How to Correctly Upsize EGC

[Sunny_92]

What is the correct way to upsize the EGC when you add extra sets of wire to account for voltage drop? Please tell me if I’m doing it correctly in the example below.

The load being fed requires a 1600A circuit, which initially would require a 1600A breaker and (4) sets of (3) #600 CU & (1) #4/0 GRD. However, the feeder must be increased to (8) sets of #600 CU to meet voltage drop requirements.

Since the total area of the ungrounded conductors was doubled, the equipment grounding conductor must be doubled as well. So then, would the appropriate feeder be (8) sets of (3) #600 CU & (2) #4/0 GRD?

Or would you still use (1) #4/0 EGC per conduit, because the total area of all the EGCs is greater than (2) #4/0?

 

[steve66]

For parallel runs, you need a full size ground in each conduit. Since you are doubling the feeder size, you have to double the grounds.


That would be two 4/0 in each conduit, or using table 8 in chapter 9, that could be one 500 kcm ground in each conduit.

 

[Sunny_92]

For parallel runs, you need a full size ground in each conduit. Since you are doubling the feeder size, you have to double the grounds.


That would be two 4/0 in each conduit, or using table 8 in chapter 9, that could be one 500 kcm ground in each conduit.

That clears things up, thank you!

 

[Carultch]

What is the correct way to upsize the EGC when you add extra sets of wire to account for voltage drop? Please tell me if I’m doing it correctly in the example below.

The load being fed requires a 1600A circuit, which initially would require a 1600A breaker and (4) sets of (3) #600 CU & (1) #4/0 GRD. However, the feeder must be increased to (8) sets of #600 CU to meet voltage drop requirements.

Since the total area of the ungrounded conductors was doubled, the equipment grounding conductor must be doubled as well. So then, would the appropriate feeder be (8) sets of (3) #600 CU & (2) #4/0 GRD?

Or would you still use (1) #4/0 EGC per conduit, because the total area of all the EGCs is greater than (2) #4/0?

This is a blindspot in the NEC that I wonder about too. Since you are changing the number of sets for curtailing voltage drop, you will get a different answer depending on what you consider your starting point size. Is your starting point size 4 sets of 600 kcmil? Or is your starting point size 8 sets of #3/0?

To meet the wording of the NEC without question, I would count the 8 sets of #3/0 as your starting poing size (minimum size that has sufficient ampacity for the intended installation), which would mean you have an upsize ratio of 600/168 = 3.57.

If the NEC intended to allow ANY configuration as "the minimum size that has sufficient ampacity for the intended installation", even if it is less sets than you are installing, I believe it would need different words to allow this, even though a 1 to 2 upsize ratio in this situation would make just as much sesne as a 1 to 3.57 ratio.

Now if you do end up increasing the quantity of sets so much, that each set would hypothetically be less than 1/0 as the minimum local size, then you would go no lower than 1/0 as your starting point for this calculation. Because even though 13 sets of #1 Cu would be 1600A, it would not be a legal installation. So your upsize ratio would be based on 13 sets of #1/0 to 13 sets of what you are installing instead.

 

[kwired]

For parallel runs, you need a full size ground in each conduit. Since you are doubling the feeder size, you have to double the grounds.


That would be two 4/0 in each conduit, or using table 8 in chapter 9, that could be one 500 kcm ground in each conduit.

Can you parallel two or more conductors to effectively make a larger conductor for use as an EGC? I know you can't for multiple cables/raceways as each needs to have full sized EGC. I am not really finding anything that specifically prohibits it but at same time not finding anything that specifically allows it either.

310.10(H) mentions ungrounded and grounded conductors in parallel but says nothing about equipment grounding conductors. I am finding nothing in 250 other then the need for full size EGC in each raceway or cable containing other conductors in parallel.

 

[Carultch]

Can you parallel two or more conductors to effectively make a larger conductor for use as an EGC? I know you can't for multiple cables/raceways as each needs to have full sized EGC. I am not really finding anything that specifically prohibits it but at same time not finding anything that specifically allows it either.

310.10(H) mentions ungrounded and grounded conductors in parallel but says nothing about equipment grounding conductors. I am finding nothing in 250 other then the need for full size EGC in each raceway or cable containing other conductors in parallel.

What calculation would you use to "add up" those EGC's?


Add up the kcmil?
Add up the 250.122 OCPD amp values?

 

[ggunn]

Can you parallel two or more conductors to effectively make a larger conductor for use as an EGC? I know you can't for multiple cables/raceways as each needs to have full sized EGC. I am not really finding anything that specifically prohibits it but at same time not finding anything that specifically allows it either.

