Battery Cable size

1 Posts

Mar 2, 2005 06:18 pm

I've used your Wire Sizing: A Practical Guide to determine battery cable sizing and I would like your opinion if I sized it correctly for my Inverter.

1) the battery bank is 48 volts @ 450 amps
2) Inverter is the Xantrex 5548 Dual Power Panel.

My assuption is that my house load can draw the full 11KW of power. Therefore 11KW @48 volts = ~230 amps.
Mt longest cable will be 5 feet, the shortest 1 foot. I am also interested in only a 1% voltage drop.

VDI = (230x5)/(1x48) = 24 rounded up

So I could use #2 AWG wire for all connections. correct?

Thanks in advance for any help.

Gary

Thomas Allen Schmidt

679 Posts

Mar 6, 2005 08:07 am

Re: Battery Cable size

I can see that your question is directed at the Alt E Store.

I would like to give my opinion on the subject.

If you take a look at the specifaction chart on the sales page you can see that Trace reccomends 4/0 (pronounced - four ought.) http://shop.altenergystore.com/itemdesc~ic~XANPP-SW5548/D~eq~~Tp~.htm

What I have done in the past is to take the maximum continuous wattage and divde that by the inverters low voltage disconnect set point. which in this case would be 5500/42=131 amps. Now this is where it can get monotonous. Just how much amperage a wire can safely handle (per NEC) depends on several factors. Those being the size and type of wire and its insulation and if it is suspended in free air or in side of conduit and how many current carrying conductors are in that conduit etc. etc. and so on and so forth. I was confused by the statement made in the notes of that "Practical Guide."
"in raceway in freee air"
The first thing to my mind was, which one? In a raceway or in free air? A raceway in free air doesnt make any sense to me. To my mind in a raceway means inside of an enclosure. A box of varible dimensions and or configurations and conduit, and free air to be like but not limited to the grid, from pole to pole. The same size and type of wire can have different amperage ratings in these two scenarios. Alt-E covers themselves nicely though by including the part about refering to the NEC.
That is after all what the inspector is going to use. If the wire size and type is wrong and you tell the inspector "thats what Alt-E said" it will not make any difference.

I know I am probably coming accross as being belligerent but its for a good reason, the NEC falls under the jurisdiction of the NFPA National Fire Protection Association. The key word here being fire. http://www.nfpa.org/index.asp

Back to that figure of mine, 132 amps. I going to give an example. #2AWG CU THHN, uhhm you know what, something just came up so I am going to leave now but I leave you something to research while I am gone. http://www.sandia.gov/pv/docs/John_Wiles_Code_Corner.htm

Thomas Allen Schmidt

679 Posts

Mar 6, 2005 02:06 pm

Re: Battery Cable size

LAST EDITED ON Mar-06-05 AT 02:16 PM (EST)

Ok, where was I? Oh ya!
#2 AWG THHN according to table 310.16 in the NEC is good for 130 amps.
(In table 310.17 its good for 190 amps, but I not going to go there.)

http://www.houwire.com/catalog/technical/article310.asp

Now when I use Alt-E's VDI I must not be doing something right. When I try there example it figures out the same, but when I try -
131 amps x 10 foot that = 1310
Then
3% x 42 that = 126
Then
1310 / 126 = a VDI of 10.40
Then looking at the chart its says #6 wire. Well all I have to say to that is a great big, Huh! Looking at gmiller 261's figures he starts out with 260 amps which is both 5548's together and still winds up with a VDI of 24 which the chart says is #2 AWG but we all ready know that #2 AWG THHN is only good for upto 130 amps.
So my question is. Why does Xantrex suggest 4 aught wire? If I am right that the 5548D power panel has two main DC breakers, one for each 5548 and the smallest ones they sell are for 175 amps then wouldnt 2 aught work for relatively short distances considering the 5548's only pull 131 amps at there lowest voltage under full load?
Or do they calculate the surge wattage instead of the continuous wattage?

We need some help with your VDI calculations Alt-E Store.

Thomas Allen Schmidt

679 Posts

Mar 6, 2005 02:14 pm

Re: Battery Cable size

One other question.
If, taught is pronounced the same as, taut
then way isnt, thought spelled, thaught or even, thaut. Oh nevermind.

Matt dean

5 Posts

Apr 24, 2006 10:54 am

Re: Battery Cable size

In the owners manual for the SW Series inverters,Xantrex states that the SW5548 can draw 147 amps,based on 85% efficiency and a low battery.
44VDC is the minimum input voltage for tis inverter.
The NEC says that the continuous load on a wire can
not ecxeed 80% of it's rating,so we need to size the wire for 184 amps.For a 1% voltage drop at 5',we have (184x5)/(1x44)This gives us a VDI of 20.9.
Looking at the chart,we see that #2 would meet the voltage drop criteria,but does not have adequate
ampacity.2/0 would meet the ampacity requirement.
So why does Xantrex recommend 4/0?The ampacities in the table are for wire types rated at 90 degrees C.If wire with a lower temperature rating is used,then a larger size is required.If any devices used with the wire,such as breakers fuseholders,or terminals,have a lower temperature rating than the wire then the wire must be used at
the lower rating.

Thomas Allen Schmidt

679 Posts

May 4, 2006 07:33 pm

Re: Battery Cable size

Hmmm...March 2005...

The address for wiewing Article 310 of the NEC has changed. Its now -
http://www.houwire.com/products/technical/article310_16.html

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Battery Cable size

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