Upsizing Equipment Ground for Voltage Drop

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SunFish

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I am looking at upsizing my AC conductors coming out of my inverter from a #6 to a #4. 250.122(B) says I need to increase the EGC size proportionally according the the circular mil area of the conductors. My ocpd is 60 A so I'm looking at a #10 EGC.

From Chapter 9 Table 8:
#4 circular mils = 41740
#6 circular mils = 26240
#8 circular mils = 16510
#10 circular mils = 10380

41740/26240 = 1.5907013..... x 10380 = 16511.5

Since technically 16511.5 is greater than #8 @ 16510 c-mils I should have to go up to a #6 AWG conductor.

Here is my issue, #4 is only rated up to 95 A at 90 deg C and 85 A at 75 deg C. So the largest overcurrent protection device one would have is less than 100 A and Table 250.122 would let you use a #8 EGC to protect #4 on say a 75 A breaker.

Why is #8 adequate as an EGC for #4 but when I upsize from #6 to #4 for voltage drop #8 is technically insufficient.

Am I allowed to round my ratio of #4 c-mills/#6 c-mills to an even 1.59? If I did this would make #8 adequate.

Am I ok to use #8 as my increased EGC size if I increase my line conductors from #6 to #4 for voltage drop concerns or do I have to use #6 since I'm technically 1.5 c-mils over the limit on #8.
 
Smart $

Smart $

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The cmil conversions of AWG sizes are rounded to four significant figures. So you can also round your calculation result to four significant figures, IMO.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
SunFish said:
Can you clarify?
Click to expand...
The cmil sizes used in NEC Chapter 9, Table 8, and by manufacturers are rounded to four significant figures per ASTM B258

The table on this web page show the AWG to cmil conversions rounded to 6 significant figures: https://sizes.com/materials/wire_BandS.htm

The original AWG sizes were designed to be a proportional increase or decrease in area. For example, a change in two gauge sizes, say #10 to #8, is the same change proportionally as #6 is to #4. When you use rounded area equivalents, the proportionality gets lost when you carry out calculations beyond the starting number of significant figures.
 
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