310.10(H) mentions ungrounded and grounded conductors in parallel but says nothing about equipment grounding conductors. I am finding nothing in 250 other then the need for full size EGC in each raceway or cable containing other conductors in parallel.

I don't see why not as long as the conductors are large enough to parallel. I don't think that the NEC stipulates that they have to be CCC's.

 

[kwired]

What calculation would you use to "add up" those EGC's?


Add up the kcmil?
Add up the 250.122 OCPD amp values?

good question.

doubling kcmil - only allows you to use 2-1/0 in place of a single 4/0 - probably cost less to run the single 4/0
tripling kcmil allows you to use 3-1/0 in place of a single 300kcmil. That one might be six one way half a dozen the other, but maybe still leaning toward using the 300 instead of three 1/0.

 

[steve66]

This is a blindspot in the NEC that I wonder about too. Since you are changing the number of sets for curtailing voltage drop, you will get a different answer depending on what you consider your starting point size. Is your starting point size 4 sets of 600 kcmil? Or is your starting point size 8 sets of #3/0?

To meet the wording of the NEC without question, I would count the 8 sets of #3/0 as your starting poing size (minimum size that has sufficient ampacity for the intended installation), which would mean you have an upsize ratio of 600/168 = 3.57.

If the NEC intended to allow ANY configuration as "the minimum size that has sufficient ampacity for the intended installation", even if it is less sets than you are installing, I believe it would need different words to allow this, even though a 1 to 2 upsize ratio in this situation would make just as much sesne as a 1 to 3.57 ratio.

Now if you do end up increasing the quantity of sets so much, that each set would hypothetically be less than 1/0 as the minimum local size, then you would go no lower than 1/0 as your starting point for this calculation. Because even though 13 sets of #1 Cu would be 1600A, it would not be a legal installation. So your upsize ratio would be based on 13 sets of #1/0 to 13 sets of what you are installing instead.

Good point, and I agree, that would pretty much ensure you met any inspectors interpretation.

I guess my argument for doubling the grounding conductor is that the feeder COULD be (4) sets of 600KCM, and that is a minimum necessary to meet all the code requirements. So adding 4 more sets doubles the conductor KCM, so doubling the grounding conductor should be enough. Nothing in 250.122 says our calculation has to start and end with the same number of conduits.

Said another way, nothing in 250.122 says we have to start with any particular feeder configuration, so I should be free to choose any starting configuration that has just enough ampacity to match the OCP.

In many ways, I see this code requirement as overkill. We generally need one 4/0 to trip a 1600A OCP, if table 250.122 can be believed. But since we need multiple conduits, we now have four 4/0's. And since we are increasing the feeder sizes, we are up to eight 4/0's. Depending on how you do the calculations - maybe even twelve 4/0's.

If you happen to be running EMT, it seems like it might make sense to just use the conduit as the ground and forget all the headaches. I guess my point is that there should be an exception that if you don't need the ground wire in the first place (if the conduit is a legal ground), then you don't have to upsize the ground conductor for voltage drop.

 

[GoldDigger]

... I guess my point is that there should be an exception that if you don't need the ground wire in the first place (if the conduit is a legal ground), then you don't have to upsize the ground conductor for voltage drop.

Which would be at odds with the more general principle that even if you are not required to install a wire EGC, if you choose to install it it must meet all of the requirements for a wire EGC anyway.

 

[infinity]

IMO you cannot parallel two EGC's to make one larger one. I see nothing in T250.122 that says you can use an equivalent size with paralleled EGC's.

 

[Carultch]

Said another way, nothing in 250.122 says we have to start with any particular feeder configuration, so I should be free to choose any starting configuration that has just enough ampacity to match the OCP.

Functionally, I agree with that interpretation, and it is what I'd like to see spelled out in the NEC, so that it is not ambiguous on whether or not it should pass.

If you happen to be running EMT, it seems like it might make sense to just use the conduit as the ground and forget all the headaches. I guess my point is that there should be an exception that if you don't need the ground wire in the first place (if the conduit is a legal ground), then you don't have to upsize the ground conductor for voltage drop.

It may not be enforceable, but the steel tube institute does provide data on length limits for where metal raceways are reliable EGCs.
https://steeltubeinstitute.org/stee...eettheNECRequirementsofEquipmentGrounding.pdf

 

[kwired]

Good point, and I agree, that would pretty much ensure you met any inspectors interpretation.

