Power mains question: wire gauge

[Cydrome Leader]

Unbelievable. With the housing market in the tank and commodities
through the roof, what the housing market really needs *more* help
from UncleS. <sheesh>

it's a tough call as to what sucks more, mexi-narco pipes, chinese copper
plated lead or the government meddling.

 

[John S]

---
From the OP's example we have a 240VAC 1/2HP motor with a startup
current of 15A being fed through 200 feet of #14 AWG wire.

1HP is 746 watts, so if his motor delivers 1/2HP at the shaft and it's
80% efficient it'll be taking about 467 watts from the mains under
full load.

True, but the current will be higher because of power factor.

200 feet of #14 solid copper looks like about half an ohm, so we have,
for the startup case:

15A--->
240AC>---[0.25R]---+ <------+
| |
[16R] 233V
| |
240AC>---[0.25R]---+ <------+

Because of the wire resistance, instead of 15A in the circuit we'll
have 14.55A, and that will drop 233V across the motor.

233V is 97% of 240V, so there's only a 3% loss in the wire.

In the full load case we have this:

1.94A--->
240AC>---[0.25R]---+ <------+
| |
[123R] 239V
| |
240AC>---[0.25R]---+ <------+

so the loss is less than half a percent.

But, that was not what I was curious about. What I want to know is, if
the running current results in a 3% drop, doesn't that result in a 18%
drop when the current goes up 6 times?

 

[rickman]

Agreed, sorta. PVC isn't too bad, though. This whole thing has
gotten absurd.

Really? S.E.D having an absurd conversation? What's your point?

Rick

 

[rickman]

You guys are all pretty ridiculous calling each other names and such,
but at least you know how to trim a post. Thanks. It makes it easier
to get through this crap.

Rick

 

[josephkk]

t at HD. #10 is "special" and

Oh, look! Grainger wants $141.10 for 250' of 12/2, TWICE what HD gets
for it.

It is really strange person that thinks that Grainger or McMaster-Carr are
the low price outfits. Their prices resemble Radio Schmuck versus online.

?-/

 

[josephkk]

That was two years ago. Tell me what you paid last week.


The point stands. Cost isn't just material. It's volume, as well.
Expensive commodities sitting on the shelf cost more than those that
fly off the shelf.


Get real, Michael. You lost.

Tough chips Keith, Michael is right on this one.

?-)

 

[DaveC]

Why not put the compressor where the power is, and run an air hose?

Then we could argue about ths size of the hose.

That's the best idea I've heard yet.

Sadly, he had already located the compressor, installed the air lines, hooked
it up before calling me to wire it. He placed it right next to the main
air-user, the mechanic's bays. (It's an auto repair shop.)

Dave (OP)

 

[josephkk]

I'm curious about this. Doesn't a motor draw about 6-10 times FLA during
start? If so, wouldn't that cause a 18-30% drop during start if you have
a 3% drop normally? Or, looking at it the other way, if you size for 8%
starting drop, your normal operating drop would be only 1.3%.

It depends on several variables, including motor size, motor winding
class, number of phases, design of motor start system, and some other
things. Very small motors may start at 2x FLA, some very large (full
voltage start) motors may require 20 to 30 x FLA.

?-)

 

[[email protected]]

Tough chips Keith, Michael is right on this one.

Bullshit.

 

[[email protected]]

It is really strange person that thinks that Grainger or McMaster-Carr are
the low price outfits. Their prices resemble Radio Schmuck versus online.

So show us your numbers instead of more diarrhea of the mouth, as is
your normal MO.

 

[[email protected]]

You guys are all pretty ridiculous calling each other names and such,
but at least you know how to trim a post. Thanks. It makes it easier
to get through this crap.

Buy a scroll wheel.

 

[Jasen Betts]

Buy a scroll wheel.

They don't make them that that go fast enough to deal with the sewage
you produce

 

[[email protected]]

They don't make them that that go fast enough to deal with the sewage
you produce

You lefties are always projecting your inadequacies.

 

[rickman]

You're fuckin' whiner, just like a lefty.

At least your rants are short (I mean all of you). Thanks, makes it
easier to breeze through the muck.

Rick

 

[[email protected]]

At least your rants are short (I mean all of you). Thanks, makes it
easier to breeze through the muck.

Instead of incessantly whining, you could just ignore me. No, you
lefties are too stupid to do that.

 

[Tom Horne]

Of course not.  Michael is being silly with his suggestion of running
#6 wire for the compressor.  I thought I'd join in.  For anything I've
done, 1/2" Type-L copper is fine.  That's probably what I'll use again
but it's getting rather pricey.

Well I'm not interested in being quarrelsome but would you mind
explaining why you would use copper piping for air lines. Copper has
gotten more pricy rather than recently pricey so it would seem to be
rather a waste to use it for air. Help me out here. What am I
missing?

 

[Tom Horne]

1HP is 746 watts, so if his motor delivers 1/2HP at the shaft and it's
80% efficient it'll be taking about 467 watts from the mains under
full load.

200 feet of #14 solid copper looks like about half an ohm, so we have,
for the startup case:

         15A--->
240AC>---[0.25R]---+ <------+
                  |        |
                [16R]     233V
                  |        |
240AC>---[0.25R]---+ <------+

Because of the wire resistance, instead of 15A in the circuit we'll
have 14.55A, and that will drop 233V across the motor.

233V is 97% of 240V, so there's only a 3% loss in the wire.

In the full load case we have this:

         1.94A--->
240AC>---[0.25R]---+ <------+
                  |        |
                [123R]    239V
                  |        |
240AC>---[0.25R]---+ <------+

so the loss is less than half a percent.

  That is right on the mark.

  It was always fun to un a 100 foot extension cord to a construction
work area and run a device like a circular saw or such.  Then try it with
two 100 foot cords together for 200 feet.  It (the difference) is very
noticeable.

  Most of those cords are #14 at those lengths (or they'd better be), but
at the price difference for raw wire and a buried run, I'd say that
dropping to #12 or eve the #10 run was not an improper suggestion,
considering it was going to be a permanent install, and the device being
powered may not be, and wiring a service outlet is what we are doing.

  So, yes, as you illustrate, #14 is sufficient, even good, considering
what I have seen houses and such wired with.

  At 200 feet away, and not knowing if the device will be the only item
being powered, I would at least examine the cost tables for the job using
#10, #12 and #14 Thhn, no conduit.

  As far as the conduit goes though, if one gets small diameter PVC, and
all the fittings, no 200 foot pull needs to be done.  The segments and
fittings can all be fed down the wire bundle and build the conduit in
place right on the wires, then bury the finished run.

In North American Practice the electrical codes forbid assembling
conduit around wire. The conduit must be built as a complete assembly
and then you pull the wire from pull point to pull point.

 

[[email protected]]

Well I'm not interested in being quarrelsome but would you mind
explaining why you would use copper piping for air lines. Copper has
gotten more pricy rather than recently pricey so it would seem to be
rather a waste to use it for air. Help me out here. What am I
missing?

A reasonable alternative.

 

[Tom Horne]

A reasonable alternative.

I've seen the pipe fitters install hundreds of feet of black iron on
jobs with a lot of air in use such as assembly lines. What is wrong
with that? Is it too expensive to install to be cost effective? I
have been out of the big job work for over a decade now.

 

[[email protected]]

I've seen the pipe fitters install hundreds of feet of black iron on
jobs with a lot of air in use such as assembly lines. What is wrong
with that? Is it too expensive to install to be cost effective? I
have been out of the big job work for over a decade now.

Black iron is incredibly expensive and impossible to work with.