Feeding mutliple motors which same feeder...Article 430.24?

[Senselessticker]

I’m wanting to power “8” - 1 hp motors from the same feeder/circuit (480V 3ph). A single control panel will control all 8 motors. All 8 motors will start and stop at the same time. Concerning sizing the feeder.....Do I simply determine conductor feeder size according to NEC Article 430.24? (Assuming I do not have to de-rate the cable for any other reason)

NEC 430.24 (2005) reads:


Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity for the other loads.

It seems to me that the conductor should be sized to compensate for the motors all starting at the same time. Any thoughts or code references?

Many Thanks for any feedback!

Sense

 

[dpc]

Feel free to use larger conductor than NEC requires. The NEC is not a design guide - it is just giving minimum requirements, usually related to avoiding fires. The reference you cite for sizing the feeder is definitive.

Eight 1-hp motors starting simultaneously is going to be somewhere between starting a 10 hp and a 15 hp motor.

Unless you have unusually high starting torque requirements or long runs, I'm guessing that the #12 you will come up with per NEC will work fine. But I'd probably upsize to #10 just to be ready for that 9th and 10th motor.

 

[itsmoked]

Be careful here.. We just had a thread where someone did this and is having lots of problems because wire run length was ignored. Large starting currents can cause large voltage drops on longer runs.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

Hi folks;
I agree with both dpc and itsmoked.
There is some interesting and pertinent information in the "Cowern Papers, Motor Basics" on the Baldor Motor web-site.
As well as "NEMA design letters", designating starting torque and current characteristics, there are "NEMA loched rotor code letters" for motors.
The code letter usually applied to 1 horsepower motors is "L".
This indicates 9.0 - 10.0 KVA/HP.
Eight 1 horsepower motors starting together will require 82 - 90 KVA.
The code letter usually applied to 15 HP motors is "F".
This indicates 5 - 5.6 KVA/HP.
One 15 horsepower motor starting will require 75 - 84 KVA.

I have designed similar control schemes. My feeders and motor starter, (if one starter is used to start the motors as a group) would be based on one 15 HP motor.

In one scheme I was using 5 HP motors. I protected each motor with a manual motor starter. I started the motors in two groups and used starters based on the starting KVA rather than the lessor figure of combined horsepower.
respectfully