440 volt 40 hp motor wire size

[dm9289]

I need to replace some wiring that was burnt due to a fault and over fusing and I had a couple of questions on how to arrrive at the wire size.

The motor is 440v 3phase 40hp. Its FLA is 52A. It is in an envoronment that can get as hot as 125 degrees. so I was going to use THHW or THHN.

First do I have to make the wire capability 125% of the load it is a continuosly running motor? that would make the amperage 65A?

I also thought for temperature I would have to derate the wires ampacity by multiplying it by .76 for a correction factor?

Here is where it gets even trickier the wire runns in conduit for about 70ft with 5 wires total, 3 for the 440v and 2 for control power 120v for a door switch. Would this be considered 5 current carrying conductors cut it down to 80%?

Tricky part 2 the last 30 ft runs in an open cable tray ( grated with large circular holes) to a motor control center with many wires in it would I have to derate for that?

Finally how would I fuse it. Thanks for the refresher course!!!!!

Dave

 

[iwire]

I need to replace some wiring that was burnt due to a fault and over fusing and I had a couple of questions on how to arrrive at the wire size.

The motor is 440v 3phase 40hp. Its FLA is 52A. It is in an envoronment that can get as hot as 125 degrees. so I was going to use THHW or THHN.

I come up with 4 AWG THHN.

52*1.25=65 amp conductor needed

4AWG --- 95*.76=72 amps after temp correction.

If you are using standard breakers you could go as high as 150 amps.

52*2.5=130 amps but you can round up to the next standard size so you could use a 150 amp breaker.

 

[hardworkingstiff]

From what I seem to have learned yesterday I believe I can make a case for #6.

If you look at the Application examples on page 310 and 93 of the 2008 handbook, the point is made that the code does not require double-derating where continuous loads are supplied by a conductor that is also subject to ampacity adjustments.

52-amps *1.25% = 65-amps required for a continuous load. (A #6 at 75C is 65-amps so it's OK).

52-amps / .76 = 68.4 or 68-amps required for temperature compensation (can use the 90C column so #6 is OK because it's rated 75-amps at 90C).

Based on my new understanding, a #6 is adequate per the NEC and the conditions stated.

Edit: This is assuming the motor and OCPD are rated for use at 75C. Being it's 600-volt equipment I think that assumption should be safe.

 

[dm9289]

Would I have to derate for the amount of conductors or the pretty full vented cable tray

 

[hardworkingstiff]

Would I have to derate for the amount of conductors or the pretty full vented cable tray

I don't know the answer to your question, but...

If that is a concern, then another concern would be whether adding 3 more CCCs would put the existing derating calculation into a more strict %?

 

[Rockyd]

Might want to give consideration to 392.9 in it's entirety for your situation.

 

[augie47]

Single conductor THWN smaller than 1/0 is not approved for cable tray.
If you are going to use a 3 condcutor with ground TC cable, you need to look at Art 392 for spacing and derating. If you meet the requirements lof 392.9, the the 3 conductor cable would not need to be derated.

 

[dm9289]

Thanks all I read thru the many referals in 392 where does it say I what size single conductor for the cable tray?

Also for fusing the motor does anyone know where those guidelines are?

 

[mattsilkwood]

Single conductor THWN smaller than 1/0 is not approved for cable tray.
.

I learned that one the hard way awhile back. Thats what I get for trusting the boss and not looking it up myself.

 

[cadpoint]

Wait, your in the Show me State!

 

[augie47]

Thanks all I read thru the many referals in 392 where does it say I what size single conductor for the cable tray??

392.3(B)(1)

Also for fusing the motor does anyone know where those guidelines are?

If the fuse is for SCGF protection, Art 430 Part IV, 430.52 (Normally 175% for DE fuses)

 

[tom baker]

For fusing the motor, if you are referring to the branch circuit short circuit and ground fault protection, I would only use dual element time delay fuses, size at 125% of motor FLA.
Is this really a 440V motor? That utilization voltage has not been used for many years.
What is the system voltage? I would be concerned putting a 440v motor on a 480 volt system

Normally a motor for 480 is a 460 volt motor.