Wire size from battery to starter

Dayton Murdock · #1 10-18-2005, 07:24 AM

Hi RV Builders

What size wire should I use from the battery to the starter?

tinman · #2 10-18-2005, 07:57 AM

How long of a run is required? For a "standard" installation... that is to say that the battery is mounted on the firewall, the norm would be 2 AWG with crimped and soldered terminals. I had great success with the welding cable because it is very pliable and easy to run.
Tinman

Dayton Murdock · #3 10-18-2005, 12:54 PM

I want to know if can I use a smaller size i.e. 4 or 6 ga. wire for the starter application.

mlw450802 · #4 10-18-2005, 02:59 PM

Quote:

Originally Posted by Dayton Murdock

I want to know if can I use a smaller size i.e. 4 or 6 ga. wire for the starter application.

Dayton,
I did a ROUGH calculation on 4 gage wire assuming a 500 amp starting current.
It looks like the heating rate in that gage will be approximately 8 degrees F per second. The assumption is totally without heat transfer considerations to the surroundings and the rate is proportional to the square of the actual current. (I^2*R)
If the real starting current is only 250 amps then the heat up rate will be ~2.8 deg per second.

10 seconds of cranking will increast the starter cable 80 degreesF in the first case and 28 degF in the second.

These numbers are very ROUGH and I will check more closely when I get home.

-mike

Davepar · #5 10-18-2005, 03:16 PM

Using a smaller wire will cause the wire to get hotter and provide less voltage to the starter. Not sure if it's a large or small effect. There are formulas (in the Aeroelectric book) to calculate the amounts, but I don't have them in front of me right now.

(Oops, I see Mike is already on the case.)

osxuser · #6 10-18-2005, 03:27 PM

I ran #2 in that Whitmen Tailwind I rewired, it did seem a bit like overkill, but that is one of those things I want to be strong. If your just going on a short run #4 would probably work fine, but if your going from a aft mounting forward I'd use #2.

az_gila · #7 10-18-2005, 09:48 PM

Quote:

Originally Posted by Davepar

Using a smaller wire will cause the wire to get hotter and provide less voltage to the starter. Not sure if it's a large or small effect. There are formulas (in the Aeroelectric book) to calculate the amounts, but I don't have them in front of me right now.

A calculator is here...

http://www.stealth316.com/2-wire-resistance.htm

Voltage drops are not too bad for 500 amps and 5 ft.
0.4 volts for #2 and 0.65 for #4

The weight savings is only 6 oz. Given the high currents involved here, it probably isn't a good spot to save weight...

gil in Tucson

vic syracuse · #8 10-19-2005, 02:59 PM

I used #4 on the RV-10 with the PC 625 battery in the back, and the B&C starter. It works fine. So far the coldest start has been around 30 degrees with no problems. I agree with everyone's posts regarding it SHOULD be #2, but #4 was alot easier to work with. I silver soldered all connections.

Vic

Will Womack · #9 10-19-2005, 03:53 PM

In the certificated world we have to keep all runs to a maximum of 2% voltage drop. On a 14VDC system that means you can only drop 0.28V no matter how much current or how long the run.

Ref AC 43.13-1B Sec. 5 11-66 b.

Davepar · #10 10-20-2005, 10:14 AM

That same paragraph refers to table 11-6, which states that the allowable voltage drop for continuous operation in a 14V system is 0.5V and for intermittent operation it's 1V. I wonder if the starter could be considered intermittent.

Link to AC 43.13 chapter 11

Chapter 11 has a lot of advice on wire sizing that I'm still trying to digest. Luckily we don't have to worry about huge bundles of wire at 50,000'.