[rescue] Big computer, single phase, three phase

Ethan O'Toole ethan at 757tech.net
Fri Feb 12 13:35:40 CST 2010

> Safety first, be very careful.  You won't get a second chance.

Yes. I'm pretty cautious, especially working around live panels. 
Interestingly enough there used to be something really big in the back 
room, that is no longer there. There are three large AC power lines that 
run to the main breaker panel, where the breaker is missing. One thought 
is to use that, the other thought is to check the wire gauge on a 3 phase 
drop that used to go to a ?diato? machine. The panel breaker is only 20 
amps, the outlet only 15. But the wiring looks like it is well big enough 
to be used for more. Dunno.

When I had the old office a friend helped (did most of the work, he's a 
commercial electrician) install a 36kVA UPS system I had gotten from 
surplus. I still have the breaker panel and monitoring hardware I used 
with it. Figured I'd reuse that.

I know unbalanced is very hard on the step down transformer (and they make 
K rated ones for environments where people are using lots of 120vac 
instead of 3 phase motors). Back at NASA the PDUs were always pretty 
balanced, but they kept each computer system on the same legs IIRC.

> That being said, this is okay to do and encouraged to keep the phases in
> balance.  Read up on grounding and bonding.  Bond all supplies together.  DO
> NOT let the neutral float or you can fry equipment or yourself with over
> voltages.

Ahhh shit. Yea. That takes out the 3 wires, there is no ground. I'll check 
the other power distro tonite and see if the wire is the right size to 
handle an upgrade to 30 amps over 20.

> Double check your voltage between L1&L2, L2&L3, and L1&L3.  Double check the
> voltage between each phase and neutral (which is tied to ground at the
> breaker box / load center ONLY) to be sure you're running at the voltage you
> think you are.  This has saved my butt a few times when I discovered I
> needed to go to a step down transformer to get the 240 V single phase I was
> after.

I'll check. I'm going to permanantly install this toy I used to run at the 
old office:


We had them built into the PDUs at NASA and I thought it was so cool I got 
one on eBay.

> Consult the NEC.
> Did I mention you should bond your supplies together?  Call me if you need
> to.

What do you mean? Ground them all together on the computer? It's all one 
chassis, so there shouldn't be an issue here. Well 2 cabinets but they are 
physically bolted together.

Thanks for the advice!

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