250.122(B) minimum intended size

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hhsting

Senior Member
I have load of 290A. The breaker used is 300A breaker. The cable size used due to voltage drop is 500kcmil cu at 75C


Equipment grounding conductor should be increased but what would be the minimum intended size cable be for 250.122(B)? Anyone know?

The engineer is saying minimum cable size is 300kcmil not 250kcmil cu at 75C. Is he correct or incorrect?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
350 kcmil is the minimum size for 300 amps with a load of 290 amps. Look at 310.15(B)(16).
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
This is always questionable but yes if the calculated load is 285 or less than 300 kcm cu can be used for a 300 amp breaker so the engineer is incorrect
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I have load of 290A. The breaker used is 300A breaker. The cable size used due to voltage drop is 500kcmil cu at 75C


Equipment grounding conductor should be increased but what would be the minimum intended size cable be for 250.122(B)? Anyone know?

The engineer is saying minimum cable size is 300kcmil not 250kcmil cu at 75C. Is he correct or incorrect?
How could you expect to use a cable with an ampacity of 255 amps for a 290 amp load ???????????????????????????????????????????????????
Even the 300 kcmil is too small for a 290 amp load.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
How could you expect to use a cable with an ampacity of 255 amps for a 290 amp load ???????????????????????????????????????????????????
Even the 300 kcmil is too small for a 290 amp load.
That's correct, as mentioned 350 kcmil in post #2 would be the minimum size but I wonder why this question is even being asked. Reject and send the plans back with a note for the engineer to re-calculate the coductor size.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The smaller the 'base' that you start from, the greater the % change to the size actually used, and thus the greater the % change required for the EGC.

So the engineer has an incentive to select the largest plausible base size for the circuit conductors. If the engineer selects a smaller base then necessary then the EGC will be larger than required and that should be just fine.

With a load of 290A and 75C terminations, neither 250 kcmil nor 300 kcmil are correct. The minimum is 350 kcmil. So the use of 300 as the base for this calculation should be acceptable, though noted as an error in case this value is used elsewhere.

Jon
 

Carultch

Senior Member
Location
Massachusetts
The smaller the 'base' that you start from, the greater the % change to the size actually used, and thus the greater the % change required for the EGC.

So the engineer has an incentive to select the largest plausible base size for the circuit conductors. If the engineer selects a smaller base then necessary then the EGC will be larger than required and that should be just fine.

So suppose we had a 300A circuit that we needed to upsize to 600 kcmil Cu, for mitigating voltage drop.

Does this mean that we can strategically use a 400A OCPD to protect this circuit, when a 300A OCPD would otherwise suffice, so that the starting point for the calculation of proportionally upsizing the EGC is 500 kcmil Cu instead of 350 kcmil Cu?

Using a 300A OCPD in this example, would mean 350 kcmil Cu + #4 Cu EGC is the starting point, that gets upsized to 600 kcmil Cu + #1 Cu EGC.
Using a 400A OCPD in this example, would mean 500 kcmil Cu + #3 Cu EGC is the starting point, that gets upsized to 600 kcmil Cu + #2 Cu EGC.

Not necessarily the best example to show what could happen, but you can see that there is a possibility to reduce the EGC by doing this, that might feel like a loophole to this code rule. This assumes your load device allows you to do this.
 
Last edited:

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
I pretty much ignore this rule, especially with the addition of the exception in the 2020 code.

I have played a bit with the numbers and no matter what, the fault current that will flow in the circuit with an upsized ungrounded conductors and a table size EGC is always greater than the fault current that will flow with the minimum permitted size of ungrounded conductors and a table sized EGC.
 

hhsting

Senior Member
The smaller the 'base' that you start from, the greater the % change to the size actually used, and thus the greater the % change required for the EGC.

So the engineer has an incentive to select the largest plausible base size for the circuit conductors. If the engineer selects a smaller base then necessary then the EGC will be larger than required and that should be just fine.

