EGC, 250.122 A&B

[tshephard]

Discussion on another group got me to looking in the 2002. If 250.122 A sets the EGC size by overcurrent device rating, and I then increase conductor size for voltage drop, which rule dominates?
Seems to me if the OCPD didn't change, and the current didn't change, why does the EGC need to change?
Any code guys to help me understand why and when to apply thses rules?

 

[cpro123]

Re: EGC, 250.122 A&B

250.122 B Increase in size. Says Where ungrounded conductors are increased in size so shall be the EGC.

Chris

 

[roger]

Re: EGC, 250.122 A&B

Think of it like this, if you have to increase the wire size to overcome voltage loss in order to deliver the current, the EGC size would also need to be increased to deliver the fault current.

All conductors have to grow in proportion with one another to perform their tasks.

Roger

 

[dereckbc]

Re: EGC, 250.122 A&B

If you increase the size of the conductors to overcome voltage drop, it is logical to assume it is from excessively long conductor runs which adds impedance to the circuit. Thus the long length also increases the ground fault impedance. So you increase the EGC size in proportion to the circuit conductors to compensate. Otherwise it is possible that the OCPD may not operate in a timely fashion.

 

[tom baker]

Re: EGC, 250.122 A&B

The note to table 250.122 used to state "may" not it says shall, and it was also changed from increased in size for voltage drop to increased in size. In order to comply with 250.4(A) the size of the equipment grounding conductor has to be sufficent to allow the OCPD to clear. Your EGC size needs to allow the OCPD (if a TM ckt breaker) to operate in the instanenous region, or typically at 5-10 x its rating.