Egc sizing

Djelite

Senior Member
Location
Ny
Occupation
Electrician
In the code book article 250.122(table) is used to size egc based on the ocpd rating. If you increase the ungrounded conductors size from its minimum required the egc must be resized as well. If you change the ocpd rating to a higher one you dont need to increases ocpd rating, why?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
One simple reason: someone could see the larger wires and decide to put in a larger breaker.

Another reason is to make sure the fault current is high enough to operate the OCPD quickly.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
Licensed Electrician
One simple reason: someone could see the larger wires and decide to put in a larger breaker.
And then it would be a compliant installation.
Another reason is to make sure the fault current is high enough to operate the OCPD quickly.
How can that be? I put 8 AWG on a 15A breaker with a 10 AWG egc or I put 8 AWG on a 50A breaker with the same 10 AWG egc. The former is a violation and the latter is compliant.

Now I barely made it past ninth grade algebra and I never took a science class in my life
 

Djelite

Senior Member
Location
Ny
Occupation
Electrician
One simple reason: someone could see the larger wires and decide to put in a larger breaker.

Another reason is to make sure the fault current is high enough to operate the OCPD
One simple reason: someone could see the larger wires and decide to put in a larger breaker.

Another reason is to make sure the fault current is high enough to operate the OCPD quickly.
But you are putting a larger ocpd on larger ungrounded conductors to avoid increasing the ocpd( due to voltage drop) even thou all egc are sized based on the rating of its ocpd
 
Location
Alabama
Occupation
Electrician
This actually comes up quite a bit. I have used 10 AWG on 20AMP homeruns where it was longer than 70ft. A recent inspector asked me to put some sort of identifier on the wire and breaker panel that the circuit should not exceed 20AMPS.
 

bwat

EE
Location
NC
Occupation
EE
There is no valid technical substantiation for 250.122(B).
...except for when the ungrounded conductors were increased for voltage drop due to long run.

Outside of that, agreed. The way it's worded can make for some very strange results, such as ActionDave's post #3.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...except for when the ungrounded conductors were increased for voltage drop due to long run.

Outside of that, agreed. The way it's worded can make for some very strange results, such as ActionDave's post #3.
Show me the math that supports that.

There have been many posts over the years that show there is no real technical substantiation for that rule, especially when doing what Dave said is compliant. If the table sized EGC is adequate to clear the circuit with a larger OCDP, then it is adequate to clear the circuit with the smaller OCPD and larger conductors.
 

Djelite

Senior Member
Location
Ny
Occupation
Electrician
S dsws are
Show me the math that supports that.

There have been many posts over the years that show there is no real technical substantiation for that rule, especially when doing what Dave said is compliant. If the table sized EGC is adequate to clear the circuit with a larger OCDP, then it is adequate to clear the circuit with the smaller OCPD and larger conductors.
exactly my point
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If the table sized EGC is adequate to clear the circuit with a larger OCDP, then it is adequate to clear the circuit with the smaller OCPD and larger conductors.
I agree and the concept of increasing the EGC is even more ridiculous when the raceway is metallic and qualifies as an EGC.
 

bwat

EE
Location
NC
Occupation
EE
If the table sized EGC is adequate to clear the circuit with a larger OCDP, then it is adequate to clear the circuit with the smaller OCPD and larger conductors.
Agreed. No argument there whatsoever. We're really talking about two different scenarios when you say it that way. The word "If" there is very important.

This is technically not a true statement: "If the EGC is adequately sized per the table and section 250.122, then the circuit is adequate to clear a fault". Put 10 AWG ungrounded and EGC on a very long 30A CB circuit and short them at the end. Yes, it was sized to code, but it won't clear.
 

kwired

Electron manager
Location
NE Nebraska
Agreed. No argument there whatsoever. We're really talking about two different scenarios when you say it that way. The word "If" there is very important.

This is technically not a true statement: "If the EGC is adequately sized per the table and section 250.122, then the circuit is adequate to clear a fault". Put 10 AWG ungrounded and EGC on a very long 30A CB circuit and short them at the end. Yes, it was sized to code, but it won't clear.
Won't clear is maybe wrong. Yes there is a point where you will limit current enough that it won't, but until you reach that length, you simply increase the time it takes to trip, primarily because you reduced the amount of fault current with the added resistance of the conductors.

Short length from source that maybe would trip nearly instantaneously might take many seconds to trip if it were a 1000 foot run, but eventually will trip as long as actual current is over the device rating, and something else doesn't fail first.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If the table sized EGC is adequate to clear the circuit with a larger OCDP, then it is adequate to clear the circuit with the smaller OCPD and larger conductors.
While the above is true, as pointed out in some installs the table sized EGC may not be adequate to clear the fault, whatever the OCPD size.

So a rule that uses the condition "if the ungrounded conductors are upsized due to VD" as a proxy for "situations where the EGC is really long and the table size may not be big enough, whatever size the OCPD is" has a technical substantiation.

However, if upsizing for VD is not a requirement, then relying on that proxy fails to satisfy the "practical safeguarding" intention of the NEC, since with the larger OCPD the EGC would be allowed to be downsized.

Cheers, Wayne
 
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