Wire size for very short run @40 amps

[Butch(OH)]

I'm working on an old 32Volt DC lighting plant and need to wire in a shunt for a new ammeter. The run from the buss to the shunt is very short, less than 3". The unit is rated at 40 amps. None of the on-line tables are any good for a DC run this short and the way the shunt is made and where I need to put it means using over sized wire is not the easy way out. Anyone know how do do the calcs so I can use the smallest acceptable wire size?

 

[M-MAN]

I would use a 10g and call it good. Most of us use that with alternator conversions.

 

[stevieb49829]

The "OMNI calculator" I found on the net says 8 AWG. But that assumes the 40 amp draw. Is there a way of figuring the actual draw through the gauge? Steve

 

[stevieb49829]

calculator

 

[Butch(OH)]

The "OMNI calculator" I found on the net says 8 AWG. But that assumes the 40 amp draw. Is there a way of figuring the actual draw through the gauge? Steve

I wish I was that smart Stevie, LOL. At this point I only have this pic, gauge is on the way here.

 

[stevieb49829]

I wish I was that smart Stevie, LOL. At this point I only have this pic, gauge is on the way here.

Assuming your new gauge is in series with the load (Your wires in and out of the gauge is what powers the load, with no parallel wires going directly to the load) then just look at what you intend to power. Just add up all the amp ratings on anything that could possibly be running at the same time, and that will give you a better estimate of the actual load on the wires. 8 gauge is designed to handle the 40 amp maximum. 10 gauge as M Man suggests is probably closer to what you actually need, but if you use the full 40 amps at any time, your 3" long 10 ga. wire will act as a fuse and get hot. Maybe to the point of frying. Which is what fuses do.

I ran the calculator using a 30 amp load, and it recommended 10 gauge wire. So if you pull less than the full 40 amps, the 10 gauge is probably suitable. steve

 

[Loosehandle]

The delcos were only 750 watts? So 24 amps.

I'm working on an old 32Volt DC lighting plant and need to wire in a shunt for a new ammeter. The run from the buss to the shunt is very short, less than 3". The unit is rated at 40 amps. None of the on-line tables are any good for a DC run this short and the way the shunt is made and where I need to put it means using over sized wire is not the easy way out. Anyone know how do do the calcs so I can use the smallest acceptable wire size?

 

[Butch(OH)]

The delcos were only 750 watts? So 24 amps.

This isn't a Delco. It's an R A Lister and Co. They called it a 1 1/2 KW lighting plant and the tag says 40 amps 32-40 Volts

 

[Butch(OH)]

Assuming your new gauge is in series with the load (Your wires in and out of the gauge is what powers the load, with no parallel wires going directly to the load) then just look at what you intend to power. Just add up all the amp ratings on anything that could possibly be running at the same time, and that will give you a better estimate of the actual load on the wires. 8 gauge is designed to handle the 40 amp maximum. 10 gauge as M Man suggests is probably closer to what you actually need, but if you use the full 40 amps at any time, your 3" long 10 ga. wire will act as a fuse and get hot. Maybe to the point of frying. Which is what fuses do.

I ran the calculator using a 30 amp load, and it recommended 10 gauge wire. So if you pull less than the full 40 amps, the 10 gauge is probably suitable. steve

I don't know if I can explain it very well but how it works is you have a bank of batteries, In my case it will be 3-12 Volts in series for 36 Volts Those are connected to the control box which also has 2 output leads for lights or whatever else they had that used DC power. You can just use the batteries to run things a while and then run the engine as needed to both charge batteries and power the load up to the capacity of the engine and generator head. What goes through the ammeter is only what's coming from or going to the batteries that could be nothing or 40 amps coming from, or going to. The OEM ammeter didn't use a shunt and was bolted to the busses with 3/16 brass threaded rods.

 

[buickanddeere]

10 gauge solid . Better if it is bare .

 

[sotxbill]

Since your gauge shows a shunt.... the wire from the shut to the voltage meter can be very very small as in 14/16 gauge. However the load wire in to the shunt and out to the load,,, must be the actual wire going out and should be sized for 40 amps.. The shunt by design and be located on the load wire, with small wires running back to the meter as the meter is only measuring voltage, and passing milliamps of current. That way the shunt is located in the main feed wire going out.

 

[JMOR]

10 Ga wire at 40 amps will drop 0.04 volts per foot and the wire will 'fuse' (melt) at somewhere around 300 amps. Is that coil of wire beneath the ammeter the original shunt?

 

[Butch(OH)]

10 Ga wire at 40 amps will drop 0.04 volts per foot and the wire will 'fuse' (melt) at somewhere around 300 amps. Is that coil of wire beneath the ammeter the original shunt?

No, that is the output contractor. There is no external shunt as built 100% of the amp flow goes through the OEM meter. I actually have a wiring diagram of anyone is curious.

 

[Butch(OH)]

No, that is the output contractor. There is no external shunt as built 100% of the amp flow goes through the OEM meter. I actually have a wiring diagram of anyone is curious.

A better picture showing the control board ammeter still attached. Those long screws are the conductors along with mounting. below it is the output contractor.

 

[stevieb49829]

A better picture showing the control board ammeter still attached. Those long screws are the conductors along with mounting. below it is the output contractor.

Another way to look at this: The mount/conductor screws look to be about a 10-32, and they are carrying the current in and out of the ammeter. If you match the wire size to the screw size, you'll be OK. If my guess at 10-32 is correct, 10 gauge wire is good.

PS: I like your shop. There's never enough grinders, or welders!!! steve

 

[Butch(OH)]

Another way to look at this: The mount/conductor screws look to be about a 10-32, and they are carrying the current in and out of the ammeter. If you match the wire size to the screw size, you'll be OK. If my guess at 10-32 is correct, 10 gauge wire is good. steve

They are 1/4" LOL,. British overbuilt!

 

[Tramway Guy]

10 gauge solid . Better if it is bare .

I believe stranded is a better choice!! Less sensitive to vibration and easier to use crimp or lug terminals, as they don’t loosen as easily.

 

[stevieb49829]

They are 1/4" LOL,. British overbuilt!

To be safe, use #8. A DC short can put a lot of load on any of that circuit. Or use #10 and rely on it to be your fuse in the circuit. It is a farmer thing, right? steve

 

[Janicholson]

I'm working on an old 32Volt DC lighting plant and need to wire in a shunt for a new ammeter. The run from the buss to the shunt is very short, less than 3". The unit is rated at 40 amps. None of the on-line tables are any good for a DC run this short and the way the shunt is made and where I need to put it means using over sized wire is not the easy way out. Anyone know how do do the calcs so I can use the smallest acceptable wire size?

The wires now going to the original Ammeter are correct gauge to go to the shunt. No issue at all. Jim

 

[randyrenfro7]

Why not a solid copper bar 1/8 x 1/2" and drill holes in it if I am seeing this right.