I guess my argument for doubling the grounding conductor is that the feeder COULD be (4) sets of 600KCM, and that is a minimum necessary to meet all the code requirements. So adding 4 more sets doubles the conductor KCM, so doubling the grounding conductor should be enough. Nothing in 250.122 says our calculation has to start and end with the same number of conduits.

Said another way, nothing in 250.122 says we have to start with any particular feeder configuration, so I should be free to choose any starting configuration that has just enough ampacity to match the OCP.

In many ways, I see this code requirement as overkill. We generally need one 4/0 to trip a 1600A OCP, if table 250.122 can be believed. But since we need multiple conduits, we now have four 4/0's. And since we are increasing the feeder sizes, we are up to eight 4/0's. Depending on how you do the calculations - maybe even twelve 4/0's.

If you happen to be running EMT, it seems like it might make sense to just use the conduit as the ground and forget all the headaches. I guess my point is that there should be an exception that if you don't need the ground wire in the first place (if the conduit is a legal ground), then you don't have to upsize the ground conductor for voltage drop.

Overkill depends on the application. IMO the biggest issue is it is hard to write a one size fits all rule here. I have seen in field irrigation applications where a 1500+ foot run with a ground fault at the far end burns up equipment before it blows fuses (properly sized for the conductors for the overcurrent protection level. Too much resistance in that long of a run limits current in fault conditions. Bolted faults will eventually clear, but those arcing faults that clear a fuse in other applications don't pull enough current in this kind of application to blow the fuse and something else burns open first.

If you are using metal raceway as the ground and you need to double the size of ungrounded conductors, you will have bigger EGC anyway because of increased size of raceway or number of raceways needed, you just happen to bypass the need to calculate what size EGC is necessary.

 

[steve66]

IMO you cannot parallel two EGC's to make one larger one. I see nothing in T250.122 that says you can use an equivalent size with paralleled EGC's.

Looking at 310.1(H), I agree.

In my first post, I didn't see any reason why 310.1(H) wouldn't apply, but looking at it, there isn't anything in that paragraph that would include EGC's. (It does list conductors for "grounded circuits", but that would still be a current carrying wire, not an EGC. Correct?)

And 250.122 even has an exception for cables that allow the EGC's to be "sectioned" as long as their total cmils add up to meet table. But no such exception exists for conductors in conduit.

 

[ggunn]

Looking at 310.1(H), I agree.

In my first post, I didn't see any reason why 310.1(H) wouldn't apply, but looking at it, there isn't anything in that paragraph that would include EGC's. (It does list conductors for "grounded circuits", but that would still be a current carrying wire, not an EGC. Correct?)

And 250.122 even has an exception for cables that allow the EGC's to be "sectioned" as long as their total cmils add up to meet table. But no such exception exists for conductors in conduit.

Yeah, I saw that as well. I don't know why the rules for paralleling conductors would exclude grounding conductors, but they mention everything else by name.

 

[kwired]

Looking at 310.1(H), I agree.

In my first post, I didn't see any reason why 310.1(H) wouldn't apply, but looking at it, there isn't anything in that paragraph that would include EGC's. (It does list conductors for "grounded circuits", but that would still be a current carrying wire, not an EGC. Correct?)

And 250.122 even has an exception for cables that allow the EGC's to be "sectioned" as long as their total cmils add up to meet table. But no such exception exists for conductors in conduit.

Exactly what prompted me to say what I did in post 5. EGC's aren't included, but aren't specifically excluded either.

 

[Jberg73]

My way around this has always been upping the size of the EGC to meet whatever my conductors are not based on minimum required but based on actually used. I may be going overboard doing it that way, but I've never failed an inspection don't it like that.

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[kwired]

My way around this has always been upping the size of the EGC to meet whatever my conductors are not based on minimum required but based on actually used. I may be going overboard doing it that way, but I've never failed an inspection don't it like that.

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pretty simple with 15-30 amp circuits, the EGC has to be the same size as the ungrounded conductors. When you have over 30 amps it gets more complicated as well as more questionable on the logic at times. Take a 40 amp circuit with 8 AWG conductors and a 10 AWG EGC as "normal". Now increase to 6 AWG ungrounded conductors - as worded you should have to increase the EGC, even though if you had started out with a 60 amp circuit and 6 AWG, the #10 EGC is still "normal".

 

[Jberg73]

I'm commercial/industrial, I'm pulling 2500kva services.

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[kwired]

I'm commercial/industrial, I'm pulling 2500kva services.

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Is services all you do? No EGC until you get to the load side of a service, but not a lot of point to having a service if there is not going to be any feeders /branch circuits supplied.