Motor equipment grounds

[tpd]

When sizing equipment ground for a motor and the overcurrent device is much higher than the conductor rating, so the equipment ground size from Table 250.122 is actually more than motor conductor size. The exception say's E.G.C doesn't have to be larger than motor conductors. So another electrician in my shop tells me he knows there is another exception that allows for a smaller E.G.C. I can't seem to find it anywhere. Is this true? Also, we use H.M.C.P's. They are instantanous trip with a dial for different motor amperages. Like say a 100Amp HMCP you can increase the trip setting to fit a 75 hp. 3 phase motor or lower it for a smaller motor. So is the E.G.C. on these sized according to the calculation of motor full load from N.E.C. current times the setting? which would always end up using the exception and it would end up being the same as the motor leads. Or is it sized according to the 100Amp rating of the H.M.C.P? All motors in this facility use multi-conductor cable and the E.G.C's are smaller than the motor leads. Hope this isn't confusing to read!

 

[erickench]

A motor has two OCPDs. One for short circuit-ground fault and the other for overload. Why not just use the overload rating to size the EGC? It has a smaller rating.

 

[kwired]

A motor has two OCPDs. One for short circuit-ground fault and the other for overload. Why not just use the overload rating to size the EGC? It has a smaller rating.

250.122 says:

(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.

you can not size EGC by motor overload size. it has to be able to carry ground fault current that is why it is there in the first place

 

[ike5547]

Like say a 100Amp HMCP you can increase the trip setting to fit a 75 hp. 3 phase motor or lower it for a smaller motor. So is the E.G.C. on these sized according to the calculation of motor full load from N.E.C. current times the setting?

The branch circuit conductors are not sized on the OCPD or the overload setting. They are sized based on the horse power of the motor.

The OCPD rating is permitted to be increased by a percentage not higher than necessary to allow it to open during a fault with conductors sized by the HP value.

The fault current will have to travel through the smaller ungrounded conductors regardless of how large the EGC is.

 

[ike5547]

The OCPD rating is permitted to be increased by a percentage not higher than necessary to allow it to open during a fault with conductors sized by the HP value.

Although technically the OCPD (ground fault and short circuit protection) is based on the full load current of the motor.

I didn't say it had to make sense.

 

[kwired]

Lets say we have a 25hp, 3phase, 460 volt motor.

Table 430.250 says this motor must have conductors sized for 34 amps with motor overload protection at a maximum of 125% = 42.5 amps. (many motors will be lower this is a maximum)

If the EGC were allowed to be sized according to motor overload setting then a #10 cu EGC would be allowed.

Lets say the same motor is protected by maximum allowed inverse time breaker of 90 amps. Based on 90 amp OCPD minimum EGC is #8 cu.

Now lets use the maximum allowed instantaneous trip breaker of 250 amps, the EGC does not need to be larger than motor circuit conductors but I don't believe I would want the #10 cu that would equate to motor overload setting to be used to interrupt this fault.

just some ideas to throw out there.

 

[ike5547]

Although technically the OCPD (ground fault and short circuit protection) is based on the full load current of the motor.

I didn't say it had to make sense.

My mistake. According to 430.6(A)(1) ground fault and short circuit protection for a single motor are based on the tables -- not the name plate.

 

[ike5547]

Now lets use the maximum allowed instantaneous trip breaker of 250 amps, the EGC does not need to be larger than motor circuit conductors but I don't believe I would want the #10 cu that would equate to motor overload setting to be used to interrupt this fault.

I think they're assuming that a faster reaction time will prevent significant damage to the conductor and equipment.

 

[kwired]

a smaller EGC may cause lower fault current and therefore longer reaction time.

 

[don_resqcapt19]

A motor has two OCPDs. One for short circuit-ground fault and the other for overload. Why not just use the overload rating to size the EGC? It has a smaller rating.

I think the reason would be that the EGC has nothing to do with an overload. It is there to provide the fault return path in the event of a ground fault and must be sized based on the rating of the device that provides the ground fault and short circuit protection for the circuit.

 

[erickench]

So why have have an EGC larger than the circuit conductors? I think I'll make a proposal. Fault current would be very high and the ratings of both OCPDs would'nt be high enough. It's the AIC rating that matters.

 

[kwired]

No one has said the EGC needs to be larger than circuit conductors. AIC rating does not matter for circuit protection or conductor sizes, AIC rating is important when there is a high level of fault current available. You want the device to be able to interrupt a fault without blowing up the device and surrounding equipment.

The whole idea of sizing a EGC is to provide a low enough impedance to allow a high enough fault current to operate the overcurrent device quickly and to have a large enough conductor that it is not compromised by the fault so that it will work again if needed. A fault is supposed to be a short time versus normal load on circuit conductors that could be continuous, therefore in many cases the grounding conductor is allowed to be smaller than the circuit conductor

 

[erickench]

Just one question. Was there ever an NEC cycle that used the motor overload ratings to size the EGC?

 

[don_resqcapt19]

Just one question. Was there ever an NEC cycle that used the motor overload ratings to size the EGC?

Yes, in the 2005 and earlier codes, the EGC for a motor circuit that used a instantaneous trip circuit breaker or motor short circuit protector for the ground fault and short circuit protection was sized based on the rating of the motor overload protective device. 250.122(D)

 

[erickench]

Okay I think I see the problem. The short-circuit ground-fault OCPD is located on the supply side of the circuit and the overload OCPD is located on the load side close to the motor. If there is a fault between these two OCPDs the upstream OCPD will trip and not the overload. But they went ahead and made the change even though the EGC would be oversized. It's just a dilemma that no one has been able to figure out yet.

 

[don_resqcapt19]

I don't see how the EGC is oversized. It is sized just like any other EGC...based on the size of the circuit short circuit and ground fault protective device. A motor overload does not provide short circuit and ground fault protection. It would be slower to react to a ground fault or short circuit than would a correctly sized fuse or breaker.

 

[ike5547]

I don't see how the EGC is over-sized, under-sized or sideway-sized. The system works. It has worked for many years.