With a load of 290A and 75C terminations, neither 250 kcmil nor 300 kcmil are correct. The minimum is 350 kcmil. So the use of 300 as the base for this calculation should be acceptable, though noted as an error in case this value is used elsewhere.

Jon

How is 300kcmil copper at 75C not correct? Greater than 290A??
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Not sure about some engineers, had one that provided a design that had a #8 on the plans that was having a 200A panel. His claim was it was only drawing 31 Amps total loads and didn't need any bigger, ignoring that elsewhere on plans it also indicated it was feeding the 200A main breaker panel. Field installer deviated from drawing and installed 250kcmil. Their claim was it was easier to just do it right than argue any more with the designer. I guess they've had multiple go around with this guy.
Not sure if the AHJ would inspect to "as designed" or "as installed".
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Not sure about some engineers, had one that provided a design that had a #8 on the plans that was having a 200A panel. His claim was it was only drawing 31 Amps total loads and didn't need any bigger, ignoring that elsewhere on plans it also indicated it was feeding the 200A main breaker panel. Field installer deviated from drawing and installed 250kcmil. Their claim was it was easier to just do it right than argue any more with the designer. I guess they've had multiple go around with this guy.
Not sure if the AHJ would inspect to "as designed" or "as installed".
Unless the adopted code says that they will inspect to the design, they are not permitted to do that. They can only inspect to the legally adopted code.

Yes, I am aware of many areas that illegally inspect to the design and no one calls them out on the issue. If my area did that, an area where the inspectors are paid by the taxpayers, I would object to the illegal spending of the taxpayer dollars.

It is the owner's responsibility to have the project inspected to the design.
 

romex jockey

Senior Member
Location
Vermont
Occupation
electrician
Equipment grounding conductor should be increased but what would be the minimum intended size cable be for 250.122(B)? Anyone know?
what would be the minimum size for an OCPD to properly function comes to mind hhsting

it always seems the AIC math is important when we're close to the xformer, not so much when far away.....
Not sure about some engineers, had one that provided a design that had a #8 on the plans that was having a 200A panel. His claim was it was only drawing 31 Amps total loads and didn't need any bigger, ignoring that elsewhere on plans it also indicated it was feeding the 200A main breaker panel. Field installer deviated from drawing and installed 250kcmil. Their claim was it was easier to just do it right than argue any more with the designer. I guess they've had multiple go around with this guy.
Not sure if the AHJ would inspect to "as designed" or "as installed".
On larger jobs here, architecturals are 'approved' by the state, inclusive of all trades involvement.


ie> they're inspecting the design

~RJ~
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
what would be the minimum size for an OCPD to properly function comes to mind hhsting

it always seems the AIC math is important when we're close to the xformer, not so much when far away.....

On larger jobs here, architecturals are 'approved' by the state, inclusive of all trades involvement.


ie> they're inspecting the design

~RJ~
My question is whether the "final inspector" be it a third party or AHJ will be looking at compliance from perspective of "as designed" or simply "as installed" and would deviation from design, even if it made it more compliant or safer, would or should cause a Failed inspection?
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
I have load of 290A. The breaker used is 300A breaker. The cable size used due to voltage drop is 500kcmil cu at 75C


Equipment grounding conductor should be increased but what would be the minimum intended size cable be for 250.122(B)? Anyone know?

The engineer is saying minimum cable size is 300kcmil not 250kcmil cu at 75C. Is he correct or incorrect?

Sounds like you already took care of the conductor size. Why do you care about the minimum size if they have already oversized the conductors?

You have too much going on in such a short post. Are you concerned at all about the equipment grounding conductor size? If so, you asked that question weeks ago, so refer to your old posts.
 

romex jockey

Senior Member
Location
Vermont
Occupation
electrician
My question is whether the "final inspector" be it a third party or AHJ will be looking at compliance from perspective of "as designed" or simply "as installed" and would deviation from design, even if it made it more compliant or safer, would or should cause a Failed inspection?
if i understand correctly, we're talking 'as builts' Fred?

~RJ~
 
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