Equipment Grounding Conductor Size

[nightfox1925]

The NEC and CEC has published Tables for the selection of minimum size of equipment grounding conductors based from branch circuit ampere ratings. There is also a table based from service entrance conductor size. What was the basis for these sizes?

I am having the technical impression that equipment grounding conductors (as well as grounding conductors that runs with power cable runs) should be sized to withstand the maximum ground fault available. Hence, the sizes in these tables may not be sufficient to withstand the calculated max ground fault (say in solidly grounded systems).

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

It has to carry the line-to-ground fault current long enough to clear the upstream phase overcurrent device.

These tables have been used for many decades. If there was a significant problem with the short-circuit capacity, it probably would have come to light before now.

That being said, there is nothing in the NEC that prohibits installing larger ground wires than called for in the table. In fact, if phase conductors must be increased in size due to voltage drop concerns, the ground conductor cross-section must be increased an equivalent amount.

 

[davidbeach]

dpc, it used to be that if you increased the phase conductors for voltage drop you had to increase the ground proportionally. Now (last couple of code cycles I believe) it is that if you increase the phase conductor you have to increase the ground proportionally, doesn't matter anymore why you increase the phase conductor size.

 

[dpc]

David,

Thanks for the clarification. 250.122(B) covers this in the 2008 NEC.

 

[nightfox1925]

Thanks for the prompt response. My idea is to compute for the available ground fault on the circuit of application and depending upon conductor temperature ratings and OCPD tripping time, I use the Thermal Equation of copper conductor described in ANSI/IEEE Std. 242-1986 to determine the minimum conductor size. This is the ideal approach right? What if my ground fault is large enough to encroach the minimum size the code requires?

Curiousity drives me to wonder why there is no clause in the code that says that "Grounding conductors indicated in these tables should be verified to withstand the available maximum ground fault current". The IEEE Std 242-1986 is clear enough...why don't the code do the same?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

If you want to go to that trouble, I don't see any problem, as long as you don't go BELOW the minimum sizes listed in the NEC.

The NEC tries to provide reasonably straightforward (?) guidance for non-engineers. Simple tables are much easier to understand and enforce than a calculation procedure where every assumption and variable will be subject to interpretation and disagreement. Pity the poor inspector who would have to review all of your calculations to figure out the proper ground wire size.

I think you'll find that the sizes listed will be adequate for nearly all situations. But maybe you'll prove me wrong.

I do a lot of coordination studies and when plotting the phase conductor damage curves against the protective device clearing times, I've almost never seen a problem with conductor damage.

I realize that the EGCs are smaller, but I'd still not expect a problem unless you've got a monstrous source transformer.

 

[nightfox1925]

Thanks dpc.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[gepman]

Thanks davidbeach for pointing this (code change) out. I looked back and this changed between the 1999 and 2002 code. Also changed at the same time was an exception which allowed the EGC to be sized according to the rating of the ground fault trip for parallel conductors.

My typical work usually does not require increasing the size of the conductor for voltage drop but it is very common for ambient temperature considerations.

It seems that there is a little inconsistency since conductor size is also based on the insulation rating of the phase conductor. You could have the same size EGC for different size phase conductors for the same amperage if one was based on 60 deg. C and the other based on 75 deg. C. It would seem that they should just have a table for the EGC size based upon the phase conductor size (equivalent size for parallel installations). It would save the calculations every time you upsized a conductor.