5 Ton A/C Unit-- wow

[Dennis Alwon]

I just read this from another site and could not believe the response the person got from the electricians who run the site.

This is the response from a question that asked what size wire for a 5 ton unit 135' away with a mca of 33.9 amps. Residential thru a hot attic of say 122? max. I guess a 2/0 would work also... I think a #8 would work am I nuts??? Don't answer that. :lol:

Typically, 6/3 would be used in this installation scenario. Technically, at 135' you would have a 33.9 ampacity rating and still be within the recommended 3% voltage drop guideline utilizing a #8 conductor, but at 136' you would need a #6 so we'll just say #6 for all intents and purposes.

What I'm about to say next is topic of debate, but it's a topic I insist on taking into consideration on all of my jobs. The ambient temperature of the attic is often overlooked when calculating the circuit ampacity. Table 310.16 says that #8 NM is rated at 55amps but when you make the adjustment for an ambient temperature of 114-122 degrees (which is a very reasonable assumption for a typical attic):

55 x .58 = 31.9

You can see that this circuit should only be rated at 31.9 amps. For this reason I must recommend and would install 4/3 here.

70 X .58 = 40.6

As you can see, #4 would adequately meet the ampacity requirements for your circuit, with the ambient temperature of the attic taken into consideration.


Read more: Im haveing a 5 ton a/c system installed and will have to run - JustAnswer http://www.justanswer.com/electrica...-system-installed-will-run.html#ixzz1wA33I7ae​

 

[Dennis Alwon]

Sorry I had HP-- changed it to Tons.

 

[stickboy1375]

I just read this from another site and could not believe the response the person got from the electricians who run the site.

This is the response from a question that asked what size wire for a 5 ton unit 135' away with a mca of 33.9 amps. Residential thru a hot attic of say 122? max. I guess a 2/0 would work also... I think a #8 would work am I nuts??? Don't answer that. :lol:

How did he come up with 6-3 as his very first wire selection?

never mind, he started with 8, then went to 6 for voltage drop...

 

[stickboy1375]

Dennis, i'm with you, i would run number 8AWG all day long.

 

[kwired]

The guy has right ideas but is not consistant.

He says 8 NM is good for 55 amps which is 90?C value and correct to use for starting point for deration. I don't have a clue where he came up with .58 multiplier for ambient temperature correction IMO it should have been .89 for the temperature he used.

He then skips over possibility of using 6 AWG and then derates a 4 AWG using the 60? ampacity value:?

 

[Gac66610]

thats an old question ...
i would probably run a #6 but i have been accused of 'over wiring a house'

if it could be installed under the insulation the ambient temp would not be as high
been in a lot of attics after insulation, its always cooler under the blanket of insulation

 

[augie47]

Looks to me like he's applying ambient correction to the 60? rating rather than the 90?.
At that distance, with a motor involved i might install a #6

 

[kwired]

Looks to me like he's applying ambient correction to the 60? rating rather than the 90?.
At that distance, with a motor involved i might install a #6

He is using 90? for one conductor and 60? for the other, as well as the wrong adjustment factor for the temperature he is adjusting to.

 

[Dennis Alwon]

I saw the flip flopping. Using 60C for derating etc. Totally off base with his recommendation, IMO

 

[stickboy1375]

I saw the flip flopping. Using 60C for derating etc. Totally off base with his recommendation, IMO

Does anyone derate for attic wiring? And if so, do they take an average of the summer temps or highest temp possible?


Location will reflect answers I'm sure.

 

[Dennis Alwon]

Does anyone derate for attic wiring? And if so, do they take an average of the summer temps or highest temp possible?


Location will reflect answers I'm sure.

I have not known anyone that derates for temp in an attic for residential but it probably should be done. Of course if you can satisfy 310.15(A)(2) then it is not an issue.

 

[kwired]

Does anyone derate for attic wiring? And if so, do they take an average of the summer temps or highest temp possible?


Location will reflect answers I'm sure.

I think one should derate if there is an increased ambient temperature. Determining what that ambient should be could be difficult. Not every attic will be same. Type of roofing makes a difference, construction of roof makes a difference, type of ventilation makes a difference. If conductors are installed within/below thermal insulation they are not subject to same temperature as if above insulation.

 

[Dennis Alwon]

I think one should derate if there is an increased ambient temperature. Determining what that ambient should be could be difficult. Not every attic will be same. Type of roofing makes a difference, construction of roof makes a difference, type of ventilation makes a difference. If conductors are installed within/below thermal insulation they are not subject to same temperature as if above insulation.

So which one would you say is hotter above the thermal insulation or above it? I would think it is hotter above yet we must derate se cable to 60C when it is in insulation but not necessarily above it.

 

[infinity]

I want to know why an AC unit needs 3 conductor cable.:roll:

 

[Dennis Alwon]

I want to know why an AC unit needs 3 conductor cable.:roll:

If you read the rest of the dialogue he admitted he made a mistake. The entire conversation is at the bottom of the quote I posted.

 

[ActionDave]

I saw the flip flopping. Using 60C for derating etc. Totally off base with his recommendation, IMO

Whatta ya bet this expert gains credibility with the DIYers because of his convoluted reasoning. DIYers always like to think they have honed in on the one exclusive bit of info that the rest of the world doesn't pay attention to.

 

[stickboy1375]

Whatta ya bet this expert gains credibility with the DIYers because of his convoluted reasoning. DIYers always like to think they have honed in on the one exclusive bit of info that the rest of the world doesn't pay attention to.

As much as I love being a DIY person, some people are WAY over before they even begin.... so any help is useless from the get go.

 

[Dennis Alwon]

To be fair he had the right ideas but either got mixed up with the de rating column or truly believed he had to use the 60C column. 110.14(C) should clarify it. I did write them a response but the question was posted 3 years ago. They did not respond to me.

 

[stickboy1375]

To be fair he had the right ideas but either got mixed up with the de rating column or truly believed he had to use the 60C column. 110.14(C) should clarify it. I did write them a response but the question was posted 3 years ago. They did not respond to me.

I wonder if somewhere a lonely #4 wire is strung in an attic? lol....

 

[kwired]

So which one would you say is hotter above the thermal insulation or above it? I would think it is hotter above yet we must derate se cable to 60C when it is in insulation but not necessarily above it.

take your pick both are above

Seriously, it really depends. If conductor is loaded to 75? ampacity it will be hotter than a a conductor loaded to 60? ampacity. Properties of the insulation in either case will make a difference of how much heat remains in the vicinity of the conductor. How much heat is above the insulation can vary because of many conditions.

I want to know why an AC unit needs 3 conductor cable.:roll:

Because the AC unit is a three phase unit