Conductor and Component sizing

[jenkinscody]

Hey guys, I am trying to put together a project but have hit a few bumps. The basis is I am running a 75hp motor with a soft starter that is 460VAC, 3ph... According to my text, the conductor size of the motor must be about 25% above rated amperage of the motor. I calculated about 96 rated, therefore was going to use 4AWG Copper THHN for the motor. My text also said to size a circuit breaker 300% of your rated full load amps of motor so I came out with nearly 300A?? I sized a 250A circuit breaker. My question is first of all if this sounds correct and also, would I run Wire from my incoming 460 to my circuit breaker that is capable of handling 250AMP, and then drop down to $AWG wire to my soft start and there-on to my motor? Any advice?

 

[rbulsara]

You may want to review your design with your seniors. Where does your "text" come from? It appears that you are in the NEC world. NEC has detailed requirements for a motor branch circuits, follow that.

Where did you get ampacity of #4 AWG? Did you check NEC and temperature limitations for terminations in 110.14. Do a voltage drop check as well.



Rafiq Bulsara

http://www.srengineersct.com

 

[jenkinscody]

My "text" would be just an UGLY's book. The allowable apacity of 4AWG wire is 140Amps.

 

[rbulsara]

UGLY's book is not the code book nor a recognized source for technical information. Use proper resources.

Rafiq Bulsara

http://www.srengineersct.com

 

[PaultheBigFinn]

You need to use Article 430 of the NEC. Table 430.250 gives indicates 94A for a 75 Hp motor and the conductor should be sized for 125% of that, or 117.5A. That's just a hair over the 75 degree table 310.16 rating of 115A for #2 AWG. I would use #1 AWG.

The NEC requirs the motor to have thermal protection and by table 430.52 allows a MAXIMUM rating of either 300%, 175%, 800% or 250% for a synchronous moter depending on the type of device providing short circuit protection. For a circuit breaker I would use 250%.

BUT that is a maximum rating intended to allow a short duration starting current. When you have a soft starter the starting current is lower. It depends on the device.

All of this said, I agree that you should review this with your seniors.

 

[wareagle]

Quote"My "text" would be just an UGLY's book. The allowable apacity of 4AWG wire is 140Amps."

That is not what table 310.16 says. #4 at 75C = 85 amps
BigFin is correct. Use #1 if the voltaage drop is ok.
Re Check you Uglys book.

 

[rcw retired EE]

Jenkinscody-

I assume you used page 74 of the Ugly's reference. Note that it is a table of allowable ampacities of "Single Insulated Condutors in Free Air", based on an ambient temperature of 30 C (86F). It is a direct copy of a table from the NEC, minus all of the notes and other modifcations. Unless you are going to run all of these wires on an open pole line, you can't use that table.

Wire in conduit is shown on a different table in the NEC, and is also included in Ugly's (page 73 in my copy.)

Bottom line, like others said, get help from someone who understands the NEC and motor circuits.