2011 NEC 250-122

[orion]

We have a contractor that proposes to use four 3/c IAC cable consisting of 3 -750kcm AL and 3-2/0 AL grounds for a 2000A circuit ( we'll adjust the over current device to 1900A ). NEC 250-122 specifies a 400kcmil AL conductor for a 2000A circuit. Table 8 properties list the 2/0 as 133,100 circular mils. Multiplying x3 conductors = 399,300 circular mils. We stated that the 3-2/0's do not meet the circular mil area per the NEC. The contractor now has furnished a letter from the cable manufacture stating "Our cable is UL listed under UL 1569. UL 1569 in turn calls out ASTM B801 ?Standard Specification for Concentric-lay Stranded Conductors of 8000 series Aluminum Alloy for Subsequent Covering or Insulation? to define the physical and electrical characteristics of the conductor. Section 15 of ASTM 801 states ? The cross-sectional area of the conductors shall be not less than 98% of the cross-sectional area as specified in Column 1 of Table 1.? Per code the ground size for a 2000 amp panel is 400kcmil. 98% of 400kcmil is 392kcmil. Our cable construction has 3-2/0 ground phases. Through our manufacturing tolerance we allow for 0 to minus 1% therefore the minimum size of our 2/0 is: 0.99x133,100 = 131,769. Because we have 3-2/0 then the combined circular mil is 395,307 circular mils which is more than 392kcmil.

In other words a conductor that has a cross section of 392kcmil can be called a 400kcmil because it falls within the 2% tolerance. And because the combined circular mil of 3-2/0 is 395,307 CM you can call it a 400kcmil. So in the end 3-2/0 meet the requirement of a 2000 amp panel.

My belief is the cable meets the intent, HOWEVER, no where in the NEC do I find where "allowable percentages" of a cable is permitted for use. Should 3-3/0's be the minimum requirement at 167,800 mil x3 = 503,400 circ mil?

 

[xformer]

We have a contractor that proposes to use four 3/c IAC cable consisting of 3 -750kcm AL and 3-2/0 AL grounds for a 2000A circuit ( we'll adjust the over current device to 1900A ). NEC 250-122 specifies a 400kcmil AL conductor for a 2000A circuit. Table 8 properties list the 2/0 as 133,100 circular mils. Multiplying x3 conductors = 399,300 circular mils. We stated that the 3-2/0's do not meet the circular mil area per the NEC. The contractor now has furnished a letter from the cable manufacture stating "Our cable is UL listed under UL 1569. UL 1569 in turn calls out ASTM B801 ?Standard Specification for Concentric-lay Stranded Conductors of 8000 series Aluminum Alloy for Subsequent Covering or Insulation? to define the physical and electrical characteristics of the conductor. Section 15 of ASTM 801 states ? The cross-sectional area of the conductors shall be not less than 98% of the cross-sectional area as specified in Column 1 of Table 1.? Per code the ground size for a 2000 amp panel is 400kcmil. 98% of 400kcmil is 392kcmil. Our cable construction has 3-2/0 ground phases. Through our manufacturing tolerance we allow for 0 to minus 1% therefore the minimum size of our 2/0 is: 0.99x133,100 = 131,769. Because we have 3-2/0 then the combined circular mil is 395,307 circular mils which is more than 392kcmil.

In other words a conductor that has a cross section of 392kcmil can be called a 400kcmil because it falls within the 2% tolerance. And because the combined circular mil of 3-2/0 is 395,307 CM you can call it a 400kcmil. So in the end 3-2/0 meet the requirement of a 2000 amp panel.

My belief is the cable meets the intent, HOWEVER, no where in the NEC do I find where "allowable percentages" of a cable is permitted for use. Should 3-3/0's be the minimum requirement at 167,800 mil x3 = 503,400 circ mil?

IMHO... I don't think you will find any allowable percentages like that in the NEC as currently written. AN install of this type would require the 3/0's in order to be code compliant. However, with proper documentation and AHJ approval, a variance could be granted.

