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infinity

infinity

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dragsterjack said:
when doing the calculation for the increased grounded conductor if the cir. mil. is in between sizes do you have to go to the next larger size?

Thanks
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If you have only increased the size of the grounded conductor there is no required increase for the EGC. I assume that you meant grounding conductor?
 
Smart $

Smart $

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Commercial cmil sizes, those published in the NEC, are rounded to four significant figures. Round the result of your calculation to the same before determining the next larger size need be used. In cases where the upsized of the ungrounded conductor is an AWG size, you can simply match the change in gauge number.
 
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GoldDigger

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Smart $ said:
Commercial cmil sizes, those published in the NEC, are rounded to four significant figures. Round the result of your calculation to the same before determining the next larger size need be used. In cases where the upsized of the ungrounded conductor is an AWG size, you can simply match the change in gauge number.
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Some examples have been posted here, I believe, in which the size to gauge ratio is such that going up the same number is marginally too small.

mobile
 
Smart $

Smart $

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GoldDigger said:
Some examples have been posted here, I believe, in which the size to gauge ratio is such that going up the same number is marginally too small.
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I would like to see those examples. Got a link or three?

The original AWG sizing formula made any change by gauge number difference the same ratio no matter the size. With commercial sizes being rounded to four significant figures the ratios should hold true as long as the calculations use not more than four significant figures. One place where that may not make a difference is when a change in gauge sizes results in an AWG size that is not sold commercially, like 5AWG... and using 4AWG will be required.
 
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Carultch

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Smart $ said:
I would like to see those examples. Got a link or three?

The original AWG sizing formula made any change by gauge number difference the same ratio no matter the size. With commercial sizes being rounded to four significant figures the ratios should hold true as long as the calculations use not more than four significant figures. One place where that may not make a difference is when a change in gauge sizes results in an AWG size that is not sold commercially, like 5AWG... and using 4AWG will be required.
Click to expand...


One example:

Local size:
#4 ungrounded with #8 EGC.

Increase in size for voltage drop:
#1/0 ungrounded

KCMIL(#8) * KCMIL(1/0) / KCMIL(#4) = 106/41.7*16.5

Result: 41.9 kcmil

This is most closely the size of #4, but is marginally larger. Blindly applying the rule to round up would result in #3, even though #4 still meets the intent of the rule.

The formula ultimately takes the form of A = K*B^(-C*n), where K, B, and C are constants. So an incremental change in gauge size (n), means a multiplicative change in cross sectional area (A). The marginal differences are due to rounding errors in your KCMIL data.
 
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kwired

kwired

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dragsterjack said:
when doing the calculation for the increased grounded conductor if the cir. mil. is in between sizes do you have to go to the next larger size?

Thanks
Click to expand...
Yes, unless you can find another way to install the remaining circular mils required within the same conductor.
 
Smart $

Smart $

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Location
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Carultch said:
One example:

Local size:
#4 ungrounded with #8 EGC.

Increase in size for voltage drop:
#1/0 ungrounded

KCMIL(#8) * KCMIL(1/0) / KCMIL(#4) = 106/41.7*16.5

Result: 41.9 kcmil

This is most closely the size of #4, but is marginally larger. Blindly applying the rule to round up would result in #3, even though #4 still meets the intent of the rule.

The formula ultimately takes the form of A = K*B^(-C*n), where K, B, and C are constants. So an incremental change in gauge size (n), means a multiplicative change in cross sectional area (A). The marginal differences are due to rounding errors in your KCMIL data.
Click to expand...
You best check to see if your calculator is functioning properly. :happyyes:

1/0AWG@105600cmil ÷ 4AWG@41740cmil × 8AWG@16510cmil = 41769.4298035458cmil

Rounded to four significant figures the result is 41770cmil. I'm away from my desktop and don't have access to my AWG records but I believe this is a case where rounding of the original cmil values to commercial sizes resulted in one being rounded up and the other being rounded down... then the proportional calculation amplifies the rounding. All in all, I believe the manufacturing tolerance is greater than this the 30cmil difference.
 
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Carultch

Senior Member
Location
Massachusetts
Smart $ said:
You best check to see if your calculator is functioning properly. :happyyes:

1/0AWG@105600cmil ÷ 4AWG@41740cmil × 8AWG@16510cmil = 41769.4298035458cmil

Rounded to four significant figures the result is 41770cmil. I'm away from my desktop and don't have access to my AWG records but I believe this is a case where rounding of the original cmil values to commercial sizes resulted in one being rounded up and the other being rounded down... then the proportional calculation amplifies the rounding. All in all, I believe the manufacturing tolerance is greater than this the 30cmil difference.
Click to expand...


The reason for this rounding error is that this data source states 1/0 is 106 kcmil, instead of 105.6 kcmil.
https://en.wikipedia.org/wiki/American_wire_gauge
 
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