wire gauge with a small duty cycle

i've got a source at 24VDC with a .5ohm load, or about 48 amps. I know i'd need about an 8 or 10 gauge wire for that, but my duty cycle is only going to be about 10%. I've looked around and havent found anything, but is there a chart or formula associate with figuring this out?

Thanks

justinlw said:

i've got a source at 24VDC with a .5ohm load, or about 48 amps. I know i'd need about an 8 or 10 gauge wire for that, but my duty cycle is only going to be about 10%. I've looked around and havent found anything, but is there a chart or formula associate with figuring this out?

Thanks

Click to expand...

I don't know of one, but what's the frequency of the duty cycle?.

I would average out the current...like if it's 100A at 10% duty cycle I would size the wire (minimally) at 10A. Kind of like using the RMS value of an AC signal to size the wire rather than the peak-to-peak measurement. Wires aren't like transistors where they have a high loss intermediate switching stage that needs to be accounted for so I think it should be alright if you do this.

I don't think it's that huge a problem as long as the frequency isn't in the MHz range. I don't think the skin effect really matters until the frequency gets that high.

If you need a wire sizing chart:
**broken link removed**

Also consider that if you use a thinner gauge wire, there will be more volt drop due to the resistance of the wire. You will not have 24volts at the load.

What is the minimum voltage the load requires?
What is the load?
Is the voltage critical for the correct operation of the load?

JimB

Keep in mind, if the frequency of the signal is 1khz and the duty cycle is 10% the the 'pulse' frequency will actually be 10khz.

justinlw said:

i've got a source at 24VDC with a .5ohm load, or about 48 amps. I know i'd need about an 8 or 10 gauge wire for that, but my duty cycle is only going to be about 10%. I've looked around and havent found anything, but is there a chart or formula associate with figuring this out?

Thanks

Click to expand...

Try this link...
**broken link removed**

dknguyen said:

I would average out the current...like if it's 100A at 10% duty cycle I would size the wire (minimally) at 10A. Kind of like using the RMS value of an AC signal to size the wire rather than the peak-to-peak measurement. Wires aren't like transistors where they have a high loss intermediate switching stage that needs to be accounted for so I think it should be alright if you do this.

I don't think it's that huge a problem as long as the frequency isn't in the MHz range. I don't think the skin effect really matters until the frequency gets that high.

If you need a wire sizing chart:
**broken link removed**

Click to expand...

The problem is low frequency, which I'm pretty sure is the reason Nigel asked. If the pulse width is long relative to the thermal time constant, duty cycle is irrelevant.

Ron H said:

The problem is low frequency, which I'm pretty sure is the reason Nigel asked.

Click to expand...

Yes it was, a very low frequency will overheat the cable when it's turned ON.

Current flowing thru a wire heats the wire. Assuming voltage drops are tolerable then a significant limiting factor is temperature. If the wire gets hot enough it will deform or simply melt. The insulation may degrade or burn before that.

In free air, the conductor will settle at a temperature where the heat input as a result of the current flow will equal the losses to the surroundings via conduction and radiation. Add insulation, pack wires into conduit, etc and the amount of current to be handled is reduced. All of this only helps to illustrate how complex the situation might be.

Wire can be had with insulations that tolerate higher temperatures. If your problem requires it - and the situation can tolerate the voltage drops you might find some ampacity increase for a given wire size (with due consideration for other issues discussed here) by going to a high temp insulation. You allow the wire to get hotter but the insulation tolerates it. There are tables/charts for this.

Thanks guys.

The wires will be run under the floow in a trough(few inches of clearance all around). Its probably gonna be turned on for about <0.3s twice every 7s(worst case, more like <0.3s every >14s)

Oh voltage drop. We're running the wires about 130", so Vdrop is an issue. With 16gague, we'd have a 2.5ish drop. The power supply we are going to use puts out 30V, so we might be able to handle as much as a 6V drop.

48 amps * 6v drop = 288 watts dissipated in the wire!
I REALLY don't think you want a 6v drop
Even at 10% duty or so, that's still a lot of instantaneous power. If it doesn't burn up your wires within the 0.3s that it's on each time, it's at least going to cause a lot of heating, and your wire will be doing a lot of thermal cycling, which is probably NOT going to help it survive for a long time.
Spending a little more for heavy enough wires is a LOT smarter than setting your floor on fire...

Why?

What is the circuit driving the wire?

Is it a transistor or a relay?

What about the load?