EGC Size Question

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Natfuelbilll

Senior Member
A similar situation/question.

The job calls for a #3 conductor on a 20A circuit. The engineer shows the correct size ground based on the proportional rule found in 250.122(B). The #3 was chosen to account for voltage drop.

The contractor wants to install #2 because of greater availability instead of the #3.

The 2014 NEC is clear about increasing the ground size but the 2014 NECHB commentary discusses this increase is needed based on "to compensate for voltage drop or for any other reason related to proper circuit operation" reasons. A "commercial" reason would not dictate an increased ground size...

The question is does the ground need to be adjusted again based on the contractors choice to install larger wire? Why or why not?
 
charlie b

charlie b

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This posted in a related thread, but I have moved it to a thread of its own.

Let us assume (absent any given information to the contrary) that the circuit is not installed in an environment exceeding a 30C ambient temperature, and that there are no more than 3 current-carrying conductors in the conduit. Therefore, given that the OCPD is a 20 amp breaker, I would infer that a #12 THHN wire would have sufficient ampacity for the circuit. The minimum EGC for a 20 amp circuit is a #12. If the wire size used is a #3, then the EGC must be increased proportionally. In this instance, the math is easy. The EGC must be a minimum of #3. If instead the wire size used is a #2, the EGC must be a minimum of #2.
 
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Carultch

Senior Member
Location
Massachusetts
Natfuelbilll said:
A similar situation/question.

The job calls for a #3 conductor on a 20A circuit. The engineer shows the correct size ground based on the proportional rule found in 250.122(B). The #3 was chosen to account for voltage drop.

The contractor wants to install #2 because of greater availability instead of the #3.

The 2014 NEC is clear about increasing the ground size but the 2014 NECHB commentary discusses this increase is needed based on "to compensate for voltage drop or for any other reason related to proper circuit operation" reasons. A "commercial" reason would not dictate an increased ground size...

The question is does the ground need to be adjusted again based on the contractors choice to install larger wire? Why or why not?
Click to expand...

Good question.

The intent of this rule is precisely to adjust the size of the EGC proportionally to any increase in the size of the main conductors for the purpose of curtailing voltage drop. However, no codes in the year of 2014 or prior have been written with language that excludes other reasons for increasing size for this rule. Such as using left over wire from a previous job, using larger wire due to availability, unifying the wire size with other circuits in the system, equipment manufacturers requiring oversized wire because of termination capacity, assuming 60C terminations even though 75C terminations are present, or others.

NEC2017 will specify for voltage drop only.
NEC2014 specifies any increase from "the minimum size that has sufficient ampacity for the intended installation".

In this particular case, when an inspector sees the configuration of the circuit, the inspector will not be able to know that the main wires only had to be increased to #3, and that #2 was selected for another reason. Therefore, the correct course of action is to select an EGC sized according to the size #2.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Carultch said:
Good question.

The intent of this rule is precisely to adjust the size of the EGC proportionally to any increase in the size of the main conductors for the purpose of curtailing voltage drop. However, no codes in the year of 2014 or prior have been written with language that excludes other reasons for increasing size for this rule. Such as using left over wire from a previous job, using larger wire due to availability, unifying the wire size with other circuits in the system, equipment manufacturers requiring oversized wire because of termination capacity, assuming 60C terminations even though 75C terminations are present, or others.

NEC2017 will specify for voltage drop only.
NEC2014 specifies any increase from "the minimum size that has sufficient ampacity for the intended installation".
Click to expand...
How would an inspector know that you increased the CCC size because that's what you had on the truck rather than to reduce the voltage drop from (for example) 3% to 2%? You got the benefit of that Vd reduction whether you wanted it or not and a fault has lower impedance irrespective of your motivation.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
ggunn said:
How would an inspector know that you increased the CCC size because that's what you had on the truck rather than to reduce the voltage drop from (for example) 3% to 2%? You got the benefit of that Vd reduction whether you wanted it or not and a fault has lower impedance irrespective of your motivation.
Click to expand...
In many cases, he wouldn't unless you tell him... that is, an on-site inspector. Plan check should catch it, but not always. Sometimes compliance is just a matter of conscience.
 
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Natfuelbilll

Senior Member
Further discussions of the electromagnetic reasons for increasing the EGC based on increased CCC sizing will help us understand this further.

Anyone able to help explain this with calculations why the EGC size increase is to be at least directly proportional to the CCC?
 
kwired

kwired

Electron manager
Location
NE Nebraska
ggunn said:
How would an inspector know that you increased the CCC size because that's what you had on the truck rather than to reduce the voltage drop from (for example) 3% to 2%? You got the benefit of that Vd reduction whether you wanted it or not and a fault has lower impedance irrespective of your motivation.
Click to expand...
Because that is what you told him:)

Smart $ said:
In many cases, he wouldn't unless you tell him... that is, an on-site inspector. Plan check should catch it, but not always. Sometimes compliance is just a matter of conscience.
Click to expand...
Final inspection is usually also plan check on most of my work.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Carultch said:
Interesting how thinner raceways can act as a sufficient EGC for a greater distance than their thicker counterparts. I would intuitively think that RMC would outrank both IMC and EMT in its performance as an EGC.
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I found that interesting as well.
 
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