Quote:
My winch
claims to draw 435A at maximum Line pull.
(ref) Warn provides a 2ga battery run, which
according to this chart is good for ~180A continuous draw. Maybe they are considering duty cycle as part of this, I don't know.
Yes, they do consider duty cycle. Current carrying capacity for a wire is based on how much the wire will heat up at a given current. Or, more specifically, how much heat a wire can dissipate. The goal is to prevent thermal runaway on the conductor which means the conductor is generating more heat than it can dissipate.
At 180 amps, a 2 gauge wire generates around 5 watts per foot. It will get warm, but it will reach a point of equalibrium there the temperature will stabilize and the wire will dissipate as much heat as is generated.
At 500 amps, that same wire generates 39 watts of heat per foot. The wire can easily pass this current = for a while. But, it will eventually heat up to where it burns up the insulation, then maybe, even the copper itself.
Here's where duty cycle comes into play. Warn doesn't list duty cycle for truck winches, but they do for the industrial winches. I picked a 12VDC 12000 lb winch for an example.The duty cycle at no load was 3.1 minutes out of every 10 minutes, or 31%. At that load, it is rated at 107 amps.
What we're going to do now is to calculate the ECTC or Equalivalent Continuous Thermal Current. Take that 107 amps and multiply it by the 31% duty cycle to get an ECTC of about 33 amps. That is saying if you free spool this winch for 3.1 minutes out of every 10 minutes, it will have the average heating effect on the power leads the same as running 33 amps through them continuously. At 33 amps ECTC, this will generate about 0.2 watts per foot of wire average.
At maximum load, this winch draws 481 amps with a duty cycle of 5%.(one half minute operation out of every 10 minutes). 481 x 0.05 = for an ECTC of about 24 amps.
Now it seems like we should be able to have a higher duty cycle than that because the ECTC values are so low. You have to remember, these values are averaged over 10 minutes. In the latter case, that is 30 seconds of heating and 570 seconds of cooling, so the temperature will rise faster in the second example than it would in the first example because of higher peak currents.so the ECTC is derated to allow for that. It isn't just the cables heating up, it's also the winch motor.
Fuses also operate under the same ECTC theory as motors and wire. However, since they have less thermal mass, and are designed to control overcurret situations, their allowable duty cycle time periods are much shorter. It also varies by fuse type and purpose. Some fuses blow as soon as they see hardly any overcurrent at all, and some will allow overcurrent for a longer period. The fuse manufacturers publish charts of each fuse's current vs. time rating.
In a nutshell, that's why fuses can be rated lower than the peak ratings on a load device, in this case, a winch.
The above caculations have some bearing in the real world, but are very simplistic. Because winch load is often prolonged and widely varied in respect to load current, the best test is to see if things are overheating.