EXHAUST FAN LOAD

[mstrlucky74]

Trying to figure out the load of an exhaust fan. It is fed from a 3 phase 480v panel. It has a 3p-40A breaker and the panel schedule says the KVA connected and demand is 17.5 but they are calling for 4#10'a and the run is 350'. Seems impossible to run #10's. Correct? Thanks.

 

[iwire]

Seems fine, I come up with less than 2% voltage drop.

Votage drop: 5.94
Votage drop percentage: 1.24%
Votage at the end: 474.06

http://www.calculator.net/voltage-d...nce=350&distanceunit=feet&amperes=17&x=47&y=3

 

[mstrlucky74]

Seems fine, I come up with less than 2% voltage drop.





http://www.calculator.net/voltage-d...nce=350&distanceunit=feet&amperes=17&x=47&y=3

Thanks... How do you get 17 amp load?

 

[Strathead]

Thanks... How do you get 17 amp load?

He oops it. It is actually 21 amps 17500/1.73/480 But even at 21 amps the VD would be small enough.

 

[mstrlucky74]

He oops it. It is actually 21 amps 17500/1.73/480 But even at 21 amps the VD would be small enough.

1.73 represents what? Thanks.

And why are they using a 40a breaker

 

[Strathead]

1.73 represents what? Thanks.

And why are they using a 40a breaker

It represents the square root of three. Basically three phase power has advantages over three separate feeds. If you have a 10 amp reading on phase A 10 amp on phase B and 10 amp on phase C. the power used is not the addition of these, or 30 amps, because some of the power is shared. (not really the scientific explanation, but easier on the brain for our discussion. That factor is the square root of three. The proof of that is a bit deeper so you will have to take my word for it or, believe me some of the brains here will boggle your mind with the nuance.

40 A breaker is table 430.52 which states 250% of the full load current is an acceptable breaker size. See for motors, wire is sized for the FLA times 125% for continuous duty. Then the breaker is sized for the table. This is why the "rule of thumb" that a #12 wire is a 20A maximum circuit is not correct!

 

[mstrlucky74]

It represents the square root of three. Basically three phase power has advantages over three separate feeds. If you have a 10 amp reading on phase A 10 amp on phase B and 10 amp on phase C. the power used is not the addition of these, or 30 amps, because some of the power is shared. (not really the scientific explanation, but easier on the brain for our discussion. That factor is the square root of three. The proof of that is a bit deeper so you will have to take my word for it or, believe me some of the brains here will boggle your mind with the nuance.

40 A breaker is table 430.52 which states 250% of the full load current is an acceptable breaker size. See for motors, wire is sized for the FLA times 125% for continuous duty. Then the breaker is sized for the table. This is why the "rule of thumb" that a #12 wire is a 20A maximum circuit is not correct!

A field guy is saying you can't have #10's and a 40a breaker:?

 

[Strathead]

A field guy is saying you can't have #10's and a 40a breaker:?

He is wrong. Have him prove it. I thought is was odd when I first discovered this. It was actually a Mike Holt lesson in ECM I think that pointed it out. I am betting one of the moderators can provide you with the entire lesson.

 

[mstrlucky74]

He is wrong. Have him prove it. I thought is was odd when I first discovered this. It was actually a Mike Holt lesson in ECM I think that pointed it out. I am betting one of the moderators can provide you with the entire lesson.

But if you don't mind can you break it down a little more why the 40a breaker is okay. I have the code book in front of me but just could not follow and arrive at the calculation. I would like to be able to explain it. Just a quick calculation for the 40a breaker. Thanks a lot.

 

[Strathead]

But if you don't mind can you break it down a little more why the 40a breaker is okay. I have the code book in front of me but just could not follow and arrive at the calculation. I would like to be able to explain it. Just a quick calculation for the 40a breaker. Thanks a lot.

There are other guys here who can probably reel it right off their heads. One of them surely will soon I hope. I don't do this much, but it is basically like I told you. I can't readily find the section for the size of a motor wire, but from memory it has to be sized per the full load current of the motor as per the name plate or tables 430.247 to 430.251 times 125% continuous rating. Once you size the wire, you then size the overcurrent protection per 430.52. It is as simple as that. You will find nothing in the code book that specifically prohibits the wire from being protected this way. I assume that you are hung up on the tables 310.15, but they are the allowable ampacities not the allowable breaker sizes. Code section 240.4 (G) is the section that specifically allows you to size the OC the way I described above.

Hope this helps or someone better at this than me chimes in.

 

[mstrlucky74]

There are other guys here who can probably reel it right off their heads. One of them surely will soon I hope. I don't do this much, but it is basically like I told you. I can't readily find the section for the size of a motor wire, but from memory it has to be sized per the full load current of the motor as per the name plate or tables 430.247 to 430.251 times 125% continuous rating. Once you size the wire, you then size the overcurrent protection per 430.52. It is as simple as that. You will find nothing in the code book that specifically prohibits the wire from being protected this way. I assume that you are hung up on the tables 310.15, but they are the allowable ampacities not the allowable breaker sizes. Code section 240.4 (G) is the section that specifically allows you to size the OC the way I described above.

Hope this helps or someone better at this than me chimes in.

Ok, thanks.

 

[GoldDigger]

But if you don't mind can you break it down a little more why the 40a breaker is okay. I have the code book in front of me but just could not follow and arrive at the calculation. I would like to be able to explain it. Just a quick calculation for the 40a breaker. Thanks a lot.

The protection for the wire is coming from two different directions:
The motor overloads will keep the current from exceeding the capacity of the #10 (or whatever). This protection is provided at the load end of the wire run.
The 40 amp breaker is there to protect against shorts in the wiring, and will protect the #10 wire just fine since any short at all will draw more than the trip point of the 40 amp breaker.
At least that is how Mike Holt explained it in the video. If you search the forum you will probably be able to find the link to the video. It was within the last month or so.

Found it: http://forums.mikeholt.com/showthread.php?t=152085&p=1470297&highlight=#post1470297

 

[mstrlucky74]

The protection for the wire is coming from two different directions:
The motor overloads will keep the current from exceeding the capacity of the #10 (or whatever). This protection is provided at the load end of the wire run.
The 40 amp breaker is there to protect against shorts in the wiring, and will protect the #10 wire just fine since any short at all will draw more than the trip point of the 40 amp breaker.
At least that is how Mike Holt explained it in the video. If you search the forum you will probably be able to find the link to the video. It was within the last month or so.

Found it: http://forums.mikeholt.com/showthread.php?t=152085&p=1470297&highlight=#post1470297

This is for ac units though

 

[GoldDigger]

This is for ac units though

The same principle applies to any motor load for which the manufacturer states a maximum OCPD size in their installation instructions.
If the manufacturer just gives you the motor nameplate figures, then you have to justify it using the various tables as mentioned earlier. But the principle remains the same IF there is overload protection at the motor.