I have a pure curiosity question for the NANOG crowd here. If you run
your facility/datacenter/cage/rack on 120 volts, why?
I've been running my facility at 208 for years because I can get away
with lower amperage circuits. I'm curious about the reasons for using
high-amp 120 volt circuits to drive racks of equipment instead of
low-amp 208 or 240 volt circuits.
208 isn't all that great. On one hand, a 20A 208V circuit is vaguely
more convenient than a 30A 120V circuit because it is delivering a bit
more power to the rack (3328 vs 2880), and it's likely to work with a
lot of modern equipment containing autoranging power supplies.
On the flip side, with 120, you don't have to have "odd cords," and it
is somewhat easier to "right-size" power for a rack (20A, 30A, 2x20A),
so for an average rack that isn't crammed with high power webhosting
1U's (etc), a customer might actually find that the ability to right-
size the power feed is more flexible with 120V.
And I don't like not having anywhere to plug in my power screwdriver's
recharger... I suppose I should see if I can find someplace that has
a transformer of an appropriate size, or does anyone already have the
part number for something that can provide a few hunderd milliamps of
120V from 208? 
... JG
Joe Greco wrote:
208 isn't all that great. On one hand, a 20A 208V circuit is vaguely
more convenient than a 30A 120V circuit because it is delivering a bit
more power to the rack (3328 vs 2880), and it's likely to work with a
lot of modern equipment containing autoranging power supplies.
>
On the flip side, with 120, you don't have to have "odd cords," and it
is somewhat easier to "right-size" power for a rack (20A, 30A, 2x20A),
so for an average rack that isn't crammed with high power webhosting
1U's (etc), a customer might actually find that the ability to right-
size the power feed is more flexible with 120V.
I don't find it makes much difference, really. People are used to working with 120 only because that's how we roll in the USA; scary high voltage is for the oven and dryer. I like odd cords; it makes the protected power stuff blazingly obvious and slightly harder to plug dumb things into a UPS branch circuit because hey, a plug is a plug, right?
And I don't like not having anywhere to plug in my power screwdriver's
recharger... I suppose I should see if I can find someplace that has
a transformer of an appropriate size, or does anyone already have the
part number for something that can provide a few hunderd milliamps of
120V from 208?
True, you do lose the convenience outlet factor. I made up for it by placing standard 120V outlets (utility/generator only) along the walls. It works out because I hate those stupid "wall warts" with a passion. I go out of my way to buy products that come with a corded transformer, especially if it has a C14 connector on it.
If you're adept at electrical stuff you can always get a small transformer, put it in a box, stick a C14 on the high side and a 5-15 on the low side. Nothing fancy required.
~Seth
We always seem to have an odd device or three that needs 120V, like a
wallwart for an external modem or LCD KVM console, or some legacy
specialized gear (CBORD comes to mind). For those we have been
providing a single circuit/PDU in the room that runs at 120V and
running extension cords as necessary. Everything else is 208V.
Once upon a time, Joe Greco <jgreco@ns.sol.net> said:
And I don't like not having anywhere to plug in my power screwdriver's
recharger... I suppose I should see if I can find someplace that has
a transformer of an appropriate size, or does anyone already have the
part number for something that can provide a few hunderd milliamps of
120V from 208?
Isn't 208V usually provided as a connection across two phases of a 3
phase circuit? In that case, you get 120V by going between one phase
and neutral (no transformer required).
You need a NEMA 14 (4 wire) connector to get two phases, neutral, and
ground (provides 1 208V circuit and/or 2 120V circuits) or a NEMA L21
(5 wire) connector to get all three phases, neutral, and ground
(provides 3 208V circuits and/or 3 120V circuits).
Yes, you are correct Chris. The loss from getting 240 from two legs is
due to the fact that it is at 120 instead of 180 deg's.
Indeed it is. If you want to see it yourself, measure the voltage between "hots" on different circuits. I see 208-212V depending on the legs (they aren't evenly loaded.) This is easier to do in a data center, but with a long extention cord it can be done with the office. (of course, having the building wiring diagram(s) makes for a short hunt.)
--Ricky