Anyone advising against extension cords must live REALLY close to a power
plant, cause all wiring is essentially an extension cord

The short answer: You can add 10 feet of 12 gauge on all three machines, no
problem, but I agree with Greg that it is cleaner just to put longer cords
directly to the electrical connections in the machine. No extra plugs to
get dirty, trip over, and pay for.

The longer answer: If you really are curious, what you care about is a)
voltage drop, and b) internal heating.

Internal heating is usually not an issue unless the cable is wound into a
tight bundle or run in an enclosed space (like conduit). For stretched out
cords, the heat generation is distributed over the length of the cord. You
could (in theory, at least) run 15A of current thru miles of 14ga wire, and
it would never overheat -- but the voltage drop would limit you way before
that.

Voltage drop: this is where length is important. Lower gauges are bigger
wires, so their resistance is lower; for the same current, they have less
voltage drop per length. 12 is about 1.7 ohms per 1000' (suprisingly
little) while 14 is 2.6 ohms per 1000'. So a run of 14 will drop 50% more
voltage than a run of 12. Ratings are pretty conservative: 100' of 14gauge
at 15A will drop a reasonable 8V (4 on the hot, 4 on the neutral). 1000'
would lose 80V -- which clearly would not work in a 110V system.

This is why power transmission lines are high voltage (lowering the current
for the same power), At low voltage, the losses would be way to large to
tolerate.

As far as prepackaged extension cords: One reason the cords may be rated is
15A is not the cable but the connectors (the plug itself). Connectors carry
a rating, just like cable does, or outlets, etc. Usually, the rating has to
do with the internal construction and regulatory testing. Often an plug
carrying too much current will heat up long before the cord it is attached
to. You will notice it when you grab the plug to unplug it -- it is warmer
than expected. Vacuum cleaners are notorious for this, as the plugs have a
tendancy to get uplugged with a quick yank on the cord (tsk, tsk). Time to
fix the plug or get a new one.

FWIW,

Matthew


"Mark Wells" wrote in message
ink.net...
So I know I'm over-thinking, but this will give a chance for some of the
folks with electrical religion to let off some steam.

I have 3 machines that all need short (10 ft) extension cords:
* 1.5 HP dust collector
* 1 HP band saw
* 1 HP jointer
All run on 110 volts.

I've read everything from "don't ever use extension cords with woodworking
machines" to "14 gauge, 50 ft cord is fine."

The dust collector says that its max draw is 18 amps. I can't find any
extension cord, no matter what gauge, that says it is rated for more than
15 amps. That seems odd to me since an extension cord doesn't seem
fundamentally different than Romex to me.

Here's what I was thinking
* dust collector - 10 gauge, 10 foot extension cord
* band saw - 12 gauge, 10 foot extension cord
* jointer - 12 gauge, 10 foot extension cord

I was going to get 10 gauge for all of them, but the 10 is twice as much
as the 12 and I'm not convinced I need the extra weight.

Any thoughts?

Mark