250.122(B) increasing ground size again

[Strathead]

This paragraph is so poorly written, I can't believe it stands! There are so many situations where there is no benefit or justification to increase the grounding conductor size just because the ungrounded conductors are sized larger. In what universe does it make sense that 50 foot run of 3-3/0 THWN and a #6 ground are OK for a 200A breaker, but change the breaker to 150A and the ground has to somehow be larger? Or A metal conduit run where I can pull the ground out of the conduit and be legal, but if I leave it in, it needs to increase in size?

That is a rant however. Coming up against this code section yet again, I had a couple other questions that it doesn't properly address. First, when are the ungrounded conductors considered increased? If, for example, I don't take advantage of the 240.4 exception, is that increasing the size? If I don't have proof that the lugs are rated at 75º so I use the 60º table then find out later that the lugs are rated at 75 have I violated the code because my conductors are larger?

So on a serious note. In your opinion, can we use the 60º table ampacities and the actual current rating without taking exception 240.4 to determine whether the conductors are oversized? If not, why not? Take the 1/0 example. If I use the 75º table to determine the ampacity, then the ground must be increase to #4. If I use the 60º table which has the exact same cross section of current carrying copper, then I can go all the way down to a 125A breaker without oversizing the ground conductor.

 

[iwire]

It may change in 2017 but if you price the work correctly you make more money when the work costs more.

 

[Strathead]

It may change in 2017 but if you price the work correctly you make more money when the work costs more.

That is true, but we are all trying to do things as economically as possible, from the lender to the customer to the contractor. The 200A to 150A breaker example was a real life example. The AC unit was spec'd at 200A. It arrived requiring a 150A MOCP. The feeders were already installed over 100 feet through three floors. There is no universe where it makes sense that we had to pull new feeders, or a new ground in order to downsize the breaker. Even though I get paid for it, I don't like a customer getting raked over the coals like that.

 

[don_resqcapt19]

I still say that the EGC should be sized off a table that works like T250.66...that is the EGC size is related to the size of the ungrounded conductors and not to the rating of the OCPD....that makes this issue go away. I know that there was a proposal a couple of cycles ago to do this, but CMP 5 rejected it.

My understanding is that the 2017 will go back to the old wording where the EGC is increased in size only if the circuit conductors were increased in size because of voltage drop. That still doesn't make any sense....in fact it probably makes more sense to have a larger EGC on circuits where the ungrounded conductors are just oversized without a voltage drop issue....there will be more fault current for the EGC to clear in that case as opposed to where the ungrounded conductors were increased in size because of voltage drop.

It also gets us back to the issue of the installer saying..."no, Mr. inspector, the size increase was not because of a voltage drop issue...I just ran out of the smaller wire, and I had the larger wire on site". One of the very reasons, the rule was changed a few cycles ago to apply to any size increase and not just size increases because of voltage drop.

 

[electrofelon]

That is true, but we are all trying to do things as economically as possible, from the lender to the customer to the contractor.

I hear you on your complaints, however considering the above statement, perhaps not pulling a wire EGC at all in many cases is a good option. I understand plans/specs may call for it.

 

[iwire]

That is true, but we are all trying to do things as economically as possible, from the lender to the customer to the contractor.

To an extent sure. But if it is a code rule it is a code rule.

Even though I get paid for it, I don't like a customer getting raked over the coals like that.

In this case the customer is not getting raked over by the code they got raked over by miscommunication over the feeder size. (Not saying it was you, just saying someone dropped the ball or made an assumption.)

 

[Carultch]

I still say that the EGC should be sized off a table that works like T250.66...that is the EGC size is related to the size of the ungrounded conductors and not to the rating of the OCPD....that makes this issue go away. I know that there was a proposal a couple of cycles ago to do this, but CMP 5 rejected it.

My understanding is that the 2017 will go back to the old wording where the EGC is increased in size only if the circuit conductors were increased in size because of voltage drop. That still doesn't make any sense....in fact it probably makes more sense to have a larger EGC on circuits where the ungrounded conductors are just oversized without a voltage drop issue....there will be more fault current for the EGC to clear in that case as opposed to where the ungrounded conductors were increased in size because of voltage drop.

It also gets us back to the issue of the installer saying..."no, Mr. inspector, the size increase was not because of a voltage drop issue...I just ran out of the smaller wire, and I had the larger wire on site". One of the very reasons, the rule was changed a few cycles ago to apply to any size increase and not just size increases because of voltage drop.

There's also "more fault current" on a shorter length circuit, than a long circuit of the same conductor size.

Larger conductors do not inherently increase the available fault current, above what is already available on a circuit of negligible length. They just don't decrease it as much as a smaller conductor would.

 

[jap]

This paragraph is so poorly written, I can't believe it stands! There are so many situations where there is no benefit or justification to increase the grounding conductor size just because the ungrounded conductors are sized larger. In what universe does it make sense that 50 foot run of 3-3/0 THWN and a #6 ground are OK for a 200A breaker, but change the breaker to 150A and the ground has to somehow be larger? Or A metal conduit run where I can pull the ground out of the conduit and be legal, but if I leave it in, it needs to increase in size?

That is a rant however. Coming up against this code section yet again, I had a couple other questions that it doesn't properly address. First, when are the ungrounded conductors considered increased? If, for example, I don't take advantage of the 240.4 exception, is that increasing the size? If I don't have proof that the lugs are rated at 75º so I use the 60º table then find out later that the lugs are rated at 75 have I violated the code because my conductors are larger?

