Grounding conductor sizing for VD

[dklecka]

In our area there is some disagreement about how to adjust the grounding conductor for voltage drop (VD). 250.122(B) calls for it to be increased the same percentage as the ungrounded conductors. In many sizes that makes sense, but it is extreme especially when considering #12 & #10 conductors. Many Engineers revert to Table 250.122 which makes sense but is a technical violation of the NEC.

The equipment ground seldom becomes a part of the ground fault path except at the equipment, maybe that is why it is called the equipment grounding conductor. Think backhoes & Sawzall’s. When a fault occurs at the equipment the ground path should be the same regardless of the source voltage since when there is a fault the current flow approaches the AIC and the source voltage in response approaches zero.

An example might help to illustrate this disconnect:

A 15FLA 120V/1ph circuit on a 20A breaker 215’ from the panel would require #6 phase/neutral conductors for 3% VD and the ground conductor would also need to be #6 per 250.122(B). Conversely a 46FLA 480V/3ph circuit on a 60/3 circuit breaker 215’ from the panel would require #6 phase conductors and a #10 ground per table 250.122 and 3% VD considerations would not be applicable. In the event of a ground fault at the equipment the AIC would be expected to be greater on the 480V circuit but the ground path is smaller.

Recommend the following change to 250.122(B) (but better language would be appreciated): …however, the grounding conductor size does not need to exceed the size imputed from Table 250.122.

 

[Strathead]

Like many things in the code, in order to prevent the NEC from being 10 volumes, some things need to be generic. There is some validity in your concern, but your solution could cause problems depending on the exact situation.

 

[infinity]

Just use a metal raceway and then you can completely ignore 250.122(B).

 

[tortuga]

Recommend the following change to 250.122(B) (but better language would be appreciated): …however, the grounding conductor size does not need to exceed the size imputed from Table 250.122.

Even if you delete or change that section as long as your using a standard inverse time circuit breaker or fuse to provide the 'ground fault protection' you have to provide ground fault protection that functions. In general we have to provide 3 main types of protection overload, short circuit and ground fault, a licensed electrical professional is legally on the hook for that, see 250.4(A) [2-5], 110.7 and 110.10.
With a long enough run of wire sized per 250.122 you could basically create a situation where a inverse time breaker would never trip due to a ground fault.
I could see adding an exception to 250.122(B) where you could use 250.122 all the time regardless of VD if a breaker provides at least ground fault protection of 30ma (GFPE) or 5ma GFCI etc.