Amps or Watts ????

[Shooter]

I've been having this argument for what it seems like forever with several people. My work affiliates keep telling me that there is NO WAY that we (FE boaters) are pulling 100+ amps thru 10awg wire. Maybe somebody can chime in and give me some additional ammo!

My answer is always.....Yes, we are and we are perfectly fine doing so! The reason we are able to do this is as follows:

1. Our wire lengths are very small (typically less than 10")
2. Our wire is typically fine strand which increases surface area
3. We run low voltage which equates to MUCH less POWER (P=IV, Watts) when compared to house wiring.

It's the POWER that counts, no? Any help would be appreciated. Thanks.

[ReddyWatts]

We are concerned with voltage drop. 1 foot of 10 awg wire has .002 ohms and heat, which is watts.

It calculates:
IxR=V
100 amps X .002 ohms = 0.2 voltage drop.

Amps squared X ohms = watts
100 amps X .002 ohms = 20 watts of heat

[Fluid]

Ya can't fix stupid*. Stop arguing and enjoy!






*Of course your coworkers aren't really stupid, but it may not be worth the effort to educate them.
.

[Shooter]

Reddy - Thanks. Your voltage drop in the wire equation supports #1 above (short length wire)

Fluid - no doubt! I'm living proof!

[befu]

The actual amperage a single wire can carry is rather high. What most people are used to seeing is the derated number. As you add conducters into a conduit, the amp carry rating drops dramatically. The more conductors, the more each one is derated. Nomex run through house walls also has a rating. These are wires that are installed in housing and factories. As such, they fall under restrictive codes so people can be assured they last. This is also at the rated current of motors.

Example: We have a 100hP motor here that is rated at 118 amps. But the startup draw is in excess of 600 amps. The wire must be derated so it can handle the heat from the startup draw without damaging the insulation.

Now, our boats do not have these regulations. They also tend to have wires running in open air, so single conductors. (haven't seen many boats running conduit yet!)

From my EE pocket handbook, a single 10 gauge copper wire in free air with a 302 degree F rating can handle 80 amps by code. Go to a 392 degree rating (FEP, FEPB, PFA) and it is rated at 90 amps. This does not mean it melts at 91 amps, this is the code.

Our boats do not pull that much amperage for that long (peak I mean). What a wire will safely carry and what it is rated to caryy can be two different numbers.

So yes, given good multi strand wire and high temp insulation, you can really pull some amps through your boat wires. Codes are to let people know what will work, not at what point they stop working.

Brian