[114817] in North American Network Operators' Group
Re: Why choose 120 volts?
daemon@ATHENA.MIT.EDU (Barton F Bruce)
Wed May 27 02:15:24 2009
From: "Barton F Bruce" <barton@gnaps.com>
To: "Seth Mattinen" <sethm@rollernet.us>
Date: Wed, 27 May 2009 02:14:36 -0400
X-GlobalNaps-MailScanner-From: barton@gnaps.com
Cc: nanog@nanog.org
Reply-To: Barton F Bruce <barton@gnaps.com>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Seth Mattinen wrote:=20
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
And you have been doing something that is a right step in the right directi=
on, but may well not be the best ultimate solution.
Lets half ignore codes. Not to be illegal, but they CAN be changed and you =
can often get specific exceptions if you are not just in the back room of a=
n office but are in a clearly professionally managed facility with well tra=
ined staff plugging in equipment.
Every time I look at a nameplate and I see 100-250VAC I get very frustrated=
. If only that had been perhaps 100-300VAC, I could then run it on 277VAC a=
nd that is especailly nice for many reasons.
Most large USA buildings already have 277 and probably all their flourescen=
t lighting is run off it (277VAC ballasts are readily available and what lo=
ok like rats-ass wall switches but are higher rated ones are readily availa=
ble - both even at home despot if you look hard enough), so nothing terribl=
y new has to be learned by electricians, etc.
277 is the phase leg to NEUTRAL voltage of a 277/480 WYE system that most e=
verything except small human plugged appliances use in any but the very lar=
gest USA office building. It is what typically comes in from the power com=
pany.
120/208 is the output of typically a delta/wye transformer that steps that =
down for the dumb humans to safely use and you are paying the penalty of th=
e WHOLE LOAD having to go through a second less than 100% efficient transfo=
rmer.
The beauty of 277 is that on a single breaker pole (unlike 208 where you ar=
e most likely to have 2 HOT legs and need a 2 pole simultaneous trip breake=
r) on reasonable size branch circuits that you are still allowed to plug MU=
LTIPLE loads into without individual fuses or breakers (that is "allowed to=
" - you may chose to protect each outlet in the rack, but that is not compu=
lsory) you get 277/120=3D2.31 times as much power available.
Sadly routers, servers, switches, etc. typically are rated to 250VAC, so us=
ing raw 277 won't work. But let us see how close HP/IBM/ACP and many many o=
thers are getting still using ONE breaker pole per much more efficient bran=
ch circuit. NB that as you go to larger branch circuits in AMPS, you MUST b=
e supplying just ONE load or MUST have additioanl breakers or fuses as you=
split it up.=20
We all know 120/208 and 277/480. What about another NEW pair of voltages in=
WYE connection! Lets use 240/415. It is exactly twice 120/208 (well it is =
not stated as 240/416 I'd guess since 240 x 1.73259 =3D 415.82 they just =
truncate rather than round - though 2400/4160 is a standard designation...)=
and is inside the 250V max rating of the switching power supplies. It stil=
l uses a single breaker pole. Your get EXACTLY twice as much power out of a=
240/415 WYE branch circuit as you would out of a 120/208 at the same AMPs.=
But you may save a transformer and its continuous power waste or at least =
part of it in between.=20
How do you get to 240/415 is the next issue. If you have 2400/4160 or 7,960=
/13,800 primary into your building, and you do all your own transformers, g=
etting 240/415(6) can be a single transformer step for you, and you will p=
robably have many transformers so can also create seperate 277/480 for mode=
st size AC inits and lighting, While LARGE chillers can be ordered at the h=
igher voltages, and for the relatively small amount of 120/208 you probably=
should come off 277/480 into standard 120/208 delta wye transformers becau=
se normal electricians can do that rather then the 13K gods($$$$).
But if you are a smaller building the only voltage that makes sense that th=
e utility is supporting is 277/480. Rather than take all your rack power th=
rough another transformer step with the losses and the extra heat to eject =
from the building, consider instead using buck (as in the classic BOOST/BUC=
K transformers) to knock that 277/480 down to 240/415. It can be packaged=
as a 3 phase unit for less than three singles, and will be smaller and le=
ss costly to have wired up, but the three singles may be available from sto=
ck.=20
It is the same sort of device you must have in front of a load that needs 2=
40 or 250 and can't handle 208, but in that case is wired BOOSTING rather t=
han BUCKING. FWIW an electric range burner or a hot water heater element r=
ated for 240 produces EXACTLY 75% of the heat if run on 208 (go do the math=
...), but you should NOT use boost bucks for such a simple situation becaus=
e optional heating elements can be ordered originally OR bought as replacem=
ents for less than $10 each and easily replaced in the field to give the or=
iginal 240 rated wattage on 208 supply.
