How to step down voltage from 208V to 110V

Question

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?

Solution 1

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).

Solution 2

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.

Comments

[Zoredache]

Have you looked to see if it possible to simply replace the power supplies in the 110v equipment?

[Dennis Williamson]

Have you double checked the equipment to make sure that it's not dual voltage?

[Eric Dennis]

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.

[Admin]

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.

[Eric Dennis]

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.

[Stefan Lasiewski]

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.