So on a serious note. In your opinion, can we use the 60º table ampacities and the actual current rating without taking exception 240.4 to determine whether the conductors are oversized? If not, why not? Take the 1/0 example. If I use the 75º table to determine the ampacity, then the ground must be increase to #4. If I use the 60º table which has the exact same cross section of current carrying copper, then I can go all the way down to a 125A breaker without oversizing the ground conductor.

If you read it and apply it "Word Specific" like many say you need to read the code, you don't have to increase the EGC just because you changed the Breaker from a 200 Amp to a 150 amp if the 200 amp breaker was existing to begin with.

Technically you wouldn't be "Increase the wire size" the wire was already there. You would be "Reducing the size of the existing breaker"

At least that's the way I read it and I think I could defend it.

JAP>

 

[Carultch]

There's also "more fault current" on a shorter length circuit, than a long circuit of the same conductor size.

Larger conductors do not inherently increase the available fault current, above what is already available on a circuit of negligible length. They just don't decrease it as much as a smaller conductor would.

If you don't understand what I am saying, I'm trying to say that it is misleading to say that larger conductors increase fault current.

Try the following calculations. Given 5 kA available at the source (transformer secondary), of a 120/240V single phase system. All copper conductor in PVC conduit.

Calculate the fault current for the load at the end of the following circuits:
A: 2/0 conductors, 10 ft circuit length
B: 2/0 conductors, 100 ft circuit length
C: 300 kcmil conductors, 100 ft circuit length

 

[GoldDigger]

I wonder if putting a 150A breaker or fuse at the equipment end of the feeder and keeping the 200 at source would also resolve the code issue? The feeder is no longer "oversized".

 

[Strathead]

I wonder if putting a 150A breaker or fuse at the equipment end of the feeder and keeping the 200 at source would also resolve the code issue? The feeder is no longer "oversized".

IMO, it would, but a non-fused disconnect was already installed.

 

[luckylerado]

I wonder if putting a 150A breaker or fuse at the equipment end of the feeder and keeping the 200 at source would also resolve the code issue? The feeder is no longer "oversized".

That or cherry pick a worst case design temperature.

 

[Strathead]

To this point no one has taken up the question from the perspective of "when the conductor is oversized." I think there is a valid argument that if the conductor is not oversized using the 60 degree table then it isn't oversized.

 

[luckylerado]

To this point no one has taken up the question from the perspective of "when the conductor is oversized." I think there is a valid argument that if the conductor is not oversized using the 60 degree table then it isn't oversized.

Ok, but for equipment rated over 100 amp we are told to assume 75° terminations so sizing from 60° unless otherwise required would make it oversized no?.

 

[Carultch]

Ok, but for equipment rated over 100 amp we are told to assume 75° terminations so sizing from 60° unless otherwise required would make it oversized no?.

NEC2014 says "the minimum size that has sufficient ampacity for the intended installation". Which would appear to mean that it is oversized, if it is sized at 60C when only needed to be sized at 75C.

 

[don_resqcapt19]

NEC2014 says "the minimum size that has sufficient ampacity for the intended installation". Which would appear to mean that it is oversized, if it is sized at 60C when only needed to be sized at 75C.

I agree...if it is larger than the code minimum for the application, it has been increased in size.

 

[Sierrasparky]

NEC2014 says "the minimum size that has sufficient ampacity for the intended installation". Which would appear to mean that it is oversized, if it is sized at 60C when only needed to be sized at 75C.

So now we have the choice police.

 

[Strathead]

Ok, but for equipment rated over 100 amp we are told to assume 75° terminations so sizing from 60° unless otherwise required would make it oversized no?.

This is sort of a head spinner. I am not saying we do anything that has to do with the size of the ungrounded conductors with the 60º table.

Here is my potential argument a little more comprehensively:

The ground is installed to carry enough current to hopefully trip the breaker in a fault condition. That means the code section is concerned with the circular area of the conductors not
the insulation properties. As such there is not reason we shouldn't be able to use the most friendly information regarding the conductors that is available.

 

[Carultch]

This is sort of a head spinner. I am not saying we do anything that has to do with the size of the ungrounded conductors with the 60º table.

Here is my potential argument a little more comprehensively:

The ground is installed to carry enough current to hopefully trip the breaker in a fault condition. That means the code section is concerned with the circular area of the conductors not
the insulation properties. As such there is not reason we shouldn't be able to use the most friendly information regarding the conductors that is available.

A situation I've wondered about is, when increasing size to curtail voltage drop will prompt a change in the number of parallel sets.

For instance, consider a 400A circuit which ordinarily might have a single set of 600 kcmil copper. Or it could be 2 sets of 4/0 copper in the same conduit. Then suppose that voltage drop curtailment will prompt an increase to 2 sets of 400 kcmil copper. The upsize ratio could either be interpreted as (2*400)/600 = 1.33, or (2*400)/(2*212) = 1.88. So the correct EGC could either be interpreted as #1 Cu or #1/0 Cu, depending on what you call "the minimum size that has sufficient ampacity for the intended installation".

 

[luckylerado]

A situation I've wondered about is, when increasing size to curtail voltage drop will prompt a change in the number of parallel sets.

For instance, consider a 400A circuit which ordinarily might have a single set of 600 kcmil copper. Or it could be 2 sets of 4/0 copper in the same conduit. Then suppose that voltage drop curtailment will prompt an increase to 2 sets of 400 kcmil copper. The upsize ratio could either be interpreted as (2*400)/600 = 1.33, or (2*400)/(2*212) = 1.88. So the correct EGC could either be interpreted as #1 Cu or #1/0 Cu, depending on what you call "the minimum size that has sufficient ampacity for the intended installation".

Would the parralell 4/0 combined circuilar mills not be less then a 600 Kcmil making it the "min" in this scenario?