 

[texie]

We have a contractor that proposes to use four 3/c IAC cable consisting of 3 -750kcm AL and 3-2/0 AL grounds for a 2000A circuit ( we'll adjust the over current device to 1900A ). NEC 250-122 specifies a 400kcmil AL conductor for a 2000A circuit. Table 8 properties list the 2/0 as 133,100 circular mils. Multiplying x3 conductors = 399,300 circular mils. We stated that the 3-2/0's do not meet the circular mil area per the NEC. The contractor now has furnished a letter from the cable manufacture stating "Our cable is UL listed under UL 1569. UL 1569 in turn calls out ASTM B801 ?Standard Specification for Concentric-lay Stranded Conductors of 8000 series Aluminum Alloy for Subsequent Covering or Insulation? to define the physical and electrical characteristics of the conductor. Section 15 of ASTM 801 states ? The cross-sectional area of the conductors shall be not less than 98% of the cross-sectional area as specified in Column 1 of Table 1.? Per code the ground size for a 2000 amp panel is 400kcmil. 98% of 400kcmil is 392kcmil. Our cable construction has 3-2/0 ground phases. Through our manufacturing tolerance we allow for 0 to minus 1% therefore the minimum size of our 2/0 is: 0.99x133,100 = 131,769. Because we have 3-2/0 then the combined circular mil is 395,307 circular mils which is more than 392kcmil.

In other words a conductor that has a cross section of 392kcmil can be called a 400kcmil because it falls within the 2% tolerance. And because the combined circular mil of 3-2/0 is 395,307 CM you can call it a 400kcmil. So in the end 3-2/0 meet the requirement of a 2000 amp panel.

My belief is the cable meets the intent, HOWEVER, no where in the NEC do I find where "allowable percentages" of a cable is permitted for use. Should 3-3/0's be the minimum requirement at 167,800 mil x3 = 503,400 circ mil?

This whole argument is moot as EACH cable would have to have a 400 KCM EGC to comply with 250.122(F). The code requires the EGC to be sized per Table 250.122 in EACH raceway or cable. This is one of the downsides of paralleling in separate raceways or cables.

 

[orion]

For clarification, the contractor has proposed to use five 750kcm AL per phase, not four as I stated in my OP.

The issue still remains and I believe at minimum 3 - 3/0 AL grounds would have to be installed in each 3/c cable assembly to meet the NEC's intent per circular mil of a 400kcmil AL ground. I also feel a cable manufacture referencing their allowable build properties have NO bearing on an allowable percentage for use as there is NO reference in the NEC allowing the same.

 

[xformer]

This whole argument is moot as EACH cable would have to have a 400 KCM EGC to comply with 250.122(F). The code requires the EGC to be sized per Table 250.122 in EACH raceway or cable. This is one of the downsides of paralleling in separate raceways or cables.

Good Catch Texie... :slaphead:Thanks for that reminder..

 

[orion]

This whole argument is moot as EACH cable would have to have a 400 KCM EGC to comply with 250.122(F). The code requires the EGC to be sized per Table 250.122 in EACH raceway or cable. This is one of the downsides of paralleling in separate raceways or cables.

As I posted above, utilizing three grounds within one 3/c IAC of equal circular mil would be the same as utilizing "one" ground.....see Table 8 Conductor Properties.

 

[texie]

For clarification, the contractor has proposed to use five 750kcm AL per phase, not four as I stated in my OP.

The issue still remains and I believe at minimum 3 - 3/0 AL grounds would have to be installed in each 3/c cable assembly to meet the NEC's intent per circular mil of a 400kcmil AL ground. I also feel a cable manufacture referencing their allowable build properties have NO bearing on an allowable percentage for use as there is NO reference in the NEC allowing the same.

Sorry, I just realized I misread your OP as you did say "in each cable".

 

[xformer]

As I posted above, utilizing three grounds within one 3/c IAC of equal circular mil would be the same as utilizing "one" ground.....see Table 8 Conductor Properties.

who's on first?.....

 

[texie]

As I posted above, utilizing three grounds within one 3/c IAC of equal circular mil would be the same as utilizing "one" ground.....see Table 8 Conductor Properties.

I agree as I stated in post#7. This complies with 310.10 (H) except for the slight circ. mill deficiency as you described. I don't think the NEC would allow this slight undersizing. That said, I think there would be some AHJs that would overlook this. I think I might be so inclined.