multipule motor feed??

[Hoyt]

Hello,

I would like to make one feed to one piece of equipment. This equipment has 3 motor connections. 2 - 20 hp motors and 1 - 5 hp motor. I would like to set a fused disconnect of 100a (my main) and this would feed the 3 disconnects of the equipment - 2 - 60a fused disconnects (for the 20 hp's) and 1 - 30a fused disconnect for the 5 hp motor.

480 volt - 3 phase equipment.

I have I don this correctly if this is my path?

Thanks,

Hoyt

 

[hillbilly]

What type fuses do you plan to use?

How are you starting the motors?


steve

 

[Hoyt]

The equipment has the equipment starters, etc.

What is meant by type of fuses...sizes?

 

[Chenley]

dual element time delay fuses or non-time delay fuses?

I'm getting different sized fuses with what you had stated with this calculator:
http://www.electrician2.com/calculators/motor_ver_1.html

 

[Hoyt]

I guess I am not sure of the type of fuses. I am thinking the time dealy. Can you explain the diff in them?

I took all 3 motors and added the FLA, I took the largest motor and multipulied by 1.25.

So 33.75 + 27 + 7.6 = 68.35 main. So that is a 100a frame fused switch with 70 am fuses. I would have 60a frame fused switch with 40 amp fuses for the 2 - 20 hp motors and a 30a frame w/ 20a fuses for the 5 hp.

Am I way off base?

 

[billsnuff]

430.62(a)

430.62(a)

if i understand this correctly, this is feeder overcurrent protection. use the largest OCPD plus the FLC of the rest of the motors, then use the next lower standard size fuse to protect the feeder.

 

[DesignerMan]

if i understand this correctly, this is feeder overcurrent protection. use the largest OCPD plus the FLC of the rest of the motors, then use the next lower standard size fuse to protect the feeder.

That would be correct- the largest OCPD used (40A)+ FLC of remaining motors (27A & 7.6A)= 74.6A. Next lower standard size would be 70A.

However, depending on the type of motor and the fuses used you could increase this. For example, a 20HP squirrel cage motor with Time Delay fuses would be 175% of FLC (27x1.75=47.25A). Next size fuse is 50A.

This is how I typically protect motors. I also recommend the Time-Delay fuses to help with inrush current.

 

[billsnuff]

i agree

i agree

squirrel cage nontime delay fuses @ 300%........dual element TD fuses @ 175%......IT breaker @ 1100% (design B)...........Inverse Time Breaker @ 250%

UGLYs 2005 pg 35 check foot notes. also 2005 NEC 430.52 - .54

 

[LarryFine]

If Hoyt is installing a 100a disco, why not install 100a fuses and suitable conductors?

 

[DesignerMan]

If Hoyt is installing a 100a disco, why not install 100a fuses and suitable conductors?

I believe if it is strickly 3 motors- that would be considered a specific load and would therefore fall within the 430.62(A) requirements.
That's just my take on it. The control panels I've designed have always had additional loads- not just motors. And we always leave some extra capacity for future add-ons. :smile:

 

[LarryFine]

I believe if it is strickly 3 motors- that would be considered a specific load and would therefore fall within the 430.62(A) requirements.

I got the impression this is a feeder, not a BC, and the individual motors had their own OCP.

 

[DesignerMan]

I got the impression this is a feeder, not a BC, and the individual motors had their own OCP.

Yea, I got the impression it was a feeder also. 430.62(A) falls under Part V. Motor Feeder Short Circuit Protection.

I agree with what your saying Larry; if the conductors are properly protected and the disconnect is sized correctly what difference doe it make?

I'm just reading it as it's written...