millstrainer
Member
I can't find it in the NEC but someone was telling me that the maximum size Equipment Grounding Conductor for a feeder is 250 kcmil. Is this true or do you have to stick with what is in Tbl 250.122?
Maximum size of Equip Grounding Conductor
- Thread starter millstrainer
- Start date
- Status
- Location
- Massachusetts
Re: Maximum size of Equip Grounding Conductor
You are talking about two different items.
The largest size Grounding Electrode Conductor is 3/0 cu or 250 al. Table 250.66
Equipment Grounding Conductors are sized by table 250.122 as you thought.
That table shows up to a 6000 amp feeder
and 800 cu and 1200 al kcmil
You are talking about two different items.
The largest size Grounding Electrode Conductor is 3/0 cu or 250 al. Table 250.66
Equipment Grounding Conductors are sized by table 250.122 as you thought.
That table shows up to a 6000 amp feeder
and 800 cu and 1200 al kcmil
tonyi
Senior Member
Re: Maximum size of Equip Grounding Conductor
250.122 is MINIMUMS. There is no maximum.
You could use 1200kcm (or larger) on an ordinary 15A residential branch circuit if you so desired. The only max limit is one of practicality in finding boxes, devices and other fittings with the capacity to handle it.
250.122 is MINIMUMS. There is no maximum.
You could use 1200kcm (or larger) on an ordinary 15A residential branch circuit if you so desired. The only max limit is one of practicality in finding boxes, devices and other fittings with the capacity to handle it.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Maximum size of Equip Grounding Conductor
I used to do that all the time. It builds character.
- Location
- Massachusetts
Re: Maximum size of Equip Grounding Conductor
Maybe I misunderstood the question
Yeah and how much copper you want to waste, not any need for a EGC larger than the circuit conductors.
Re: Maximum size of Equip Grounding Conductor
IWIRE -
In the instances of extremely high available fault currents and overcurrent protective devices with long time current curves, it is possible tha equipment grounding conductors shown in Table 250.122 are too small.
The Insulated Cable Engineers Association, Inc has publication P-32-382, Short Circuit Characteristics of Insulated Cable, which shows the maximum allowable short circuit currents for insulated conductors.
Yes, Table 250.122 will be appropriate most of the time, but it is interesting to note the "safe" operating parameters of a 2/0 thermoplastic insulated copper conductor is exceeded at 20,000 amperes for only an 8 CYCLE time frame.
And we all thougth electrical work was getting easier...
IWIRE -
This may not always be true. Although 250.122 states "need not be larger than the circuit conductors," 110.10 requires the electrical circuit components be selected to permit the circuit protective devices to operate without damaging the circuit components.
In the instances of extremely high available fault currents and overcurrent protective devices with long time current curves, it is possible tha equipment grounding conductors shown in Table 250.122 are too small.
The Insulated Cable Engineers Association, Inc has publication P-32-382, Short Circuit Characteristics of Insulated Cable, which shows the maximum allowable short circuit currents for insulated conductors.
Yes, Table 250.122 will be appropriate most of the time, but it is interesting to note the "safe" operating parameters of a 2/0 thermoplastic insulated copper conductor is exceeded at 20,000 amperes for only an 8 CYCLE time frame.
And we all thougth electrical work was getting easier...
- Location
- Massachusetts
Re: Maximum size of Equip Grounding Conductor
If the run is so long that an EGC the same size as the circuit conductors would not operate the OCPD fast enough in response to a ground fault that would mean a short circuit would also not operate the OCPD fast enough.
If the run is so long that an EGC the same size as the circuit conductors would not operate the OCPD fast enough in response to a ground fault that would mean a short circuit would also not operate the OCPD fast enough.
Re: Maximum size of Equip Grounding Conductor
I agree that the size of the ungrounded and grounding conductors would need to be increased due to voltage dropin along circuit. But look at it from another point of view. If the available fault current were extremely high and the circuit was small it is possible to damage the conductors before the operation of the OCPD. This damage would be in violation of 110.10. This would be the time the grounding conductor would need to be increased to facilitate that operation.
I have only seen the requirement for an increase once in the many times I verified the application.
But the original question was if there was a maximum grounding conductor size and I agree the answer is no, there is not. I just presented this information as that...information.
I agree that the size of the ungrounded and grounding conductors would need to be increased due to voltage dropin along circuit. But look at it from another point of view. If the available fault current were extremely high and the circuit was small it is possible to damage the conductors before the operation of the OCPD. This damage would be in violation of 110.10. This would be the time the grounding conductor would need to be increased to facilitate that operation.
I have only seen the requirement for an increase once in the many times I verified the application.
But the original question was if there was a maximum grounding conductor size and I agree the answer is no, there is not. I just presented this information as that...information.
- Location
- Massachusetts
Re: Maximum size of Equip Grounding Conductor
Take your example above and make it a line to line fault if a given size circuit conductor could operate the OCPD fast enough for this line to line fault why would a ground conductor need to be larger to operate the OCPD for a line to ground fault.
What am I missing?
ekbrunn I am keeping an open mind but I still do not understand even for the above scenario why the grounding conductor would need to be larger than the circuit conductors.
Take your example above and make it a line to line fault if a given size circuit conductor could operate the OCPD fast enough for this line to line fault why would a ground conductor need to be larger to operate the OCPD for a line to ground fault.
What am I missing?
Re: Maximum size of Equip Grounding Conductor
Iwire ?
Thank you for being open-minded about this. I reviewed my reference and although the premise is accurate, I did not apply it properly. (Sometimes I cannot see the forest for the trees.) Without getting into the actual calculation, the reference discussed the ungrounded conductor. In essence you are correct about the grounding conductors not needing to be larger than the circuit conductors for the reasons mentioned.
Now that I beat that dead horse?
Iwire ?
Thank you for being open-minded about this. I reviewed my reference and although the premise is accurate, I did not apply it properly. (Sometimes I cannot see the forest for the trees.) Without getting into the actual calculation, the reference discussed the ungrounded conductor. In essence you are correct about the grounding conductors not needing to be larger than the circuit conductors for the reasons mentioned.
Now that I beat that dead horse?
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