motor frame grounding conductor sizing 7

[electricpete]

The NEC states that equipment grounding conductor should be sized according to the rating of the overcurrent device feeding the equipment.

It seems like the logic is that in event of a fault within the motor causing ground current to flow in equipment ground conductor, we want the cable to be protected over a range of time and currents that is bounded by the protection. i.e. the motor must trip before the cable melts.

From that logic, should we be permitted to use much smaller grounding conductors for motors that are protected by sensitive ground fault relaying?

 

[dpc]

I'm a little late to the party, but here's my $0.02:

As far the rationale for equipment grounding conductor sizing, my understanding has always been that the primary concern is assuring that sufficient fault current will flow to operate the overcurrent protective device (fuse or breaker).

The NEC requires that in cases where the phase conductor size is increased to account for excessive voltage drop in a long feeder, the equipment grounding conductor must also be increased in size, in equal proportion.

It does makes engineering sense that the equipment grounding conductor size could be reduced due to the use of more sensitive ground fault protection. In fact, the NEC actually *does* allow this in the special case of multiple conductors per phase using pre-manufactured cable such as MC cable. See 250.122(F)(2).

So, if I had an existing installation where the equipment grounding conductor was too small per NEC, for some reason, I would be willing to argue for a variance based on the use of GF protection, provided that grounding conductor was sized adequately based on the ground overcurrent setting.

But the NEC is never more illogical than it is in Article 250 so it is hard to apply logic. Unfortunately, it is the one section that local inspectors know the best.

 

[jbartos]

Suggestion: Visit

http://ourworld.compuserve.com/homepages/boyce_smith/reason.htm

for: Sizing and Purposes for Good Grounding Practices

http://www.hendersoncountync.org/inspections/National Electrical Code Changes.htm

for:
Article 250- Grounding

The rationale is in the conductor current carrying capability to trip a protective device. If the branch circuit conductors are very long, the ground conductor has to be upsized to enable passing the sufficiently large current in the ground circuit to trip the protective device. The ground conductor impedance is more important is its sizing than the thermal limitations. The insulated ground conductors inside conduits with current carrying conductors must not have the ground conductor temperature be elevated such that the ground conductor insulation is damaged and current carrying conductor insulation is thermally degraded. The calculations of smaller ground conductor sizes and baseline document do not appear to be readily available.

Peebee, thank you for congrats.

 

[electricpete]

A ground conductor will have to meet multiple criteria as has been said. The question is which of those criteria is limiting and which are associated with the article 250 table. It seems very clear that these table limits are the familiar limits that we apply to phase condctors for THERMAL protection. It also seems very clear that in most any realisic case where a conductor is sized based on thermal considerations, the conductor will not be so high resistance that it will limit current flow below protective settings in event of a solid ground fault. (If we postulate a high-resistance fault, then the high fault resistance is limiting current below the protective setting, not the cable). So thermal is the most limiting and thermal limits are provided in the table.

Thank you jbartos for providing backup to my point in your first link:

"All ground conductors must be of sufficient size to trip the
circuit breaker or fuse of the circuit that it protects, without
overheating 15 amp circuit 15 amp wire, 20 amp circuit 20 amp wire"

 

[electricpete]

jbartos - I agree with your statement: "If the branch circuit conductors are very long, the ground conductor has to be upsized to enable passing the sufficiently large current in the ground circuit to trip the protective device"

I would consider this the exception rather than the rule. Usually the phase conductors may be designed for perhaps 5% voltage drop, it is difficult for me to imagine a case where phase conductors plus ground conductor alone limit fault current below protective settings, but I'm sure it's possible. My point is that I don't believe this scenario has anything to do with the table values in article 250.

 

[ielivaz]

peebee:

of course I dont run the ground wire with phase wires.

Once the naked wire begins oxiding the CuO acts like a protective layer for the core.

Every one have seen 40+ year old distribution lines exposed to wheater and working.

 

[peebee]

ielivaz:

If you are not running your ground wire with the phase conductors, then your post has nothing to do with the equipment grounding conductor referenced in electricpete's original question. Per NEC 250.118, "equipment grounding conductors" are the grounding conductors "run with or enclosing the circuit conductors".

In addition to those required ground conductors, you can certainly continue to provide any additional ground conductors as you see fit in any way you see fit. They are not required by NEC and therefore can be of about any type, size, or configuration you please. The required equipment grounding conductors, however, must meet very specific requirements of Article 250.

 

[jbartos]

Suggestion marked ///\\\: ///The NEC revisited has the following:\\250-122 (B) – The title of this section has been changed from “adjusted for voltage drop” to “increased in size”. This essentially means that any time an ungrounded conductor is changed in size the corresponding equipment ground must also be proportionally changed in size also.

///However, there is no specific explanation how to change the ground proportionally. Apparently, if one upsizes a current currying conductor to the next size, the ground is supposed to be upsized to the next size.\\