into some standard receptacles, don't do that. Those receptacles need to be breakered for 20A at most, so that if one of the server power supplies suffers a short circuit, the breaker can trip before the receptacle melts and catches fire.

If you're using standard electrical distribution hardware, you should fit a subpanel between your 14-50 plug and your receptacles. Alternatively, some server power distribution hardware has circuit breakers built into the receptacles themselves; this is also fine.

Consider Feeding Your Servers 240V

You mention that you're using standard commodity servers -- in 2019, that means they will have universal-input power supplies designed to be used anywhere in the world, and will accept a voltage input of 100-250V. Double check the labeling on your servers, but it's extremely likely that they will be perfectly happy with 240V instead of 120V. If you do this, you will guarantee balanced current on your two hot legs, and you will make maximum use of the power you have available. Most likely, your servers power supplies will run a few percent more efficiently as well.

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    Also consider that OP's power supplies will run slightly more efficiently on 240V AC than 120V... the higher voltages mean lower currents. en.wikipedia.org/wiki/80_Plus#Efficiency_level_certifications – Criggie Aug 15 '19 at 3:38
  • @Criggie, true, I tried to hint at that with my last sentence, but I didn't have a source for the numbers -- nice link. – Nate S. Aug 15 '19 at 16:48

You need to provision power

Meaning you need to crunch the numbers.

You start at the Server power supply's nameplate. It will specify amps or VA; divide VA by voltage to get amps. If you can prove beyond a shadow that the Server will never exceed a lower number, you can use that number.

Now, since the Server is a continuous load, you multiply by 125% (1.25). That is a derate, and the more informal way to talk about it is to derate the 50A supply at 80% to 40A. Don't do that also, it's the same derate so don't apply it twice.

So for instance if you run the Server in 240V mode, and it draws 4 amps, you multiply by 125% and get 5 amps.

You must treat the server like it draws that, when computing allowable load on the circuit. So your 50A circuit can support 10 of these (not 12).

Use a PDU

You cannot hotwire straight from a 50A supply to a bunch of 15/20A receptacles and small appliances. The appliance would set the house on fire before that fat 50A breaker ever tripped. The official solution in rack mounted servers is a PDU or Power Distribution Unit. This UL listed device takes care of fusing/breakering each group of PCs in 15-20A batches.

If you aren't willing to ante up for a PDU, then obtain any common 8-space or larger subpanel. Main lug will suffice, or main breaker if you want a master off switch. Keep neutral and ground separate. Fit an array of 15A or 20A breakers to protect each branch to the servers.


Yes, it's OK. What do you think happens with all of the other 110V outlets in your house? Even something that you would normally plug into such a socket, such as a clothes dryer or a cooktop, will not always draw the same current through L1 and L2.

And despite what TimWestcott says, the electrical code doesn't care what happens beyond the wall socket — it is only concerned with "permanent" wiring that is part of the building structure.

However, he is correct in that anything that "looks like" a 15A or 20A socket should really have appropriate protection — so your rack should have an internal distribution panel with suitable circuit breakers.


I really wish they'd named this group "electronics engineering". Because your question is definitely about electrical engineering!

Based on first principles, you should be OK. But it's going to vary by region, and possibly with the personalities of your local inspectors. If it were me I'd want to consult with an electrician in my area about what will meet local codes (and make inspectors happy).

I think you may be violating one of the tenets of the electrical code, which is that you need a circuit that's fused to match the cord running from plug to load -- so if your rack breaks that 50A service into a bunch 15A sockets without further protection, it's probably not going to be up to code.

You may need a sub-panel someplace, broken out into individual 15A circuits -- either in your rack, or on the wall where you're planning on putting your 50A socket.


What you are doing fits under the umbrella of power distribution, and generally requires an electrician's license to perform. It is sometimes tricky to get right, so I recommend that you do not venture into that without the appropriate knowledge and qualifications.

Code prohibits appliances that are not certified by one of the several underwriters (UL, CSA, Intertek/ETL…) to be connected to a receptacle. Since you are building the rack yourself, the whole rack counts as such an "appliance", meaning that if you do not want to void your homeowner insurance, your setup will need to be verified and approved by an electrician (or possibly an electrical engineer).

Going with off-the-shelf products like Harper suggests seems to be a very good idea, safety-wise, but your insurance policy might still require you to get your setup verified by an electrician. Ask your insurer and a qualified electrician. Your typical contractor might not know much about server rooms, so try to look for server room experts who also happen to have an electrician contractor's license.

User Dave Tweed said that "the electrical code doesn't care what happens beyond the wall socket". This is true only to a certain extent. For example, code will say things like a type of appliance (e.g. a dishwasher or a refrigerator) will require its own dedicated receptacle or branch circuit on its own breaker. Also, code will prohibit doing things like stapling power cords to a wall to route cords to e.g. a lamp. Code will also prohibit one from connecting devices that are not certified to a receptacle; a device requires a nameplate that lists the name of the manufacturer, model number, serial number, utility voltage, sustained current use, peak current use if applicable, cycles, additional certifications (e.g. wet-location approved). Also, it is forbidden in the Canadian electrical code to run an extension cord or circuit from one building to another to power one-off receptacles or circuits, save very exceptional cases.

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