AC wire sizing

[binney]

I'm looking @ my copy of Mike Holt's "Electrical Formulas with Sample Calculations" book. And one of the examples has me puzzled.

Page 78 for those who have the book.

Here is how it reads.

Q. An air conditioner nameplate indicates the minimum circuit ampacity of 23A and a maximum fuse size of 40A. What is the minimum size branch-circuit conductor and the maximum size overcurrent protection device 75 degree C.

A. 12awg, 40A fuse

Conductor: The conductors must be sized based on the 60 degree C column of table 310.16 and 12 awg rated 25A.

Overcurrent protection:The overcurrent protection device must not be greater than a 40A fuse, either one-time or dual element.

My question is, if the terminals are rated for 75 degrees C why do we need to use the 60 degree table. In this particular example it wouldn't matter #12 is good for 25A in both the 75 & 60 degree columns.

Where does the 60 degree requirement come from?

 

[iwire]

Re: AC wire sizing

Was the example NM cable?

334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.

 

[websparky]

Re: AC wire sizing

How about 110.14 (C)(1)?

 

[roger]

Re: AC wire sizing

Hello Dave, the only problem with 110.14(C)(1) is, per the question, it gives us 75 deg C covering the questions componets.

Roger

 

[binney]

Re: AC wire sizing

the example doesn't state the type of wiring method, buth the picture shows a cut-away picture with a conduit coming in from the top

 

[binney]

Re: AC wire sizing

110.14(C)(1)(B)unless the equipment is listed and mrked otherwise

the example stated 75 degree terminals

 

[binney]

Re: AC wire sizing

good point with the NM cable 334.80, i've never seen that before, I was told years ago that the wire inside the NM cable was THHN and always used it's 90 degree rating. Sometimes we assume are mentors were always right.


The picture clearly shows a conduit.

 

[jimwalker]

Re: AC wire sizing

I would not sleep well if i didn't give that unit 10-2 and fuse at 30 or 40. I know 12-2 is ok but unless my boss told me to use 12-2 it just won't happen.How many others feel this way ?

 

[binney]

Re: AC wire sizing

If the condeser is close to the panel I run a 12-2 with no problem. If it's on the other end of the house I run a 10-2. I always use the Maximum OCPD

 

[iwire]

Re: AC wire sizing

We all understand it would be impossible to overload the conductor in this application even if on a 40 amp breaker?

Voltage drop is another issue that should be considered.

 

[binney]

Re: AC wire sizing

I was always told up 1 wire size for every 100'. Seing the AC has such a large draw at start up, I cut that # down quite a bit. Nobody like there lights dimming evertime the AC kicks in.

 

[binney]

Re: AC wire sizing

I was always told up 1 wire size for every 100'. Seing the AC has such a large draw at start up, I cut that # down quite a bit. Nobody like there lights dimming evertime the AC kicks in.

 

[jimwalker]

Re: AC wire sizing

Not to many areas that i will spend the extra few dollars but on something like this and the lights don't flicker i think it's worth it.Basically i was taught to wire as cheap as i can, often it's that $100 that im cheaper by that lands the contract.A GC looks at it as a $100 from every trade ads up and suppers usually get bonus on what they can save over what was in budget.

 

[roger]

Re: AC wire sizing

Jim,

I would not sleep well if i didn't give that unit 10-2 and fuse at 30 or 40.

please give the reason as to why this would bother you so bad?

Roger

 

[jimwalker]

Re: AC wire sizing

If that run was a 100 feet on 12-2 and fused at 40
it dam sure will have a voltage drop.And if the wire itself started to short out from maybe a staple that was too tight it might catch fire.Unlikely but is a risk.It's just a personal issue with me that if there is any chance of a fire or shock and it is that easy to solve i do it.Have never had any fires and only 2 times has anyone got shocked do to my wiring.One was from defective 3 prong range recepticle (ground terminal was defective) and another over a local rule to leave panel cover off for final inspection (they no longer ask for that )

 

[roger]

Re: AC wire sizing

Jim, 12/2 shorted with a staple at 100' would be a fault of 622 amps, wouldn't you think this would open the OCPD?

Roger

 

[websparky]

Re: AC wire sizing

Hi Guys,

OK, you can use 12AWG with a 40A fuse.

This would require a 60A disconnect at the unit, right?

 

[jimwalker]

Re: AC wire sizing

Maybe im over kill on this ,but never said the staple made good contact.
Pretty much always use that $10 pullout 60 amp disc.Cheaper than 2 pole 30 switch with bellbox and cover.

 

[iwire]

Re: AC wire sizing

No, minimum circuit ampacity of 23 amps is easily handled by a 30 amp disconnect.

440.12 asks for 115% of name plate rated current.

 

[jimwalker]

Re: AC wire sizing

Any cost differance ? have always just used the 60