In any case the 3 phase buck transformer VA rating will just need to be 3 x=
(277-240) x per-phase-AMPs on the load side. Or look at it this way: 37/24=
0 =3D 15.4% to just buck rather than the KVA of a transformer dropping the =
whole load. Remember the buck transformer's secondary in this case simply =
is 37 volts and is wired in series but 180 degrees out of phase so drops th=
e 277 to 240VAC But maybe that isn't wizest in the big picture but may work=
for you.
If you are into really big systems you really need folks that know what the=
y are doing. Those that simply tell you to buy K rated transformers may be =
missing a slick opportunity to knock out power factor problems caused by tr=
iplen harmonics related to the multigrounded neutral system. So you may not=
want to BUCK, but instead use an exceptional transformer SYSTEM.=20
Look at these folks (this is just one PDF, they have plenty more, and a few=
dollars more spent with them can bring a rapid ROI just on power factor sa=
vings alone on the utility bill let alone the dramatically lower transforme=
r losses) :
http://mirusinternational.com/downloads/CAT-EC01-08-F10a_w.pdf
NB that one of their tricks is to have the load split to separate outputs w=
ith different phase shifts within these special transformers so the trouble=
some harmonics are canceled and not reflected back into the primary. Even t=
hough newer power supplies will be more efficient and better power factor c=
orrected that years ago, this is still an issue to be very aware of.
Anyway, GOOGLE for 415 volts or 240/415 and these days you will find many h=
its from big names you already know. Maybe any "issues" with local codes ca=
n be solved or already have been.=20=20
Some folks have been bucking down from 277 to more like 250 for years (to n=
eutral, so single breaker) with the local inspector staying totally clueles=
s at to what was "accidentally" happening. He was probably aware of plugged=
in PDUs with integral 277/480 to 120/208 DeltaWye transformers in use so =
probably assumed a lot of the higher voltage breaker panels were feeding mo=
re of that.
A 30 amp 3 phase feed at 240/415 to a big power strip with 15 or 20 amp bre=
akers for smaller groups of single phase IEC outlets is a STANDARD product=
from multiple sources and at the 80% allowed loading provides 240 x 30 x 3=
x .8 =3D 17,280 VA and just two of these feeds gets you to almost 35 KVA p=
er cabinet that drives folks to liquid CO2 cooling systems that can easily =
function in a datacenter without a raised floor, and do it without CRAC cre=
ated hurricanes or huge air ducts trying to cool the room and failing.
NB that such a power strip using IEC outlets uses EXACTLY the same cords as=
you would use for 120VAC - nothing weird other than NOONE is using the "WE=
IRD" North American plugs and outlets Well, almost. The power strip could =
be built for BOTH 120/208 AND 240/415 and would require NO outlet changes. =
Breakers, shoild be picked for the higher voltage. IEC outlets in these vol=
tages are THE SAME - they just change between 15 and 20 amps and there are =
specials for high temperature usage.
The one thing that should be done is to be sure be to use S cord rather tha=
n SJ cord (600V class vs 300V class - well even better use the "O"il option=
too, simply because it lasts a lot longer - SO cord) - or use a plastic eq=
uivalent for any cords carring more than one HOT from 240/415.
The old code allowed 42 poles per breaker panel. That would be 7 cabinets w=
orth of just these handy size 30 amp 3 phase feeds for 241920 VA total - al=
most 1/4 of a megawatt. Bigger branch circuit amps can give you a lot more,=
obviously, but short of going to Square-D I Line panels, you probably don'=
t want much more load than this in a typical breaker panel. It would be "ni=
ce" to have the two 30 amp 240/415 WYE feeds to any cabinet be fed from the=
SAME breaker numbers in two totally separate breaker panels, and each serv=
er's power supplies split between the two branch circuits.=20
Just be VERY VERY certain you check each device as there are still some 120=
volt ONLY devices some idiot will want to plug in.
Three phase isn't all that weird..
