Sizing conductors for AC unit

[ronnie]

What size NM cable is required for a 19.1 mca AC unit that is 90' from the main panel. The cable will be installed in the attic where the temperature in the summer would reach 123-140 degrees F.

 

[walkerj]

10-2 should get ya there

 

[bphgravity]

1. Section 440.32

2. Section 334.80

3. Table 310.16

 

[Dennis Alwon]

1. Section 440.32

2. Section 334.80

3. Table 310.16

I would like to add article 240.4 which allows the asterik in Table 310.16 to be used.

If you consider the attic ambient to be 145F then we have to derate .58,
However there is much debate over what ambient temp. means. Is it the high temp of the year for one hour of the entire year or the temp. average for the hottest days, or the average for the summer months, or the average for the year.

We certainly know it is not 145F all day during the summer but it will peak there. I would love to know the answer to this question because I know that it is rarely considered on residential application. I don't believe I ever considered it except for the placement of transformers in the attic.

Anyway with VD and not using ambient I would say 12/2nm is good, however I would pull a 10/2.

 

[infinity]

Without the ambient adjustment factor you could #14 NM cable for this unit.

With the 145 degree adjustment factor you would need #10 NM cable.

If you could actually answer Dennis's question, maybe you could use something in between.

 

[Dennis Alwon]

Without the ambient adjustment factor you could #14 NM cable for this unit.

This is true but I did not want to suggest that especially with the VD on 90. I calculated that and it would have to be a #12. I know VD is not code but it would make sense to consider that on an a/c unit.

 

[infinity]

This is true but I did not want to suggest that especially with the VD on 90. I calculated that and it would have to be a #12. I know VD is not code but it would make sense to consider that on an a/c unit.

Good point, I left voltage drop out of my answer. Good design would not use #14 on this circuit.

 

[bphgravity]

Our jursidiction, and likely most others in Florida will begin using the ASHRAE Handbook - Fundamentals to establish area ambient temeperature.

 

[Dennis Alwon]

Our jursidiction, and likely most others in Florida will begin using the ASHRAE Handbook - Fundamentals to establish area ambient temeperature.

Can you expand on that or is it to involved?

 

[bphgravity]

See the FPN to Section 310.15(B)(2)(c) of the 2008 NEC. There has never really been an NFPA established or recommended guide for determining ambient temperatures in any one particular location.

According to the Copper Development Association, the CMP's accepted this criteria for establishing some of the new temperature correction / derating requirements found in the 2008 NEC.

The temperatures provided are based on a 30 year collection averaging the months of June through August.

The handbook can be obtained here: www.ashrae.org

A pocket guide with the same information can be obtained here: www.copper.org

The CDA has a really good presentation on this issue as they were the driving force behind many of the code changes in 2008 NEC. I suggest you contact them or your local IAEI division to see if they can arrange a seminar with them. They will do it for free!

 

[tallguy]

Here's a link to the actual table so you don't have to hunt for it. It's a bit odd though... has locations listed like Mount Washington... not sure that's useful to anyone other than the little weather observatory hut that sits on top of it.

 

[Dennis Alwon]

Here's a link to the actual table so you don't have to hunt for it. It's a bit odd though... has locations listed like Mount Washington... not sure that's useful to anyone other than the little weather observatory hut that sits on top of it.

Those tables are for wires in direct sunlight in conduit not for NM cable in an attic. Is there a table for attic ambient temp. I did not find one.

 

[tallguy]

Is there a table for attic ambient temp. I did not find one.

I couldn't find one either.

 

[bphgravity]

We will assume the 2% ambient temperature as provided in the CDA report.

 

[kbsparky]

In most residential applications, you would be dealing with a nominal system voltage of 240. If this is the case, then voltage drop would not have to be considered at all.

If you look at the nameplate on the compressor unit, I'd be willing to bet that its rated supply voltage is anywhere from 197-250. You'd have to have one heck of a voltage drop to fall below that value.

 

[George Stolz]

It's cool this came up, I just noticed the new 310.15(B)(2)(c) a couple days ago. The last I'd seen I thought it had been shot down in the ROP stage, looks like it ended up making it.

So, looking at the table linked to above:
View attachment 902

It appears to me that I will not be able to put three #12 conductors on a roof on a 20A breaker at all, ey?

Edit:

Actually, going through the numbers, it actually appears that small MWBC's would be virtually uneffected by this. Over 1/2" away from the roof, 90? conductors will still be good enough for the values in 240.4(D), I think...