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?
Best Answer
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.