Keeping it simple, if you have three separate coils on a generator that eac=
h produce AC 120 degrees out of phase with each other, and connect one end =
of each coil to a single grounded point called "neutral" , you WON'T have t=
wice the single coil voltage between any two of the other ends of the coils=
(the HOT wires), but have the square root of 3 times it instead because th=
ey are not a full 180 degrees out of phase the way single phase house power=
is, or two flash cells that when you ground the center connection between =
the cells and call it neutral, you will find 3 volts between the ends of th=
e two cell battery and 1.5 from that middle to either end BUT with opposite=
polarity.from the center.
The sign waves coming out of that 3 phase generator peak in succession at e=
ach hot leg in turn, and a three phase motor will reverse direction if you =
swap any two of the three wires.
Just as on a single phase neutral where you can reduce the neutral current =
to ZERO if you have two identical resistive loads (ie non reactive) from ea=
ch hot leg to neutral, so can you if you have three identical resistive loa=
ds each on one of the three phase hot legs to neutral.
If you put one of those resistive loads on just one phase leg, obviously th=
e NEUTRAL has the same current as the one in use phase leg. The trick quest=
ion is how much is on neutral if you had just two of the three resistors co=
nnected. Think of it this way. If all three are on, neutral has ZERO amps. =
cut ONE off, and you just CHANGED what is on neutrasl by one resistor's wor=
th of current. When we remove that resistor from the common neutral wire th=
at was reading ZERO amps, we will get a one amp reading on neutral. If we w=
ere to turn that resistor back on and shut OFF the other two, we would also=
get 1 amp on neutral, BUT there would be an exact 180 degree phase shift i=
f we were watching the two currents on a scope. Together they cancel. But b=
ut but why 180 when 3 phases are all 120 degrees apart? Those two other leg=
s ARE 120 degrees apart and each 120 apart from the third leg, but those tw=
o 120 degrees apart currents sum to being 180 degrees from the third leg (s=
till assuming each resistor is indentical).
Anything other than resistors and current may lead or lag voltage and harmo=
nics may add in ugly ways so neutral current can easily exceed that on any =
phase leg and thus we have all the power factor issues and charges from uti=
lities and mandates for better power factor corrected supplies and codes di=
ctating over sized neutrals where switching power supplies abound.
I was cheerily suggesting hot leg to neutral WYE (aka STAR) connections ( a=
nd I still do!). But actually those using two hot pole branch circuits ( or=
two hots of a three phase branch ciruuit broken into many smaller two wire=
208 ones ) are NOT using the neutral at all in each of those SINGLE PHASE =
connections and avoid the triplen harmonic currents summing in the neutral,=
and can also still use the neutral for small random 120V loads but need no=
t oversize the neutral. Actually any load including three phase DELTA stuck=
across there that avoids the neutral avoids the harmonics issues. I was ju=
st using the 208 single phase loads because they seemed to have been used m=
ore than I had suspected.
Using a three phase branch circuit (say 30 amps as that would be a handys s=
ize, but DOES NOT MATTER here) at 120/208 - whether you are using it as 3 1=
20 volt circuits or 3 208 volt circuits ( as EACH of those connection optio=
ns gets EXACTLY the same max power out of the circuit!!!) gives you EXACTLY=
1/2 the power you could get from the same ampere circuit running 240/415 (=
which would normally ONLY be running phase to neutral loads at 240V, but we=
re 415 volt capable devices available you still get the same total out - EX=
ACTLY twice what the 120/208 can deliver).
NOTE that I carefully was refering to 4 wire 3 phase legs and a neutral for=
these example branch circuits. IF you are comparing two wire single phase =
circuits from the breaker panel to the cabinet (how silly! unless it is fro=
m a PDU in an adjacent cabinet), 208 volts gets you about 13% less power on=
your two wires than you could get at the same amperage on a 2 wire circuit=
at 240 volts. Whether those wires are both hots or a hot and a neutral mak=
es no matter as it is simply the voltage between those two wires times max =
allowed amps (80% of branch ckt rating) that determines what you can get ou=
t. A switching supply simply sucks more amps at lower input voltage so gets=
the power it needs in either case, but you can inflict a larger load at th=
e higher voltage.
Thus 240/415 (240/416...) DELIVERS twice what 120/208 does amp for amp with=
full three phase wiring, and that is why it is so great.
