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I have some racks that will be fed by 208V/20A circuits. These circuits will be conditioned and battery-backed by the facility in which these racks will live. 99% of the devices in the rack will be able to support 208V input, so I plan to use these PDUs.

However, there may be one or two odd devices that will need 110V input. I know that I can use a step-down transformer to provide 110V for these devices, but that seems like overkill for such a small number of devices, plus I don't want to pay extra for the UPS functionality since my power will already be battery-backed.

Any suggestions for something I can use for these one-off 110V devices?

Stefan Lasiewski
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Eric Dennis
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    Have you looked to see if it possible to simply replace the power supplies in the 110v equipment? – Zoredache Jul 02 '10 at 20:06
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    Have you double checked the equipment to make sure that it's not dual voltage? – Dennis Williamson Jul 02 '10 at 20:08
  • I suspect that some devices will have dual power supplies. However, from experience, there are usually some devices that run on 110V only, like DSL modems used for out-of-band connectivity. Swapping out power supplies probably isn't an option for all devices. – Eric Dennis Jul 02 '10 at 23:28
  • Is it multi-phase 208 to the rack? In other words is the 208 taken from 2 hot legs? If so, one 208 leg to neutral is 110V and you may already have what you need there. –  Sep 27 '12 at 14:55

3 Answers3

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As someone who worked in the hosting/datacenter management world I beg of you,

Don't Do That!


If you require 110V power contact your facility/electrician and have a proper 110V feed run to your rack - most facilities are equipped to provide conditioned UPS/Generator backed power at 110V as well as 208V, and already have equipment in place for situations like yours.
If they don't already have a 110V supply available they'll be able to work with you to provide a stable solution for your power needs.

Adding your own step-down gear introduces a number of problems, not the least of which is added load (electrical, but probably more importantly thermal) for the datacenter infrastructure, and would almost certainly be frowned upon by the DC Operations team (If it isn't you may want to seriously consider new hosting arrangements).

voretaq7
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  • Thanks for the quick reply. I definitely wouldn't consider doing this for a large number of devices, but I don't like incurring the extra monthly cost of an additional circuit just to power one or two 110V devices. – Eric Dennis Jul 02 '10 at 23:29
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    The "right" thing to do is to get a 120V power strip to your rack. But that is expensive and takes time. The "fast" thing to do is to use one of those IEC C14 to NEMA 5-15R cords, but they are a potential fire hazard because the voltage will be much higher then what most people expect for that style of plug. – Stefan Lasiewski Apr 15 '11 at 21:36
  • Vacuum cleaners work powerfully at 208 for about 3-minutes using IEC C14 to NEMA 5-15R adapters, then fill the DC with acrid smoke. Laser Printers work for 10-minutes. Use an IEC C14 to NEMA L6-30R 3-Wire Cable to connect a Schneider Electric APC AP9626 Step-Down Transformer RM 2U L6-30P Plug 208V IN with 120V OUT NEMA 5-20 Receptacles. The XFMR weight with cables is about 112 pounds for an inconvenient but quick and TEMPORARY solution where you know the VA/VAR/Watts draw won't unbalance the 3-phases, pop a circuit breaker, and increase thermal load beyond capacity. – Jules Bartow Aug 31 '22 at 02:21
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I agree with voretaq7, just install a special 120V circuit (and associated rack PDU) for that load. Alternatively find a dual voltage power supply to replace each el-cheapo power brick you have.

Do NOT use a IEC C14 to NEMA 5-15R cable for that. It'll certainly make the copper connection, BUT it won't step down the voltage. Those cables should be banned, IMHO (but I keep one in my bag, just in case.) They are dangerous in the wrong hands as they are a fire hazard. (Since you are asking this question I assume you know those dangers, but I want others to read this.) Others: ALWAYS check the allowed input voltage. Plug in the wrong thing and you'll cause a fire. Which is NEVER good in a DC.

toppledwagon
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208V is a 3-phase system. The voltage measured between any two phase conductors will be equal. However, the voltage between any phase voltage and neutral is called the "line voltage." The line voltage is less than the phase voltage by a factor of square root of three or 1.732. So, for a phase voltage of 208 volts, measured from phase to phase, the line voltage will be 120-volts (208/1.732), measured from any phase conductor to neutral. This means, technically it is possible to have a passive adapter cord that will deliver 120V from a 208V circuit. I don't know if this is compliant with code or other safety standards though.

user2765858
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  • PDUs generally don't provide access to the Wye neutral. That would be too easy. If there is a powered up but unused 5-wire 4-pole outlet/receptacle under the floor tiles, or dropping down from the ceiling, it is possible to wire up one leg of a NEMA L21-30P or IEC 60309 Pin and Sleeve connector, leaving the other two legs not wired in the plug end of the cable with three wires to the 5-20R receptacle for 110VAC. If you're not a bonafide certified licensed insured electrician, the danger of confusing ground, neutral & hot legs, or a poor connection is definitely not worth it. – Jules Bartow Aug 31 '22 at 02:21