Sizing Equipment Grounding Conductor

[aglezb]

Dear Mr. Holt,
When sizing grounding conductor using Table 250.122 I have a concern regarding the case when, for voltage loss reduction reasons, the current-carrying conductor needs to be sized larger than the selected OCPD rating. In such cases, is it recommendable/customary to size the grounding conductor relative to the new (larger) current-carrying conductor size? For example the rating of my selected OCPD is 300A, the required conductor is a 250MCM and the minimum equipment ground conductor is a #4 AWG. If just for voltage loss reduction I need to increase the conductors from 250 to 400MCM should/can I increase the ground conductor as well? (I understand that table 250.122 does not relate to conductor area but for OCPD ratings and I don't think either that the notes to the table are applicable in my case).
If you consider the ground conductor could be sized larger can you please comment on criteria for the oversizing?
Thanks much
Alberto Gonzalez

 

[don_resqcapt19]

Take of look at 250.122(B). It requires that the EGC be increased when you increase the circuit conductors.

 

[aglezb]

Take of look at 250.122(B). It requires that the EGC be increased when you increase the circuit conductors.

Don,
Thank you very much for the fast and helpful response. I'm really pleased to have the answer and the code reference.
AG

 

[infinity]

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

 

[Dennis Alwon]

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

I see they clarified that in the 2014 NEC. I think most of us interpreted it like that in the past- I know I assumed that was what it meant so I am sure others did also

(B) Increased in Size. Where ungrounded conductors are
increased in size from the minimum size that has sufficient
ampacity for the intended installation, wire-type equipment
grounding conductors, where installed, shall be increased in
size proportionately according to the circular mil area of
the ungrounded conductors.

 

[lielec11]

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

I'm not sure I understand why increasing phase conductors for voltage drop would be different than increasing phase conductors for derating.. can you elaborate please?

 

[infinity]

I'm not sure I understand why increasing phase conductors for voltage drop would be different than increasing phase conductors for derating.. can you elaborate please?

If in the voltage drop scenario (determined partially by circuit length) an undersized EGC could theoretically be too small to open the OCPD ahead of the circuit. If the conductors were increased in size to compensate for derating the size of the EGC wouldn't really matter because it would still serve it's purpose if installed according to T250.122.

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

I see they clarified that in the 2014 NEC. I think most of us interpreted it like that in the past- I know I assumed that was what it meant so I am sure others did also

I'm not so sure that intended result was to have the increase only for voltage drop but that's the way it worked out.

 

[luckylerado]

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

The commentary in the 2014 handbook seems to disagree with this statement.

250.122(B)

"Generally, the minimum-sized EGC is selected from Table 250.122
based on the rating or setting of the feeder or branch-circuit
OCPD(s). Where the ungrounded circuit conductors are increased
in size to compensate for voltage drop or for any other reason
related to proper circuit operation, the EGCs must be increased
proportionately. In some cases, use of a conductor with a higher
insulation temperature rating allows for compliance with ampacity
adjustment and correction requirements without having to
increase the circular mil area of the conductor."

 

[infinity]

For voltage drop compensation you would need to increase the size of the EGC proportionally as Don mentioned. If you were using larger conductors for another reason, say for derating purposes, then the 2014 NEC would not require an increase in the EGC.

The commentary in the 2014 handbook seems to disagree with this statement.

250.122(B)

"Generally, the minimum-sized EGC is selected from Table 250.122
based on the rating or setting of the feeder or branch-circuit
OCPD(s). Where the ungrounded circuit conductors are increased
in size to compensate for voltage drop or for any other reason
related to proper circuit operation, the EGCs must be increased
proportionately. In some cases, use of a conductor with a higher
insulation temperature rating allows for compliance with ampacity
adjustment and correction requirements without having to
increase the circular mil area of the conductor."

Prior to the 2014 I would agree with the NECH opinion but the new wording in the 2014 is clear that the increase in size does not apply to derating because the conductor needs to be up-sized to meet the size of the OCPD. I would guess that since the wording in the 2011 NECH is seemingly the same that the authors failed to change their commentary to reflect the new code wording in the 2014.

