100 amp branch circuit wire size

[boofit]

On a commercial project what size wire would be required for a 100 amp breaker on a branch circuit for a future air compressor. Ran in conduit and pulling thhn. # 2 Thhn is rated at 95a on 60 degree but 115 at 75 degree. I was under the impression that the 75 degree chart was used since it is THHN however I've been told it may need to be #1 for continuous duty. LIttle help would be appreciated.

 

[augie47]

110.14 shows using the 60° rating for installs 100 amp or less UNLESS the equipment terminations are rated 75° which is likely the case in your situation so #2 is probanly ok with its 75° rating of 115 amps.
The compressor normally would not be considered a continuous load but motor branch circuits should be 125% of the HP Table rating so the devil is in the details here.
Can you give us more info on the compressor such as nameplate data or HP.

 

[charlie b]

(I see that Gus posted while I was typing. But I will post anyway.)

First, you are looking at this backwards. How do you know that a 100 amp circuit will be sufficient for the air compressor?

Secondly, I doubt an air compressor will be operated at its maximum load for 3 hours or more at a time, without pausing. So it won't be a "continuous load."

Next, THHN can handle a temperature increase of 90C over an ambient of 30C (the value on which the table is based). But the point at which you land the wire (i.e., the termination) likely cannot. That is why we use the 75C column, in that most of the terminations available today are rated for 75C.

 

[boofit]

110.14 shows using the 60° rating for installs 100 amp or less UNLESS the equipment terminations are rated 75° which is likely the case in your situation so #2 is probanly ok with its 75° rating of 115 amps.
The compressor normally would not be considered a continuous load but motor branch circuits should be 125% of the HP Table rating so the devil is in the details here.
Can you give us more info on the compressor such as nameplate data or HP.

Unfortunately it's a circuit for a future air compressor and all I was told was to install a 100 amp circuit in a j box. However the # 2 vs #1 means the difference between 1 inch EMT and 1.25 EMT. If it wasnt for that I'd just pull the #1 and call it good. May just call the inspector in the AM.

 

[boofit]

(I see that Gus posted while I was typing. But I will post anyway.)

First, you are looking at this backwards. How do you know that a 100 amp circuit will be sufficient for the air compressor?

Secondly, I doubt an air compressor will be operated at its maximum load for 3 hours or more at a time, without pausing. So it won't be a "continuous load."

Next, THHN can handle a temperature increase of 90C over an ambient of 30C (the value on which the table is based). But the point at which you land the wire (i.e., the termination) likely cannot. That is why we use the 75C column, in that most of the terminations available today are rated for 75C.

1) because that's what size circuit was requested to be installed for future
2)I agree in that as well
3)75 degree is what I was assuming based on lugs as the 90 degree is for thhn.
The fact that it's a motor as Gus mentioned is my concern. If I could take it at 90 degrees based on thhn I'm sure that #2 would be big enough. But like you said at 75 degree it would need to be 125 amp to allow for the motor connection.

 

[augie47]

By Code a branch circuit size is dictated by the size of the over-current device.
You were told to install a "100 amp branch circuit" and you did so.
In a more perfect world you would have been given load data.

 

[boofit]

By Code a branch circuit size is dictated by the size of the over-current device.
You were told to install a "100 amp branch circuit" and you did so.
In a more perfect world you would have been given load data.

I agree however wanting to take care of the customers needs for future. Sometimes being the nice guy is hard lol

 

[infinity]

Since you only need 100 amps #3 THHN will work.

 

[retirede]

Secondly, I doubt an air compressor will be operated at its maximum load for 3 hours or more at a time, without pausing. So it won't be a "continuous load."

That can’t be said without knowing the application. I’ve been involved in many applications where the compressor(s) ran fully loaded 24/7.

If it’s a single, general purpose plant air application, you’re likely correct.

 

[charlie b]

I’ve been involved in many applications where the compressor(s) ran fully loaded 24/7.

There's not much the electrical world can do about that. But it is certainly a failure on the mechanical design to require a compressor to have that operational requirement.

 

[retirede]

There's not much the electrical world can do about that. But it is certainly a failure on the mechanical design to require a compressor to have that operational requirement.

There are applications that will simply take all the air they can get.

In other cases, a bank of compressors may be installed where all but one run at 100% load continuously. It’s more efficient than one large unit running partially loaded.