EGC size

[NoahsArc]

So what's the worst-case scenario here: someone upsized conductors for a 20A circuit to 250 kcmils. Keeps a 12ga ground. There's a short, are the grounds going to be less capable of getting sufficient current through the OCPD now?
I'm not seeing it I guess.

I get the technical language aspect, but I think any qualified person would disagree on the OP's instance, and I don't really understand the issue at all apparently.

 

[wwhitney]

You have to physically do something to increase the size of a conductor.

No, you don't. That's not what "increased" means in that sentence. It just means A > B.

"Increased from X" = "Larger than X".

Cheers, Wayne

 

[jap]

No, you don't. That's not what "increased" means in that sentence. It just means A > B.

"Increased from X" = "Larger than X".

Cheers, Wayne

I guess we'll just have to agree to disagree.

JAP>

 

[jap]

So what's the worst-case scenario here: someone upsized conductors for a 20A circuit to 250 kcmils. Keeps a 12ga ground. There's a short, are the grounds going to be less capable of getting sufficient current through the OCPD now?
I'm not seeing it I guess.

I get the technical language aspect, but I think any qualified person would disagree on the OP's instance, and I don't really understand the issue at all apparently.

You would not be able to upsize the ungrounded conductors on a 20a circuit to 250mcm and keep the 12ga EGC.

JAP>

 

[wwhitney]

So what's the worst-case scenario here: someone upsized conductors for a 20A circuit to 250 kcmils. Keeps a 12ga ground. There's a short, are the grounds going to be less capable of getting sufficient current through the OCPD now?
I'm not seeing it I guess.

The worst case is that they did that because the run is 4,000 ft long (the DC resistance of 250 kcmil Cu is about 1/40 of that of 12 AWG solid copper, so that would be like using #12 Cu ungrounded for a 100ft run). Now the 4,000 ft #12 Cu EGC has a resistance of 8 ohms. If you get an end of line fault to EGC, at 120V you only get 15A fault current. The 20A breaker never trips.

Cheers, Wayne

 

[NoahsArc]

The worst case is that they did that because the run is 4,000 ft long (the DC resistance of 250 kcmil Cu is about 1/40 of that of 12 AWG solid copper, so that would be like using #12 Cu ungrounded for a 100ft run). Now the 4,000 ft #12 Cu EGC has a resistance of 8 ohms. If you get an end of line fault to EGC, at 120V you only get 15A fault current. The 20A breaker never trips.

Cheers, Wayne

Makes sense, maybe just say "manage vdrop for your EGC" instead?
I don't get why the actual issue is obscured for something that is indirectly treating the issue and causing issues elsewhere...

 

[wwhitney]

You have to physically do something to increase the size of a conductor or install it.

If I have a design on paper with a 50' run and minimum size conductors, and I change the design to a 500' run and upsize the conductors for voltage drop, that increase doesn't physically do anything, either. It's just on paper.

The rule explicitly tells you what to compare the installed size to, it doesn't require any physical change to the conductor. The change here was to the "minimum size that has sufficient ampacity," because the OCPD size changed.

Cheers, Wayne

 

[Tulsa Electrician]

I have 60A fused disconnect which has 60A fuse and it is being fed from #6 awg phase/neutral with #10 awg ground.

Now they are replacing 60 fuse with 30A fuse. Do they have to replace equipment grounding conductor from #10 awg to #6 awg NEC 2017 Article 250.122(B)?

Here is how I look at it.
Replacing the fuses to a lower ampacity does affect the feeder conductors it's self. As long as the 60 amp breaker for the feeder is used. The conductors between the breaker and line side of disconnect are not increased in size. Once you put 30 amp fuses in the disconnect the rule would apply from the load side of the disconnect to the load.

I look at it like a ML panel with a 100 amp feeder and 30 amp branch circuit. With only one 30 amp breaker installed.

 

[jap]

If I have a design on paper with a 50' run and minimum size conductors, and I change the design to a 500' run and upsize the conductors for voltage drop, that increase doesn't physically do anything, either. It's just on paper.

The rule explicitly tells you what to compare the installed size to, it doesn't require any physical change to the conductor. The change here was to the "minimum size that has sufficient ampacity," because the OCPD size changed.

Cheers, Wayne

Exactly my point.
You physically have to "install" those conductors to make the rule apply.

