Motor disconnect & 430.110(a)

[m sleem]

For the most cases, the non-fused disconnect installed in motor branch. If used the fused disconnect,

1- Shall we apply 430.52 for the disconnect rating or we still have to follow 430.110(a).

2- Shall we apply 430.24 for the feeder size?

 

[augie47]

Disconnects, fused or unfused should be sized per 430 Part IX (includes 430.110)

I've seen the feeder enforced per 430.24 and I have also seen inspectors require the feeder to a branch circuit overcurrent device for a single motor to be sized by the size of the feeder OCP device,
430.24 seems to apply to multi-motor and combination loads, so some might be hesitant to allow it on one motor.
I have also seen inspectors consider the fused disconnect at the motor as "supplemental".

 

[m sleem]

Disconnects, fused or unfused should be sized per 430 Part IX (includes 430.110)

I've seen the feeder enforced per 430.24 and I have also seen inspectors require the feeder to a branch circuit overcurrent device for a single motor to be sized by the size of the feeder OCP device,
430.24 seems to apply to multi-motor and combination loads, so some might be hesitant to allow it on one motor.
I have also seen inspectors consider the fused disconnect at the motor as "supplemental".

But, if we apply 430.24 for single motor the disconnect rating would be 115% of FLA, how the disconnect will allow for the starting current which almost 600% of FLA?

 

[augie47]

Notice that 430.110(A)(1) also requires that the disconnect be rated in HP and be rated to handle the LRA, etc.
(Normally if you obtain a disconnect rated in HP it will cover all the requirements)

 

[m sleem]

Notice that 430.110(A)(1) also requires that the disconnect be rated in HP and be rated to handle the LRA, etc.
(Normally if you obtain a disconnect rated in HP it will cover all the requirements)

I'm working as a designer. I want to indicate the disconnect's size in the drawing. If indicates 115% of the FLA, would that satisfy?, or we need to add the HP rating, it seems to me that would be conflict.

 

[augie47]

The ampere rating might well be different depending on the type fuse used (dual-element or not) so, if I had a choice I would say specify HP ratings.

 

[jwelectric]

I'm working as a designer. I want to indicate the disconnect's size in the drawing. If indicates 115% of the FLA, would that satisfy?, or we need to add the HP rating, it seems to me that would be conflict.

Your answer lies in the size of overcurrent device that will be installed therein and the size of disconnects that are manufactured.

A careful read of the section you mentioned will reveal some interesting information.

430.110 Ampere Rating and Interrupting Capacity.

(A) General. The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.

In most cases it will be a lot larger than 115%

 

[m sleem]

Your answer lies in the size of overcurrent device that will be installed therein and the size of disconnects that are manufactured.

A careful read of the section you mentioned will reveal some interesting information.

430.110 Ampere Rating and Interrupting Capacity.

(A) General. The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.

In most cases it will be a lot larger than 115%

As you noted, 115% is the min rating and could be more, assume the motor fla is 100a, starting current is 4 to 6x, so when can we use 115% for disconnect rating???

Before i had the same query regarding motor overload, and the very helpful guys explained that, there are different classes of OL as per NEMA, class 10,20,30 allow 10sec,20sec,30sec respectively for 6x, i know the fuse characteristic is a quite different from the overload relay where the fuse doesn't allow for overload current in a few second.

 

[augie47]

The ampere rating might well be different depending on the type fuse used (dual-element or not) so, if I had a choice I would say specify HP ratings.

On second thought, once I turned on my brain, almost all the prints I have ever seen showed the disconnect in amperage size.
You need to check the manufacture specs for the disconnect desired, look for your HP, and specify that amperage switch.
Some of the guys who do the same every day may chime in, but with few, if any, exceptions I think the HP ratings will be the same for any manufacturer.

Here is a document that should help:
http://cmsapps.sea.siemens.com/controls/icc2010/10IndControl_ pdfs/10IC_18/10IC_18.pdf