Fuse sizing questions.

Dean W · June 2015

Hello all;
My name is Dean, and I'm a new guy working on a small battery backup system. As a long time member of a few
other (non solar) forums, it's funny being the new guy. I have done quite a bit of searching and reading the past week,
and now have a lot of things rattling in my head. Much new info, to me, and I'm having a little trouble with fitting it all
together regarding a few things. Fuses, in this question.

When I put a fuse in the battery cable on the positive side, I should select an amperage that is under the capacity of
the cable, right? For instance, my inverter comes with 8ga cables. The AWG site says the maximum amperage for
wiring with that size cable is 73 amps. In that case, should I use a 70 amp fuse, or is there a need for a certain
percentage of safety factor that I don't yet know about?

Thanks for your help, and for such a nice forum here.

Dean

mike95490 · June 2015

70A seems right to me. You want the fuse to blow before the wire melts the insulation off of it. Fuses have a bit of delay, so a brief surge can get through. Does the inverter have a suggested fuse/wire gauge pairing ?

BB. · June 2015

You can get the NEC (National Electric Code), or search for information on the web... Maximum current through the wire depends on temperature, conduit fill, insulation type, etc...

NEC tends to be more conservative--Here is a simplified NEC wire chart:

https://lugsdirect.com/WireCurrentAm...ble-301-16.htm

8 AWG cable is "good" for 40-55 amps (depending) per the NEC.

Note for NEC, they use a 0.80 derating... or take your maximum sustained current and multiply by 1.25:

73 amps * 1.25 NEC derating = 91 amp minimum branch circuit wire and breaker rating (round up to 100 amps).

Or you can use Marine wiring guides (table 3):

http://www.boatus.com/boattech/artic...-terminals.asp

8 awg
80 amp Engine Space/outside
68 amp inside wiring

And then, you also need to worry about voltage drop (too long of cable, too much voltage drop).

You did not say what the Battery Bus voltage was, but roughly a good starting point for wiring voltage drop:

12 volt -- Maximum 0.5 volt drop
24 volt -- Maximum 1.0 volt drop
48 volt -- Maximum 2.0 volt drop

If you have 73 amps on a 12 volt battery bus with 8 feet of wiring, one way run, and a maximum of 0.50 volt drop... Using a generic voltage drop calculator:

http://www.calculator.net/voltage-drop-calculator.html

For 6 AWG wire:

Voltage drop: 0.46
Voltage drop percentage: 3.83%
Voltage at the end: 11.54

So, keeping DC wiring short as possible for high current runs with low voltage DC is critical.

Many times, for DC wire runs used in Off Grid Solar Power/Battery systems, we use much heavier AWG wiring to keep voltage drop (and power losses) low vs what you would need to carry XYZ amps of current.

-Bill

Dean W · June 2015

Thank you for the helpful answers, gents.
To answer the first post question, the inverter comes with 8 ga copper cables, and the instructions say to
include an ANL fuse, but they do not say what amperage rating for it.

To the second post, the voltage is 12 DC from two 6v 220 ah batteries in series. The wire length will be less
than two feet, each way. I can use larger gauge cables. I used the example of 8 ga in my question because
that is what the inverter is supplied with. It's 900/1800 watts. I plan for a 400 watt load on it, usually in 3-4
hour run times. The batteries will be charged from the house grid power.

If I figure it correctly, a 400 watt load on the AC side should be about 34 amps draw on the batteries. Does
that sound right? In that case, should I use the formula given:
34 * 1.25 = 42.5 amps. So, should I fuse for that amount of amps (42.5), or the amount of amps the cable can carry?
(Bill, the figure of 73 amps that you used in your equations was what I found for the #8 ga cable on the AWS site.)

Sorry to belabor this. I've read a lot lately and am trying to reconcile it in my head.

Dean

Fuse sizing questions.

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