SW inverter to battery cable calculation [RE-wrenches]

[Mark Edmunds]

Hi Joel,

For simplicity and consistency we and other inverter manufacturers normally
refer installers to our calculated wiring tables in the manual, but for
background - the average dc input current can be calculated as discussed in
some of the earlier posts: Output power/(efficiency*input voltage) then
multiply by 1.25 for the appropriate NEC rating level.

For a SW5548, with worst case efficiency of 85% at a low bat. 44V input, the
average full load input current is 5500/(.85*44)=147A, multiply by 1.25 for
a minimum NEC ampacity rating of 184A required for the input cable. This
would give  #2/0 AWG as OK for use in conduit. Obviously even though a cable
of appropriate current rating is chosen, the length will also effect the
total loss. The voltage drop in the cable at full load should normally be
kept below about 0.5%, or 0.24V in this case. From good old Ohm's law
(V=I*R), the total dc wiring resistance limit can then be estimated,
0.24V/147A=0.0016 Ohm.

When you know your round trip dc cable length, take the resistance limit
divided by length to get resistance per foot that you need to keep the dc
drop under 0.5%, then multiply by 1000 to give resistance per 1000ft, the
normal number in wire tables. With longer wire run lengths you will find
that the required cable size to keep the voltage drop acceptable will be
larger than that required by the NEC anyway. In the example case above, with
total cable length (2 ways) of 20ft, the wiring resistance limit would be
0.0016Ohm/20ft*1000 = 0.081 Ohm/1000ft, which looking at a wire table gives
a required gage of about #3/0 AWG. Obviously with high current wiring bigger
is better, in terms of reducing losses, and bumping up to a more common #4/0
wouldn't hurt!

As has been discussed in some of the previous posts the true losses in the
cable are affected by the ac ripple current drawn by the inverter. The only
really accurate way to determine that effect is to actually use a true rms
ammeter to measure the total rms current flowing, though the average
technique described above will get you pretty close.


Hope this helps, John Wiles had a good "Code Corner" article on dc wiring
for inverters in HP 95 (June/July 03) check that out too.


Regards,

Mark Edmunds
Xantrex Technology

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Friday, November 28, 2003 7:43 PM
To: RE-wrenches at topica.com
Subject: Re: SW inverter to battery cable calculation [RE-wrenches]


Dear Mark,
I was not blowing the whistle on tech support which has generally been
helpful.
Besides, not everyone at Xantrex is going to know somewhat esoteric
calculations. Technical notes and bulletins from the manufacturer are always
welcome. What is the calculation?
Best regards,
Joel Davidson

Mark Edmunds wrote:

> Our technical support people are generally well versed in cable sizing and
> rating requirements for our products. We are following up with Joel
directly
> to see who he talked to and why he did not get satisfactory follow up.
>
> Regards,
>
> Mark Edmunds
> Xantrex Technology Inc.

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