 

[don_resqcapt19]

The commentary in the 2014 handbook seems to disagree with this statement.

250.122(B)

"Generally, the minimum-sized EGC is selected from Table 250.122
based on the rating or setting of the feeder or branch-circuit
OCPD(s). Where the ungrounded circuit conductors are increased
in size to compensate for voltage drop or for any other reason
related to proper circuit operation, the EGCs must be increased
proportionately. In some cases, use of a conductor with a higher
insulation temperature rating allows for compliance with ampacity
adjustment and correction requirements without having to
increase the circular mil area of the conductor."

Conductors that have been increased in size because of the derating rules are not conductors that have been "increased in size from the minimum size that has sufficient ampacity for the intended installation".

 

[Dennis Alwon]

Conductors that have been increased in size because of the derating rules are not conductors that have been "increased in size from the minimum size that has sufficient ampacity for the intended installation".

I totally agree. If the minimum size required has taken temp, fill, etc into consideration then that would be the minimum size required. I am not sure why the wording was added. Seems redundant

 

[infinity]

I totally agree. If the minimum size required has taken temp, fill, etc into consideration then that would be the minimum size required. I am not sure why the wording was added. Seems redundant

The new wording was needed because the "increased in size" wording from the 2011 and prior cycles didn't provide a starting point as to where they're being increased. One could argue that using #10 AWG conductors for a 20 amp circuit is an increase in size because for most the standard size for a 20 amp circuit is #12 AWG but that's open to interpretation. IMO the new wording adds needed clarity and eliminates the confusion for not up-sizing the EGC when applying derating factors.

 

[don_resqcapt19]

The new wording was needed because the "increased in size" wording from the 2011 and prior cycles didn't provide a starting point as to where they're being increased. One could argue that using #10 AWG conductors for a 20 amp circuit is an increase in size because for most the standard size for a 20 amp circuit is #12 AWG but that's open to interpretation. IMO the new wording adds needed clarity and eliminates the confusion for not up-sizing the EGC when applying derating factors.

They really need to either get rid of the rule, or make the starting point the table ampacity.
The physics of the issue doesn't care why you used a larger conductor. Either a larger conductor requires a larger EGC or it doesn't, but it doesn't change based on why you used more copper.

 

[Smart $]

...The physics of the issue doesn't care why you used a larger conductor. Either a larger conductor requires a larger EGC or it doesn't, but it doesn't change based on why you used more copper.

I agree.

I fail to see how so many arrived at the upsize rationale. Speaking in terms of physics rather than Code, using a larger circuit conductor for reasons other than voltage drop would allow a smaller EGC. Total ground-fault impedance is the key factor for the rule... and to begin with, it was solely for purposes of voltage drop where a wire type EGC was required (such as when a nonmetallic wiring method was used) and no other reason. Someone proposed "for any reason" along the way, the CMP jumped on board with no real substantiation, and here we are... :blink:

Other than voltage drop reasons, I do agree with the rationale that someone could upsize the OCPD in the future and the EGC may be undersized.... but then again, if the EGC would be undersized, you can't upsize the OCPD. :slaphead:

 

[luckylerado]

Respectfully, The authors do not invoke "Voltage Drop" very often and when they do it is written in clear and enforceable terms. eg. Art. 647, 690, 695.

Please help me to understand your position.

My stance is that the EGC must be increased when ungrounded conductors are increased for any reason. Please follow my line of thinking for a minute.


Given info:
240V Branch circuit, 100 amp OCP with 75 deg lugs
#3 THHN supply conductors under normal conditions
Ambient 141 deg F (65%)
4 current carrying conductors (80%)

After applying correction factors for temp and de-rating for 4 current carrying conductors, it is determined that a 2/0 THHN copper conductor is the "minimum size that has sufficient ampacity for the intended installation"

I believe that you contend that in this case the EGC sized per 250.122 remains a min #8 copper as the 2/0 ungrounded conductors have not been increased from the minimum size with sufficient ampacity.