If I have a 4 ft gate ( the conductor) and drive a 42" lawnmower ( the amperage) through it, its a 4 ft gate.
If I drive a 30" lawnmower through it, it's a 4 ft gate.
If I push a 20" lawnmower through it, it's a 4 ft gate.
If I carry a weed eater through it, it's a 4 ft gate.
If I walk through the gate, it's a 4 ft gate.

The gate does not magically get "larger" just because something smaller in going through it.

Now,

Should you decide to change the size of the gate, that's a different story.

JAP>

 

[jap]

Here is how I look at it.
Replacing the fuses to a lower ampacity does affect the feeder conductors it's self. As long as the 60 amp breaker for the feeder is used. The conductors between the breaker and line side of disconnect are not increased in size. Once you put 30 amp fuses in the disconnect the rule would apply from the load side of the disconnect to the load.

I look at it like a ML panel with a 100 amp feeder and 30 amp branch circuit. With only one 30 amp breaker installed.

What good is an upsized EGC going to do on the load side of a disconnect to the load if the line side EGC does not have the capacity of the load side EGC?

JAP>

 

[wwhitney]

Exactly my point.
You physically have to "install" those conductors to make the rule apply.

This interpretation contradicts what the rule says in plain language. It tells you to compare the installed conductor size with the smallest size that could have been installed NEC-compliantly. You always compare the installed reality with a hypothetical; never the installed reality with a past reality.

Cheers, Wayne

 

[jap]

This interpretation contradicts what the rule says in plain language. It tells you to compare the installed conductor size with the smallest size that could have been installed NEC-compliantly. You always compare the installed reality with a hypothetical; never the installed reality with a past reality.

Cheers, Wayne

I don't see the word compare stated anywhere in the rule.

The rule says when conductors are increased in size.

You do not increase the size of conductor by installing a fuse.

JAP>

 

[wwhitney]

I don't see the word compare stated anywhere in the rule.

"Increased from" is a comparison. You are completely ignoring the "from" phrase.

Cheers, Wayne

 

[hhsting]

There should be a way to do voltage drop calcs for equipment grounding conductor so that they are sufficient size with respect to when increasing phase/neutral. NEC should focus on developing those equations and let the designer handle it.

 

[augie47]

There are methods to address that. No doubt some of the engineers here can advise you or you might find a method in the Soares Book on Grounding

 

[jap]

If he decides to tap that 60 amp feeder, what size EGC would he need to install to his 30a ECB or fused disconnect?

Jap>

 

[jaggedben]

Stupidest rule in the book. It needs to say increased in size to account for voltage drop.

 

[jap]

No, you don't. That's not what "increased" means in that sentence. It just means A > B.

"Increased from X" = "Larger than X".

Cheers, Wayne

This whole time I've been picturing a 60 amp fused disconnect fed with #6 awg phase/neutral with #10 awg ground.

I am assuming there are no load side #6 ga. conductors at all.
I'm assuming they are installing #10 ga conductors from the load side of the disconnect to the load and installing 30a fuses.

Is that the way you are picturing it?

If #10ga conductors are installed from the load side of the disconnect to the load, with 30a fuses installed, I don't see any reason why the EGC would have to be upsized.

Maybe that's where we aren't coming together.

JAP>

 

[wwhitney]

This whole time I've been picturing a 60 amp fused disconnect fed with #6 awg phase/neutral with #10 awg ground.

I agree that obviously the size of an EGC depends on the upstream (supply-side) OCPD size, not on the size of any downstream (load-side) OCPD.

Therefore I think I and most of the other responders assumed the OP was about the size of the EGC downstream of the fused disconnect.

But you may be correct that the OP is about the upstream EGC size to the fused disconnect. In which case post #28 is the first answer with a correct explanation.

Cheers, Wayne

 

[jap]

So, I agree that obviously the size of an EGC depends on the upstream (supply-side) OCPD size, not on the size of any downstream (load-side) OCPD.

Therefore I think I and most of the other responders assumed the OP was about the size of the EGC downstream of the fused disconnect.

But you may be correct that the OP is about the upstream EGC size to the fused disconnect. In which case post #28 is the first correct answer.

Cheers, Wayne

If the OP was to leave the #6 conductors on the load side of the disconnect in play, and, install 30a fuses, I could see how the rule would come into play, but, not if a 30a fuse was installed and the load side conductors were changed to 30a rated conductors.

If that wasn't the case, the buss bars in our panels are rated for much more than than most of the Main OCPD's installed in them, and, we'd have to calculate our EGC for our branch circuits base on the current carrying capacity of the buss bars instead of the amperage of the Branch Circuit device.

Which would make no sense at all.

JAP>