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We are going to switch from 20 amp 120 volt to 20 amp 208 volt circuits in our colocation facility. I am new to dealing with anything other than standard 120v power, so I am unsure of some intricacies of this change.

We purchased APC AP7941 PDUs and these power cords: http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10228&cs_id=1022805&p_id=6452&seq=1&format=2

I noticed those cords are rated 10 AMP / 125 Volt. Will they be incompatible with the PDUs listed above? I was unable to find C13/C14 cords that were rated for 208V power.

Secondly, for our servers with redundant power supplies, is it possible to power cut the servers over to the new 208v PDUs without shutting them down? For example, one power supply running off a 120v circuit and the other running off of a 208v circuit?

Finally, I'm sure others have experience with a similar project. I would appreciate any help or advice anyone can offer.

Thanks.

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3 Answers 3

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WARNING: I AM NOT AN ELECTRICIAN

That said, I have used C13/C14 jumper cables labled for 125V on the same APC AP7941 208V PDUs. It's probably not a great idea, but I've done it.

Maybe consider the raw power you're drawing? 125V@10amps is 1250 Watts. Is any of your gear likely to draw more than 1000 Watts? Consult an electrician to be sure and consider the impact of one of your cheaper cables catching fire.

I note that you got the 16 AWG product. This is slightly better than the 18 AWG product.

Usually the recommendations look like this:

  • 18 AWG - 10 amps@125V
  • 16 AWG - 13 amps@125V
  • 14 AWG - 15 amps@250V

CablesToGo sells 14 AWG cables spec'd for 250V.

http://search.cablestogo.com/?Ntt=250+Volt+power+extension+cord&N=0&vno=24

But unfortunately not in short lengths. (I really like the 2' options with vertical PDUs.)

Other tip: I used to take some gaffing tape and wrap a strip about 3U long and .5" wide around the end of the plug going in to the APC PDU. This helps secure the cable. They tend to pop out if you're not using the bracket and lots of zip ties. The gaffing tape can help hold it in place.


Generally you can swap over from 120V to 208V without a hiccup. Each PSU should be converting from AC to DC independently. (MAKE SURE YOUR PSUS are not somehow connected with AC failover to eachother)

Best of luck and be safe...

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I talked to Monoprice technical support. The cables themselves say 300V on them. He said this meant the cables were stable from 120v to 300v. Our largest power supply is 900W I believe. Thank you for the advice. –  tjohn86 Jan 20 '11 at 22:43
    
@tjohn86 If the cables are marked for 300V you're all set. To put a rough amperage number on that 900W - full load at 120V you're talking ~8A, at 208V it's about 5A. (plus a bit for power factor as the wattage is rated output & your amperage input will be higher from conversion losses) –  voretaq7 Jan 20 '11 at 22:55

You shouldn't have a problem with those power cables (16AWG) -- the amperage draw will typically be lower for 208V power than for 120V power, so there will be less power forced through the cable & less worry about heating.
If you want to be proper and safe you can get appropriately-rated cables from http://www.infinitecables.com/pow_ext.html in a variety of sizes to help keep your rack/cabling clean...

Re: redundant power and mixed voltages, this depends on your equipment, how the power supplies are connected, etc. -- I've done it successfully in the past and wouldn't expect it to be an issue, but you should check with your manufacturers and go with their official recommendation (that way if something blows up you have someone you can point to).

Also it probably goes without saying (but I'll say it anyway) that you should triple-check that all your equipment can handle 208V power (if it's reasonably modern it probably can). Plugging a box in and blowing up the power supply is a bad way to discover that it had a cheap one-voltage-only / non-autosensing PSU.

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Thank you for the link to those cables. I believe I will take each box down in order to do the cut over. A brief outage is preferable to an explosion. Thanks for the advice. –  tjohn86 Jan 20 '11 at 22:44
    
the explosion is a possibility even if you take the machines down (if your power supply can't handle 208v it's going to smoke out either way) - As JoelK said below as long as the PSUs are really independent you should be fine plugging them across different voltages. –  voretaq7 Jan 20 '11 at 22:50
    
(The other good argument for taking the machines down is you get to test shutdown & startup -- the flip side of that argument is if your startup doesn't work you're screwed. Pick your poison :-) –  voretaq7 Jan 20 '11 at 22:51
    
I believe we will purchase the 14 AWG cables from InfiniteCables. Thanks again. –  tjohn86 Jan 21 '11 at 4:26

I know this is an old thread, but I just wanted to add this in case others find it in a search.

Not all multi-PSU systems can run at mixed voltage. I've run across a few Dell and Juniper systems that won't do it. The PSU light will turn on quickly then either turn off or blink.

Whatever you do, RTFM. I'd wager > 95% of the systems will do fine with multi-voltages. It's the last 5% which are "too-smart" and will disable the differing PSU.

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