Now move that circuit to a distance at which the voltage drops to 96% and the 2/0 copper is recommended to be increased to a 3/0. This is when, I believe, you contend that it would be required to apply 250.122(b) and upsize the EGC?

Do you upsize by the ratio of increase from 2/0 to 3/0 or the ratio of increase from #3 to 3/0?


Now consider the same set of givens except that the ambient temp is normal and there are 3 current carrying conductors:
The "minimum size that has sufficient ampacity for the intended installation" becomes a #3 THHN ungrounded with the same #8 EGC.


Move the circuit to the same distance eluded to above. Nothing else has changed except the ambient temp and # of current carrying conductors, which have zero effect on voltage drop (2KDI/cm),therefore the same 3/0 supply conductors from above would be recommended to compensate for voltage drop.

Now the ratio by which we upsize the EGC becomes #3 to 3/0?

Make it a feeder. Would I be in compliance if I select an EGC based on conductors that were sized using the total connected load in the VD calculation instead of the OCP rating. Say 20 amp connected load but a capacity to serve 100 amps and what effect does that have on the required size of the EGC.

What if I decide that I simply do not want to size up the supply conductors for voltage drop because it is no more enforceable in 250 or 310 than the commentary found in the handbook.

What if I install 3/0 not because I calculated a 4% voltage drop but instead just because I had some laying around in stock and I wanted to use it up.

If 250.122(B) only applies when upsizing for voltage drop, does it really even have any teeth except as when an AHJ points to 90.4 and says "because I said so"?

 

[Smart $]

... This is when, I believe, you contend that it would be required to apply 250.122(b) and upsize the EGC?

Do you upsize by the ratio of increase from 2/0 to 3/0 or the ratio of increase from #3 to 3/0?

The former.

Now the ratio by which we upsize the EGC becomes #3 to 3/0?

Correct.

Make it a feeder. Would I be in compliance if I select an EGC based on conductors that were sized using the total connected load in the VD calculation instead of the OCP rating. Say 20 amp connected load but a capacity to serve 100 amps and what effect does that have on the required size of the EGC.

If you have an 100A OCPD supplying a 20A load, you still need to run conductors with an ampacity of 91A. Whatever the minimum size is that complies is the base size for EGC upsize determination.

What if I decide that I simply do not want to size up the supply conductors for voltage drop because it is no more enforceable in 250 or 310 than the commentary found in the handbook.

Then you do not have to upsize the EGC.

What if I install 3/0 not because I calculated a 4% voltage drop but instead just because I had some laying around in stock and I wanted to use it up.

If you're back to the first provided example, then you would have to upsize the EGC per Code, but the physics of the installation do not warrant upsizing the EGC.

If 250.122(B) only applies when upsizing for voltage drop, does it really even have any teeth except as when an AHJ points to 90.4 and says "because I said so"?

No one is saying it only applies when upsizing for voltage drop... but that is how it started, back in circa 70's Code, and only where a wire-type EGC was required.

 

[luckylerado]

Prior to the 2014 I would agree with the NECH opinion but the new wording in the 2014 is clear that the increase in size does not apply to derating because the conductor needs to be up-sized to meet the size of the OCPD. I would guess that since the wording in the 2011 NECH is seemingly the same that the authors failed to change their commentary to reflect the new code wording in the 2014.

Ahh, the clarity that I so desire. Why was this ever changed. Touch? :happysad:

Commentary (ya, I know) From 2011 handbook.

250.122(B)

"The general requirement for selecting the minimum size
equipment grounding conductor is to select directly from
Table 250.122 based on the rating or setting of the feeder or
branch-circuit overcurrent protective device(s). Where the
ungrounded circuit conductors are increased in size to compensate
for voltage drop or for any other reason related to
proper circuit operation, 250.122(B) requires that the equipment
grounding conductors be increased proportionately.
Increases in ungrounded circuit conductor size for the purposes
of ampacity adjustment, correction, or both are not
required to be considered in applying the provisions of
250.122(B). In some cases, use of a conductor with a higher
insulation temperature rating allows for compliance with
ampacity adjustment and correction requirements without
having to increase the circular mil area of the conductor. "