Grounding conductor size
- Thread starter jkim780
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petersonra
Senior Member
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- Northern illinois
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- engineer
Re: Grounding conductor size
I'm not sure anyone alive knows where they came up with them.
I have never quite understood why it needs to increase in size as the incoming conductors increase in size.
The GEC is not part of a fault clearing path, so why does it increase in size? maybe the people who came up with the sizes expected someone would unintentionally use them as part of a fault clearing path. That's the only explanation I can come up with that nmakes any sense, at least to me.
I'm not sure anyone alive knows where they came up with them.
I have never quite understood why it needs to increase in size as the incoming conductors increase in size.
The GEC is not part of a fault clearing path, so why does it increase in size? maybe the people who came up with the sizes expected someone would unintentionally use them as part of a fault clearing path. That's the only explanation I can come up with that nmakes any sense, at least to me.
Re: Grounding conductor size
Thank you, bob. you read my mind. That was one of my question. Another one was about EGC table.
1. Whay EGC is based on the OCPD rating not the magnitude of current?
2. Why EGC is not same size as ungrounded conductor? I mean if your EGC is smaller than ungrounded conductor, there will be more impedance in your EGC and that will prevent your OCPD operation in timely fashion.
Can someone explain to me?
Thank you, bob. you read my mind. That was one of my question. Another one was about EGC table.
1. Whay EGC is based on the OCPD rating not the magnitude of current?
2. Why EGC is not same size as ungrounded conductor? I mean if your EGC is smaller than ungrounded conductor, there will be more impedance in your EGC and that will prevent your OCPD operation in timely fashion.
Can someone explain to me?
bphgravity
Senior Member
- Location
- Florida
Re: Grounding conductor size
The answer to both of you questions is the same. Conductors have a withstand rating. This rating determines how much fault current can be carried before the conductor fails.
The note to Table 250.122 indicates that each installation needs to be evaluated to determine available fault current and thus adjust to larger size EGC if need be.
Ungrounded conductors are sized to to have a capacity to carry current continuously without exceeding the temperature rating of the insulation. EGC's are sized to carry the fault current of a short-circuit or ground-fault which is only a short duration event, usually less then 1/2 cycle.
The code takes into consideration impedance issues, hence the requirement in 250.122(B). Keeping overall impedance low and ensuring the effective fault path is ensured by this section.
The answer to both of you questions is the same. Conductors have a withstand rating. This rating determines how much fault current can be carried before the conductor fails.
The note to Table 250.122 indicates that each installation needs to be evaluated to determine available fault current and thus adjust to larger size EGC if need be.
Ungrounded conductors are sized to to have a capacity to carry current continuously without exceeding the temperature rating of the insulation. EGC's are sized to carry the fault current of a short-circuit or ground-fault which is only a short duration event, usually less then 1/2 cycle.
The code takes into consideration impedance issues, hence the requirement in 250.122(B). Keeping overall impedance low and ensuring the effective fault path is ensured by this section.
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