Can someone tell me what is considered the maximum safe amps in an 8 ga. wire? In 120v AC I believe it is 45 amps, but in 12v they seem to run a little higher.

Thanks,
Brian

 

Technically it depends on the type of insulation the wire has. I can't seem to find any mention of ac or dc being a factor. If it will be bundled with other wires, and the insulation is rated at 140F, then NEC says 40 amps. If it will be run by itself, then they say 60 amps. I would stay on the conservative side myself, and go with the 40 amp.

 

You might find some useful/interesting info here.

 

Interesting site, but I didn't find any specifics.

As far as the NEC goes, I guess I could use that, but judging by the fuses that Ford uses, a 12V system seems to allow higher current flow. Maybe its because of the shorter distances.

I need to know how big an inverter (12V DC to 120v AC) I can run off an 8 ga wire. I'm sure the manufacturer will have that in the instructions, but I won't see that 'til after I've bought it.

 

Can you give an example of the factory over fusing a wire in the harness? I am curious.
If you want maximum performance from any electrical load on 12 volts, I would always over do it on the wire size. Run parallel conductors if you have to. Any voltage drop on a 12 volt system will kill you. That's why they got away from 6 volt systems, and probably why the military uses 24 volt systems.

 

If I get the time I'll try to be specific, but it seems I've seen #12 fused at 25 amps, #14 at 20, and #16 at 15. Of course the NEC doesn't permit #16 in home wiring, and I think the new Fords are actually metric sizes.

I just assumed that somewhere there would be a table for automotive, like there is for residential.

Anyway, I've hooked it up with a maxifuse off the battery junction box. I'm going to use a 50 amp maxifuse, which I feel comfortable with since the 8 ga wire is mostly run by itself. I think maxifuses are available at 60 amps should I latter decide I need it and the wire can handle it.

I also need to consider the ground for current at this level. I will try to ground to a substantial body structure, but maybe I should run back to a battery connection (like at the starter solenoid). Any advice?

 

The body should be a good ground if you clean the paint first and use a suitably-sized ring terminal and a screw that will hold it solidly.

 

As far as the ground, the starter relay (solenoid) won't give you a ground, other than the mounting lugs. It's only switching positive voltage. Depending on where you mount the inverter, the best ground will be on the end of the battery to engine block cable. Run an appropriate eye terminal and double stack the terminals with a star washer between the two. That way you're not running the additional load through the engine to chassis ground cable.
Just my 2¢
Greg

 

Thank you for the suggestions! I do agree that it would be better to not run the additional possible 50amp load through the existing chassis ground, but it might be overkill. Depending on how much work it takes, I might just find a substantial chassis member. Maybe I can determine the gauge of the existing chassis to battery connection. It seems unlikely that I could burn out any such connection, but I might see a substantial voltage drop at full power.

I am trying to avoid going back through the firewall. But if I have to go through the firewall, I want to avoid having to cross over the engine (My + cable goes out the access hole on the driver side to the battery junction box). The area over the rear of the engine is already difficult to access for service. Maybe I can get into the "duct" at the cowl.

I realize that the starter relay only switches positive current, but it also has a ground lug in order to energize the coil. On my 2001 F-150, this is a large threaded, uninsulated terminal that appears to connect via a thick cable directly to the battery negative post. It tests negative, but without opening the harness I cannot be sure of capacity. As I said, though, I think it is a heavy cable direct to the battery. This is on the passenger side of the engine compartment, so if I can find or make an access through the firewall there, it would be a fairly nice layout. I'm not too happy about an additional firewall hole though.

I would appreciate any further enlightenment!

 

> maximum safe amps in an 8 ga. wire?

From the West Marine chart in their catalog, 60 amps for AWG wire with a 10% voltage drop and 25 foot round trip for the circuit. This puts you solidly in the middle of the amperage rating for the wire. If you are using SAE (house) wire, take roughly 10% off of that.

 

Thanks! It's an automotive 8 ga. wire, used for high power stereo hook up.

So if I provide a good solid ground path, I should be able to handle 60 amps. Any comment on using the body vs. running two #10 (in parallel) back to the starter solenoid ground lug? I'm thinking I should do the latter.

 

There is NO GROUND terminal on the starter relay. Don't run any ground wires to ANY of its posts. But you can ground your 2 #10s to the battery, or either of the terminals on the black battery wire.

 

Really! With all due respect, are you sure? The vehicle in question is a 2001 F-150 w/auto. The starter motor relay is mounted vertically on the firewall inboard of the battery.

Here's why I think there is a ground connection on the starter relay:
1. Page 20-1 in the Ford "2001 Wiring Diagram" shows a grounded terminal (although I cannot see a wire size).
2. The coil cannot activate the starter contacts without a ground of some sort.
3. There are 2 large threaded terminals on the relay. One has an insulated cap and shows + battery voltage at all times. The other is uncapped and shows less than 1 ohm resistance to the battery negative terminal. A heavy cable seems to connect it to the negative battery terminal, although it does go through a harness which makes its exact path unclear.

Thank you for your input!

 

The relay (solenoid) is grounded internally to apply the coil. A wiring diagram will show it as a connection but there is no wire. The "other" heavy cable goes to the starter which draws a whole lot of current, hence the heavy cable and the low resistance to ground. If you hook a load ground wire to this terminal, it will ground through the starter, and probably burn it out from the constant current.
Another way to look at it...the relay is there to switch the voltage and current on and off. When the starter is cranking, the "other terminal is connected to the battery positive through the relay. If it was a ground, the battery positive would be applied to ground when the relay energized. Not a good situation. The small wire applies the switch and the big wires carry the heavy current through.
Hope this helps.
Greg

 

Thank you! Makes sense. Thanks for saving me from making a real blunder!

Obviously I'm better off to go to a battery ground terminal, but would a body connection be O.K. or would you recommend against it?

Thanks again!