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I'm looking for advice on which electrical service I should have run to a rack that I have in a data center.

The price is the same for either a 30 AMP 220V circuit or a 60 AMP 120V curcuit.

I realize that these two curcuits are delivering effectivly the same amount of usable power, however I'm curious if one is better than the other. I remember reading somewhere that 220V is more effecient than 120V. If this is true then I'm wondering if that would result in my ability to run more devices off of the 220V power vs. the 120V.

Thanks for any help!

EDIT - This will be running in the US. All servers and Cisco gear that we are running can support either 120V or up to 240V. They are auto switching power supplies. Therefore locking power cables will not be an issue and standard plugs into the servers will be used. This will also be redundent circuits so I will be balancing each one to around 40% load so that in the event one circuit goes down then the other can carry the full load at or around 80%.

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For posterity; some more technical information on the matter of '2[2|4]0 v 120' can be found @ - I'm not a hardcore EE, so I can't vouch for anything discussed there. – Jordan T. Cox Feb 3 '10 at 3:25
up vote 8 down vote accepted

You are getting the same amount of 'power' either way (120V x 60A = 240V x 30A), so from an electrical "power" standpoint it is a wash--either one will support the same amount of equipment in the rack...

You should do some research to be certain, but most modern computer equipment is equiped with switching power supplies that accommodate a wide range of voltages (manufacurers don't need to stock a separate power supply for each country; instead, they simply ship the appropriate power cord for the destination country). Given that, your equipment most likely will not care whether they have a 120V or 208V or 240V input--so long as you have the appropriate power cord to connect the system to the power distribution module.

My understanding from engineers at colocation facilities I've worked with is that most switching power supplies are a bit more efficient with a high voltage input (e.g. they produce less heat, which is wasted energy) which can save a bit on both power air conditioning load. The wire gague required to carry the same power at 240V is smaller and more flexible than the heavier wire required at 120V, which may be a convenience when routing cables within the rack.

I would confirm that all of your equipment is capable of accepting high-voltage power; if so, obtain the appropriate power distribution module and cordage to connect your equipment to the 240V 30A circuit.

Remember, too, that a 30A circuit should not be loaded up with 30A of equipment--you need to leave some "head room" to ensure that the circuit is not overloaded (I believe the rule is to stay below 80% of the maximum rated capacity (e.g. maximum 16A continuous load on a 20A circuit; no more than 24A continuous load on a 30A circuit).

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80% capacity with all your equipment on is a good general rule. To be more specific: In the unlikely event your whole rack ever loses power and then gets switched back on all at once you're going to want that inrush current to be less than the cabinet breaker's trip current or you'll just blow your breaker and be down again. – voretaq7 Feb 3 '10 at 3:41

Practically these are roughly equivalent services: 6.6KW (which is a LOT of power - I start seeing heat problems above 3.5KW in my racks), which is why the price is the same. As a co-lo customer this is all you really care about and the rest of this is all "datacenter management" stuff.

Heat and efficiency wise, 220V service is "better" -- Power supplies tend to have a better efficiency at the higher voltage (you may be able to squeeze one more server into the supply) & run slightly cooler.

If you are mounting an in-rack UPS system you can probably cut heat dissipation by 1/4 to 1/3 by using 220V power.

If your datacenter is not supplying your PDU you will have an easier time finding 220V equipment.

The real savings with 220V power comes from inductive loads (motors: Air conditioning), which if you're co-locating you don't care about.

My Bottom line recommendation: Go with 220V unless you have a bunch of 110V-only equipment.

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Note that just because I have problems around 3KW doesn't mean you will - The CoLo I'm in has some airflow issues and the racks aren't optimally arranged for cooling. I've seen datacenters with 10+ KW racks and no heat issues, so your mileage may vary. – voretaq7 Feb 3 '10 at 3:09

Certain larger devices will require the 220V circuit - AC, UPSes, etc.

Also, the larger plugs can be lockable. Larger, heavier gauge wiring as well, should be better suited to the needs of a large, full rack.

Each machine is going to be needing 120V still though, so you'll need a converter - the UPS would handle that.

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I would add that every data-center class PDU requires 220V circuits. – Zypher Feb 3 '10 at 2:51

What voltage do your devices expect? Run whichever one you have more of.

In Australia we run almost exclusively on 240v 10 or 15 amps, and (on most of my equipment anyway) the power supplies are not easily exchanged for different voltages.

30 Amps at 220v is a LOT of power. I've run entire racks on less than that.

Make your life easier and go with whichever one your equipment supports!

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Good point with the 'either way its a lot of power' idea. Even with the smaller power-capacity at 220V @ 30A (6600W) and assuming a full 1kW load for all your kit, that's still 6.6 pieces of equipment. I'd be surprised to find something that pulls more than 500W effectively doubling that to 13.2 pieces of kit. – Jordan T. Cox Feb 3 '10 at 3:02
If it's just routers and switches you could probably fill a full 42 units and still have room to spare. I've run 2 16-port gigE switches and a router on a circuit that only had one amp (long story). – Mark Henderson Feb 3 '10 at 3:36

30 amp cable is a lot cheaper than 60 amp cable for the same length, so all else being equal it's more economical to go for the 220V.

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..and more flexible – DefenestrationDay Mar 13 '12 at 4:43

Its really dependent on what your hardware is expecting.

AFAIK; 220V is frequently used as a three-phase a/c for transmitting from the power plant to the grid and subsequently the destination node. Its then stepped down to 120V single phase a/c when it reaches its destination. Three phase is more efficient for long distance transmission and more efficient motor operation [as three coils can be used to create a continuously moving magnetic field, instead of a constantly inverting field with two-phase]. Frequently a 220V line is also left for devices requiring a larger voltage to operate.

It really boils down to what your devices expect. If you have the choice of hardware, I would recommend going with 120VAC as it is more standard and prevalent (at least in the states). Additionally, given what you've said - your colo is willing to provide more 'power' if you go with 120V. You'll be getting 7200W (120V*60A) instead of 6600W (220V * 30A).

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