On 2/29/2012 12:15 PM, Luke S. Crawford wrote:
On Wed, Feb 29, 2012 at 09:10:07AM -0500, Edward Ned Harvey wrote:
From: [email protected] [mailto:[email protected]]
On Behalf Of Tracy Reed
they currently have to run me 4 20A per rack. Coudldn't they cut
that
approximately in half (power factor correction etc, as mentioned earlier)
if I
ran 208v?
Depends what you call "cut it in half."
If you have a single 20A 120V circuit, as compared to a single 20A 3-phase
208V circuit... Well, indeed you have three 120V lines bundled into the
3-phase circuit, so it's three times as many hot wires, but it's less than
twice the effective voltage, but it's able to draw a smoother current, less
spiky, which means you're less prone to blowing circuit breakers.
so, my understanding is that you can have (or at least coresite santa clara
is trying to sell me) 208v "single phase" or 208v "three phase" and
the two are rather different.
Now, I was refering to the 208v 'single phase' as 'split phase' earlier,
which I now believe is wrong. 'split phase' refers to something similar,
but the two 'legs' are 180 degrees out of phase in a 'split phase' circuit,
which would give me 240v.
But, my understanding is that if you have a 3 phase circuit around here, it's
usually 120v per leg. each leg is 180 degrees out of phase with the others.
Always. 3 phase 208V is always 120V per leg. It is definitional and
can't be anything else. They are 120 degrees out of phase with each
other. (see my earlier post with references on this topic.)
single phase 208V is taking two of those legs and delivering it to you.
instead of sending all 3. It's relatively easy to do this, but you get
better utilization of the wire out of the extra phase.
If you want to connect a single-phase load to a 3 phase circuit (such as
a server- my understanding is that all my servers have one 'hot' one
'neutral' or 'return' and one 'earthed' connector that is grounded to
the case for safety.) you can either make the hot wire hot and the 'neutral'
the 'neutral' of the 3 phase, giving you 120v between the 120v hot and the
0 neutral.
You can only do this if it is delivered with a neutral. There are two
ways to deliver 3 phase 208V, one is called
Delta and the other is called Wye (see my earlier post).
You cannot make 120V out of a delta delivered circuit unless you install
a transformer.
Alternately, you can take two legs of that 3 phase circut and put the hot
down the conductor on my server expecting hot, and the other hot (that is
180 degrees out of phase) in the conductor on my server expecting neutral.
This means my server sees 208v, as my server, expecting single phase power,
sees the difference between what it thinks is it's hot conductor, and what
it thinks is it's neutral conductor.
120 degrees. The computer just sees a potential difference between the
wires, it really doesn't know or care hot from neutral in this context.
Calling it neutral is overly confusing when there is a legitimate
neutral in a 208V 3 phase WYE delivery, so I would avoid it.
Now, what I am (perhaps incorrectly) calling single-phase 208v is
usually connected to with a NEMA L6-30 twist-lock connector, which
has three conductors, which I understand to be two hots and a safety earth.
I'll have one of these circuits to run tests on in under a week at
55 s. market in san jose, if my insurance guy gets the paperwork to
the coresite people soon. I've got two regular 'single phase' (IEC
c14 plug) servers at another provider that just does co-lo on 208v
single phase, I've got the PDU, the outlet to the PDU is NEMA L6-20,
and from my pdu to my servers, the outlet is C13.
L6-30 is for a 30A circuit, there are many other types of delivery
depending upon the gauge of your wire from 20A up to 60A. That plug is
specified for the wire per NEC.
(you can also do 208 or 220 with a NEMA plug, fwiw)
There is no machine plug (IEC or NEMA) that can do 3 phase to a host,
fwiw. PDUS will either do single phase to all, or a subset of plugs with
breakers, or they will do 3 phase to groups of single phase receptacles
(again with breakers or fuses)
The three-phase 208v that coresite santa clara wants to charge me more
for? I believe that is three 120v hots, and therefore, rather more power
than what I'm probably incorrectly refering to as 'single phase' 208v,
and something I could not plug directly into my servers without buying
new PSUs.
correct, you need a PDU for 3 phase to single phase. There will be 3
banks, an XY, YZ, and XZ corresponding
to 208V each going across all combinations of the 3 legs.
Unlike the circuit that i've actually ordered, I think this '208v
3 phase' power will have a NEMA L21 plug, which is a 5 wire plug with
3 hots, one neutral and one earth. I know much less about this, as
I've never tried to run a server on it.
That's a WYE delivered circuit. The servers do not care.
This is what I thought. But if you are right (and like I said, I think you
are, in this case) then why is the co-lo trying to charge me exactly
twice as much for a "single phase" 208v amp than for a single phase 120v
amp? two amps of 120v, in a single phase system, have 32 more VA than
one amp of 208v.
Because they are making money for doing so. (because they can). You
could try negotiating them on this point, though I'm not sure it would
be fruitful.
This is the question I was starting with, and it is possible, likely even
that they are charging more simply because they can. But, it's also
possible, even likely, that I am misunderstanding something.
You're getting there, but I do recommend seeing my earlier post. It had
some good links and explanation. But yes, it is because they can.
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