Earth size cable

I am running three 2/0 cable to to feed a Westinghouse panel, to a 11-300 size 4 bucket and starter the size motor is 3 phase 460VAC at 111 amps

What size earthing cable would you suggest to feed this motoe from the MCC .

Ainsley
 
I am running three 2/0 cable to to feed a Westinghouse panel, to a 11-300 size 4 bucket and starter the size motor is 3 phase 460VAC at 111 amps

What size earthing cable would you suggest to feed this motoe from the MCC .
And I am using a 2 inch rigid conduit to carry the cables.
Ainsley


 

charlie b

Moderator
Staff member
I interpret the word "earthing" to mean "grounding electrode conductor." Unless the 2/0 feeder's point of origin and the Westinghouse panel are in different buildings, you don't need that conductor. Are you instead talking about the "equipment grounding conductor"? That would be sized on the basis of the upstream overcurrent device. Please clarify your question.
 
I interpret the word "earthing" to mean "grounding electrode conductor." Unless the 2/0 feeder's point of origin and the Westinghouse panel are in different buildings, you don't need that conductor. Are you instead talking about the "equipment grounding conductor"? That would be sized on the basis of the upstream overcurrent device. Please clarify your question.

The three 2/0 single core cables are connected to the Westinghouse panel THROUGH a 2" rigid conduit and then feed a 100 hp motor outdoor , I am referring to grounding to the motor main frame and back to the MCC . therefore then would a 35 mm sq cable be compliant to Nec ?
 
The Westinghouse panel is feeding the motor with 460 vac and the current rating is 111. so over load would be set a little more that 111 A
 

petersonra

Senior Member
The Westinghouse panel is feeding the motor with 460 vac and the current rating is 111. so over load would be set a little more that 111 A
You have to size the equipment grounding conductor IAW T250.122. Hint - the overload relay is not considered to be the automatic over current device. It would be the circuit breaker or fuse feeding it. by code such a device could be as much 250% of the FLC of the motor, even more in some cases.

Incidentally, we do not use 35 mm^2 cables here as that is an IEC not NEC sizing, and they are not generally recognized by the NEC because we don't use IEC cables.

You could also use metallic conduit as the EGC and meet code.

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iceworm

Curmudgeon still using printed IEEE Color Books
I'm a slow poster. Adding to Bob's post:

You are a little mixed between IEC and NEMA.
My response is based on you are following the NEC 2017.
The overloads don't matter, although depending on the motor service factor, I would pick between 125% to 140%. See art 430.32.
The grounding conductor is based on the rating/type of the short-circuit and ground-fault protective device

Start with 250.122.D
(D) Motor Circuits. Equipment grounding conductors for motor circuits shall be sized
in accordance with (D)(1) or (D)(2).
(1) General. The equipment grounding conductor size shall not be smaller than
determined by 250.122(A) based on the rating of the branch-circuit short-circuit and
ground-fault protective device
.​
(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the
overcurrent device is an instantaneous- trip circuit breaker
or a motor short-circuit
protector, the equipment grounding conductor shall be sized not smaller than that given
by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse

selected for branch- circuit short-circuit and ground-fault protection in accordance​
with 430.52(C)(1), Exception No. 1.​

Then go back to 122.A
250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of
the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be
required to be larger than the circuit conductors supplying the equipment. Where a cable tray, a
raceway, or a cable armor or sheath is used as the equipment grounding conductor, as
provided in 250.118 and 250.134(A), it shall comply with​
250.4(A)(5) or (B)(4).​

So, use Table 250.122, based on the CB rating. And if the CB is an instantaneous, that would be sized per 250.122.D.2

For a thermal magnetic CB (sized per 430.52), at 250% x 111 = 278AA, 300A CB gives #4CU
For an instantaneous CB (any rating) per 43.52, use 175% x 111 = 195A, gives #6CU
 

petersonra

Senior Member
I'm a slow poster. Adding to Bob's post:

You are a little mixed between IEC and NEMA.
My response is based on you are following the NEC 2017.
The overloads don't matter, although depending on the motor service factor, I would pick between 125% to 140%. See art 430.32.
The grounding conductor is based on the rating/type of the short-circuit and ground-fault protective device

Start with 250.122.D

Then go back to 122.A

So, use Table 250.122, based on the CB rating. And if the CB is an instantaneous, that would be sized per 250.122.D.2

For a thermal magnetic CB (sized per 430.52), at 250% x 111 = 278AA, 300A CB gives #4CU
For an instantaneous CB (any rating) per 43.52, use 175% x 111 = 195A, gives #6CU
it is based on the actual rating of the device, not the maximum, but otherwise yes. i would just use the conduit if it is metallic.
 
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