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Power Cable to Air Conditioner

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LAN_403

Part Scavenger

Mon May 21 2007, 03:36AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Hey guys, we're putting in a new air conditioner. I'm pretty sure this is an easy one, but I'm not sure I've got it right.

We need a power cable for 220V @ 60A for 30-40ft. All calculations below assume 35ft.

suggests we need #5...

This calculator

suggests #10 and so does this one

and this one

Several other sites suggested #6 and #8, and the thing on the wall @ Home Depot suggested #4!

#10 seems alot small to me, #4 is really expensive. #6 is where power dissipation meets price on the Excel sheet, so I was planning on suggesting that to dad.

#6 will dissipate 120W by the calculators, by hand using a ohm/ft table, 50W
#8 will dissipate 192W by the calculators, by hand 80W
#10 will dissipate 186W by the calculators, by hand 125W

Can anyone tell me how you're supposed to calculate this? I like equations and 'splanations, but there seems to be a large variety. Why are my volt drop comparasons off?

This is our air conditioner, it will be running alot, so it needs to be efficient; but then again, we like to be frugal...but not to the point where we burn the house down! LOL My "have to know" question is:

How do I figure out what size the wire HAS/NEEDS to be? I can figure out the efficiency mess later.

...

Mon May 21 2007, 04:25AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I am quite familiar with this stuff, since we literally put in over a mile of wire in my house over the last month

Everything from 24awg communications wire to 000awg (4 sizes bigger than a #2) main service...

In any case, there are 2 things to consider when sizing wire. 1. make sure the wire doesn't melt. 2. make sure there is enough voltage at the far end of the wire to run the device.

For #1, for 60a you need a #4 wire.
For #2, a 3% drop is allowed, which at 220v is 6v. For 4awg wire at 35ft you only have .6v of drop, so you are fine there.

Likewise, you need a 4awg wire.

You could get away with a #6 if you use 75c rated wire (most is) and all of the junction boxes are rated for 75c. But if there is a single component in the system that is not rated for 75c, (most servicee panels are not) then you need the #4. But then again, who is checking...

Steve Conner

Mon May 21 2007, 08:42AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The thermal rating for the wire assumes the AC unit actually draws 60A all the time. Does it? I'd suspect that would be more of a peak figure for starting. After all, a continuous draw of 60A is 13kVA which would be something like 10 tons of refrigeration.

If you don't actually have a 10 ton air conditioner, 6AWG would surely be plenty.

Marko

Mon May 21 2007, 12:02PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

We need a power cable for 220V @ 60A for 30-40ft. All calculations below assume 35ft.

13kW compressor?

Are you sure you haven't mixed pumped heat with electric power input, because unit you are describing would be size of a car.

And your power bill would quickly turn unhappy.

IamSmooth

Mon May 21 2007, 12:23PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

What type of air handler do you have. Are you talking about a 5-ton unit? You can read the starting/running current off of the datasheet or you can just measure it with an analogue current meter. The starting current spike is so transient a digital meter will miss it. I measured mine when I was putting in a hard-start capacitor for my generator. I wanted to see the reduction of starting current. Starting current (without the cap) was 80A and the running current is 10A @220v for my unit.

Nik

Mon May 21 2007, 04:37PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Ill check my ontario electrical code book (the ontario book has more restrictions then the canadain book and should pass in most places for wire ampacity).

For TW wire (60degree C rated) its #4
For TW75 and RW75 its #6
For R90 RW90 and T90 its #6

Any wire rated for a higher temperature rating then 90degrees C its #8 wire.

So it depends on the wire you choose and what its temperature rating is. Hope I helped.

Part Scavenger

Tue May 22 2007, 01:18AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

I talked to my dad and he said that the reason we are getting a 60A line is because more than two air conditioner guys told us to run a 60A line to the air conditioners. Something about the new one would have heat strips or something. He was pretty vague about the whole thing.

So let me see if I've got this straight. I did all the calculations by hand doing the 3% from the main branch/ 5% from the source volt-drop NEC standards and I came out with what the calculators were telling me. 10-12 AWG is acceptable within these guidelines. Oh my gosh! THAT IS if you don't mind your house burning down because the insulation overheats?

Therefore, these calculators are basically useless? Because then, I looked at the insulation ratings and #4 or #6 is what I need to get and the watt dissipation, and #4 is the way to go. Just like you guys were telling me! Thanks alot, you really helped.

Marko

Tue May 22 2007, 11:28AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I talked to my dad and he said that the reason we are getting a 60A line is because more than two air conditioner guys told us to run a 60A line to the air conditioners. Something about the new one would have heat strips or something. He was pretty vague about the whole thing.

Do you know anything about the particular conditioner?

At least give us some idea of it's size, is it for a home, hospital, etc?

Just some basic information would tell if dragging monstrous 60A cable would really be worth or silly.

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