EGC sizing

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ggunn

PE (Electrical), NABCEP certified
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I know there is a thread somewhere concerning this but I can't find it.

Under 250.122(B), when one increases the CCC size because the COU derated ampacity of the original conductor choice is less than the next size down OCPD from the one on the circuit, does one have to increase the size of the EGC? I think not; it seems to me that the original choice does not meet "the minimum size that has sufficient ampacity for the intended installation", but whaddya think?
 
ggunn said:
I know there is a thread somewhere concerning this but I can't find it.

Under 250.122(B), when one increases the CCC size because the COU derated ampacity of the original conductor choice is less than the next size down OCPD from the one on the circuit, does one have to increase the size of the EGC? I think not; it seems to me that the original choice does not meet "the minimum size that has sufficient ampacity for the intended installation", but whaddya think?
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If you increase the size of the CCC's because of derate factors (ambient, rooftop adder, or bundling), you do not need to correspondingly increase the size of the EGC. Because you didn't increase above the "minimum size that has sufficient ampacity for the intended installation". To say that in less words, think of it in terms of the "minimum local size". The minimum size assuming length is not a significant design factor.

The intent of requring this increase in size, is to minimize excessive resistance in long length conductors. Therefore, it is only when you have to increase in size for curtailing voltage drop, that there is a physical reason for having to increase the EGC proportionally by the ratio of KCMIL.

There are other cases, where by omission, it seems you still might have to increase the size of the EGC to meet the wording of the NEC. Note that there is no physical reason for needing to do so. Just a need to meet the text of the NEC.
1. Using larger than necessary wire because that is what you had available, or what you want to use up from a previous job.
2. Manufacturers' instructions prescribe larger wire than necessary for ampacity and OCPD.
3. Specifying a wire from the 60C column out of ignorance, only to find out that you could've used the 75C column.
4. Not taking advantage of 240.4(B), "the next size up rule".
 
Carultch said:
If you increase the size of the CCC's because of derate factors (ambient, rooftop adder, or bundling), you do not need to correspondingly increase the size of the EGC. Because you didn't increase above the "minimum size that has sufficient ampacity for the intended installation". To say that in less words, think of it in terms of the "minimum local size". The minimum size assuming length is not a significant design factor.

The intent of requring this increase in size, is to minimize excessive resistance in long length conductors. Therefore, it is only when you have to increase in size for curtailing voltage drop, that there is a physical reason for having to increase the EGC proportionally by the ratio of KCMIL.

There are other cases, where by omission, it seems you still might have to increase the size of the EGC to meet the wording of the NEC. Note that there is no physical reason for needing to do so. Just a need to meet the text of the NEC.
1. Using larger than necessary wire because that is what you had available, or what you want to use up from a previous job.
2. Manufacturers' instructions prescribe larger wire than necessary for ampacity and OCPD.
3. Specifying a wire from the 60C column out of ignorance, only to find out that you could've used the 75C column.
4. Not taking advantage of 240.4(B), "the next size up rule".
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Great minds think alike. :D
 
Carultch said:
...
4. Not taking advantage of 240.4(B), "the next size up rule".
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I do not believe #4 is a reason for having to upsize the EGC. 240.4(B) is "permitted"... not required.

Debatable, yes. Just haven't seen any debates over it yet. ;)
 
Smart $ said:
I do not believe #4 is a reason for having to upsize the EGC. 240.4(B) is "permitted"... not required.

Debatable, yes. Just haven't seen any debates over it yet. ;)
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Wanna start one? I'll take either side. :D
 
ggunn said:
Wanna start one? I'll take either side. :D
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I don't even know why we are allowed to use the next size up rule in the first place. Intuitively, I would think that the OCPD needs to be the weak link, so that it trips before the wire fails.

It is like saying you can build a 95 psi pressure vessel with a 100 psi relief valve and call it safe. Simply because you cannot find a 95 psi relief valve and you don't ever expect the vessel to be loaded to more than 95 psi. One would think that the relief valve should open before the vessel walls exceed its allowable load.
 
Carultch said:
I don't even know why we are allowed to use the next size up rule in the first place. Intuitively, I would think that the OCPD needs to be the weak link, so that it trips before the wire fails.
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1. The wire ampacities listed in the NEC are pretty conservative.

2. The load amps cannot exceed the ampacity of the conductors.

3. Even when using the next size up, the conductors are easily still adequately protected against faults.
 
Carultch said:
I don't even know why we are allowed to use the next size up rule in the first place. ...
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In the first place, because Code says we can...

In the second place, it also says we do not have to, so that justifies first place...

In the third place, you got lost. See both first and second place. :D
 
Carultch said:
I don't even know why we are allowed to use the next size up rule in the first place. Intuitively, I would think that the OCPD needs to be the weak link, so that it trips before the wire fails.

It is like saying you can build a 95 psi pressure vessel with a 100 psi relief valve and call it safe. Simply because you cannot find a 95 psi relief valve and you don't ever expect the vessel to be loaded to more than 95 psi. One would think that the relief valve should open before the vessel walls exceed its allowable load.
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It is not saying that at all. The analogy does not hold up because a breaker acts differently than pressure valve. What the next size up rule says is that if you have 148A of load you need 148A worth of wire but you can put it on a 150A breaker. The wire is protected at its ampacity by the load calc and any short circuit or ground fault is going to be a bazillion times more than 150A anyway so there is no real reason to use a smaller size.
 
ActionDave said:
Jumper's reply was not speculation, it was all facts.
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Two of his facts are quite subjective... but even if all three were facts, stating these as the reason for allowing the next size up is still speculation unless he can offer up the authoritative documentation to back it up.

:happyyes:
 
Smart $ said:
Two of his facts are quite subjective... but even if all three were facts, stating these as the reason for allowing the next size up is still speculation unless he can offer up the authoritative documentation to back it up.

:happyyes:
Click to expand...

Serously you don't have to be so damn pedantic, it helps no one.
 
iwire said:
Serously you don't have to be so damn pedantic, it helps no one.
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Your opinion, not mine.

What was that you said about not bothering you with facts (jokingly)... :D

Just saying it is what it is. Let's treat speculation as speculation. To treat as fact is how myths start and promulgate.
 
Always fun to see a good argument over code. One thing that most people overlook is that for low voltage conductors the OCPD provides two levels of protection, the first is from overload, the second is from fault current. If you look up the OCPD requirements for high voltage conductor where the OCPD does not protect from overload you will find that the OCPD values are 5 to 6 times the conductor's ampacity because it can easily carry that much fault current for the time it takes the OCPD to operate without damage. So the upsize OCPD allowance is only affecting the low voltage conductor overload protection and that's not as rigidly defined. Remember there's no real protection against running a conductor at 100% of it's rating for more than 3 hours, an overload condition, other than by design.

I have never seen anyone do the calculation to size EGCs correctly anyway. Everyone I know just looks up a value in 250.122 and upsizes it if the CCC is upsized. Most people do not even know there is a note on that table that says, "Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table." This table is just a hard minimum, not a way to select the correct size EGC, and by its own admission is not conservative. The only way to correctly size the EGC is to do the fault current impedance calculation and match it to the OCPD trip requirements to make sure the EGC is going to be able to carry enough fault current to trip the OCPD.
 
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