Wiring Servers to 240VAC single phase 1 wire

Question

I am attempting to wire several servers up to 240v. Some servers have PSU's which are capable of adjusting to 240v from 120v, others are strictly 240v no questions asked. According to the manufacture's website I need to use 240v single phase or 240v 3 phase, and 3 Phase is not an option where I live. My question is, living inside the United States with 2 120v Hot Wires in a residential home, how do I combine those to a single 240v hot wire with neutral and ground. It doesn't say whether I can use 2 hot wires to achieve 240 or not, and I don't want to nuke thousands of dollars in equipment. I plan to install a UPS as well which might be an answer to the question. I can provide specs on the servers and PSU's as needed. Thank you!

Answer 1

Sounds like some import-stuff if it expects 240 line and neutral.

240V devices here use both hots (not "combined") and no neutral (from the house supply.) Which would mean wiring one hot to device-line and one hot to device-neutral, and ground to device-ground. Depending on how the thing is built, that's either fine, or not. If it's NRTL-listed for use in this area, it's fine.

Using an isolation transformer (1:1 winding - 240V in from both hots, 240V out isolated so one side can be grounded as a separately derived source neutral) would be a way to get 240V line to neutral, if that's needed. that will require a VA or KVA rating as large or larger than your power requirement for the server supplies, and will waste a little power as heat (nothing is 100% efficient)

If the neutral (or Line 2, which is a clue that it's designed for this market) connection on the device has no issues (safety or operational) operating 120V above ground, it can be direct wired to a US-Style hot. Server power supplies intended and listed for the North American market expect that.

Answer 2

My question is, living inside the United States with 2 120v Hot Wires in a residential home, how do I combine those to a single 240v hot wire with neutral and ground.

You can't. You're absolutely not allowed to make suicide cords like that which have two plugs on them and merge independent 120V circuits. Those things can kill you in unexpected ways. Fate favors little girls, fools, and ships named Enterprise... but electricity doesn't.

You need to have an electrician come in and wire you a proper 240V circuit. It will have whichever socket you ask for that is designed for 240V, such as the two on the right. See the family resemblance? Depending on how your existing circuits are laid out, sometimes a 120V circuit can be converted to 240V easily as swapping socket and breaker.

For 30A they also have these larger sockets. You don't need neutral (white) if all loads on the PSU are 240V.

Answer 3

Ignoring ground (don't actually ignore ground, it is critical for safety; but every circuit here needs one ground wire so it doesn't enter into the equation), a 120V circuit or a 240V circuit needs 2 wires, either 14 AWG for 15A or 12 AWG for 20A. If a circuit needs both 120V and 240V (e.g., Multi-Wire Branch Circuit (MWBC) or a standard clothes dryer or oven) then it uses 3 wires. 120V is hot + neutral. 240V is hot + hot. 120V/240V is hot + hot + neutral.

The good news is, that means you can wire up receptacles for servers using the same type of 2-wire cable as you use for standard 120V circuits. All you have to do special is:

• Mark the white wire with colored tape on both ends to indicate it is a hot wire.
• Connect black and white wires to the two poles of a double-pole breaker.
• Use the appropriate receptacle - NEMA 6-15 for 15A, 6-20 for 20A:

But there is a big catch: You usually can't just remark the wires and reuse an existing circuit. That's because in most houses each circuit (a) serves multiple receptacles in multiple rooms (and you can't do this unless you change all of those receptacles at the same time, and that would go against the idea of being able to provide lots of dedicated power to your servers) and (b) most homes have just barely enough receptacles to meet code requirements. So usually that means running a new circuit (e.g., 12/2 cable from a new breaker to a new receptacle). That is easy in an unfinished basement, much harder (but varies a lot) in a finished room on a different floor.

Based on critical information from comments, this is a Dell E3000E-S0 power supply, rated for 3000W = 240V @ 16A. Total power is proportionally lower if used on 208V or 220V. So this is a perfect match for a 240V 20A circuit @ 80% continuous derate. It is also from a reputable manufacturer that regularly produces equipment designed to be used around the world, so using it on a 240V circuit in the US is perfectly normal. The connector looks like a C19/C20 connector, which is perfectly normal for this type of device.

Answer 4

Most European appliances require two wires with 230V AC between them (240 is the same thing) and a separate ground for protective purposes.

The usual case is one of the wires to be hot and the other to be neutral, but this is not generally required. The ground is not normally expected to interact with either and its protective role works in all cases (provided adequate protection from the source side).

The US-style split phase or some other arrangements (e.g. half-waves in either direction like in the cheap "modified-sine" inverters) work acceptably well and are even legal.

Where these assumptions go wrong?

Some devices (e.g. some UPS devices) do extensive control of the input power and one of the power quality critera is if the designated neutral to be within few volts from the ground. When fed with split phase they will consider the ground disconnected and complain about the fact (up to and including not using the power source in question).

Answer 5

You cannot take the two 120 volt legs and hook them together to get 240 volts. They are different phases and hooking them toether would cause a big spark, fire and tripping breakers and blowing transformer fuses. Speaking of transformers, you could get a 120V to 240V step up transformer to run your servers. The transformers come in a variety of different wattages.