Calculating equipment grounding conductors w/o Table 250.122

[cdcengineer]

Does anyone know a formula for calculating the size of equipment grounding without the use of NEC Table 250.122? I know it's easy enough to look at the table, but I'm just curious how the table was developed.

Thanks, and Happy Friday!

 

[Dennis Alwon]

I doubt there is a formula simply because you can have many different size conductors using the same size equipment grounding conductor. I assume they looked at what size conductor is large enough to trip the breaker. In many cases I am sure that a smaller equipment grounding conductor would work but they are probably looking at worse case scenarios.

 

[cdcengineer]

Right on, thanks Dennis, I was just curious.

I know there are a few instances where I am calculating the increased egc per 250.122 (B) where the ground increases proportionately with the ungrounded conductors (upsized for vdr). When it's close, I would like to have a way to see if the smaller ground would suffice. For example, if I'm upsizing a (#4 with a #10 grd) to a #1/0 than by calculation (using NEC Table 8) it appears that the ground must be sized to 26,261 or #4 AWG. Seems close enough to a #6, but I would install a #4 ground because 26,261 is larger than 26,240 circular mils.

I know we can calculate the cm by using cm=1000k/ohms per foot. But you still end up just over the #6. The question is, because Table 250.122 includes a range of egc's by ocp, than if I'm at the low end of the range, my judgement wants me to make the call to install the smaller size egc - or at least make an educated decision other than picking a number from a table. However, nothing in the code allows this (to my knowledge). It would seem that if we could somehow calculate the egc, than we could use the calculated size as the basis for increasing for voltage drop.

Anyway, this is probably a moot point, but had my brain turning on this Friday morning. Thanks again.

 

[Smart $]

Right on, thanks Dennis, I was just curious.

I know there are a few instances where I am calculating the increased egc per 250.122 (B) where the ground increases proportionately with the ungrounded conductors (upsized for vdr). When it's close, I would like to have a way to see if the smaller ground would suffice. For example, if I'm upsizing a (#4 with a #10 grd) to a #1/0 than by calculation (using NEC Table 8) it appears that the ground must be sized to 26,261 or #4 AWG. Seems close enough to a #6, but I would install a #4 ground because 26,261 is larger than 26,240 circular mils.

I know we can calculate the cm by using cm=1000k/ohms per foot. But you still end up just over the #6. The question is, because Table 250.122 includes a range of egc's by ocp, than if I'm at the low end of the range, my judgement wants me to make the call to install the smaller size egc - or at least make an educated decision other than picking a number from a table. However, nothing in the code allows this (to my knowledge). It would seem that if we could somehow calculate the egc, than we could use the calculated size as the basis for increasing for voltage drop.

Anyway, this is probably a moot point, but had my brain turning on this Friday morning. Thanks again.

I have a proposal in that would eliminate the need to calculate upsized EGC by cmil where only AWG sizing is used.

The original intent of AWG sizing was to make the gauges proportional by cross sectional area. That got somewhat discombobulated with conventional sizing under ASTM B258, which rounds the original diameter values to 4 significant figures. The diameter rounding is then compounded on cmil values (4 sig-figs of rounded values squared). Even with the rounding error compounded, the maximum deviation from true AWG sizes in the building wire gauges is around 0.11%. We should be able to determine upsize gauge based on gauge difference. For your example: #4 upsized to #1/0 is 4 gauge sizes. #10 EGC upsized 4 gauge sizes is a #6 EGC.

 

[tom baker]

Soars book on grounding and IEEE Green book have info on how 250.122, and the insulated wire manufactures association.
the table is sized to quickly open the OCPD without damage to the EGC. Its typically 12 1/2% of the ungrounded conductor

 

[cdcengineer]

Soars book on grounding and IEEE Green book have info on how 250.122, and the insulated wire manufactures association.
the table is sized to quickly open the OCPD without damage to the EGC. Its typically 12 1/2% of the ungrounded conductor

Thanks Tom and all.