Sizing conductors

[RichB]

Ok folks--having a "Senior Moment" here:?,--Reason for the question--We are being told we have to size our service feeders to the busswork(250A) instead of the OCPD(200A)because the math in the NEC (2011) says so, so I am trying to pin our Engineers down as to why the change after using 4/0 for years, as all the other agencies do--after all the looking the only real differance is how hard the contractor needs to work(Cost is >$1 per foot between 4/0 and 250kcmil)

When determing ampacity I can use the 90 degree colunm for my derates based on temp---In this area the local ambient temp is actully only 68 degrees F--so I get to use I believe 1.11 as the factor from the tables----now when sizing to the component I use the 75 Degree colunm--so here's my moment--do I also get to apply the derate factor there also--or am I limited to what is in the table--here's my my math so far;

310.15(B)(16)--4/0 USE-2 90^o colunm=290A
310.15(B)(2)--68^oF ambient @ 90^oC=multiplier of 1.18=342.2A

310.15(B)(16)--4/0 USE-2 at 75^oC ==230A(Do I also get to use table 310.15(B)(2) here also--if so then 230*1.25=287.5 and we can use 4/0 vice 250) But if we are limited strictly to the table then 4/0 is 20A short

Thanks folks!!!

 

[jumper]

Your ampacities for 4/0 are incorrect.

 

[charlie b]

First of all, is this a service or a feeder? What I am driving at is the location of this 200 amp OCPD. Is it the main breaker on the switchboard (or whatever component has the buswork you mention), or is there an OCPD upstream of this "service feeder"?

Secondly, I would never increase the ampacity on the basis of having an ambient that is lower than 30C. You are talking about "uprating," not derating, and I think you will have a hard time justifying that practice.

Next, you have to use table 310.15(B)(2)(a), not table 310.15(B)(2)(b), since you are using the initial ampacity values in table 310.15(B)(16), and these are based on 30C.

Finally, the ampacity you use, after any and all calculations are finished, cannot be higher than the ampacity that corresponds to the 75C column, unless you can prove that the terminations are rated for higher than 75C, and I think that will be hard to do. For a 4/0 USE-2, that value is 230 amps.

 

[infinity]

Secondly, I would never increase the ampacity on the basis of having an ambient that is lower than 30C. You are talking about "uprating," not derating, and I think you will have a hard time justifying that practice.

What if the conductors were underground?

 

[iMuse97]

What if the conductors were underground?

Wouldn't the terminations have to be underground too? :?

 

[RichB]

jumper, charlieb--you are correct--even using the correct numbers for 4/0 the 75C column limits me to 230A,,which is what I thought but got myself going in circles and just confused the daylights outta myself about de/uprating

This is a service--the 200A OCPD is the first disconnect after the meter base and POCO xformer

Was just trying to find a reason to stay with 4/0 that the Eng's would buy--other than the added work to pull in 250s and that all the contractors are asking me why especially in light of the fact that everyone else(city, county PUDs etc) is pulling in 4/0

uprating based on ambient---isn't that the same as derating based on ambient--the way I read that section and the tables it would seem that that is allowed--hence the multipliers for a lower ambient and yes i realize that it is not a standard practice and would be difficult to convince people of.

Thanks guys, always a pleasure to learn from you!!

 

[kwired]

Wouldn't the terminations have to be underground too? :?

2008 NEC 310.15(A)(2) exception - you can use the higher ampacity beyond point of transition up to 10 feet or 10% of circuit length - which ever is less. If each end of the conductors is only 5 feet max out of the ground you can use the below ground ambient temperature if total circuit length is 100 feet or longer. Less than 100 feet total and the above ground portion allowed at higher ampacity gets shorter.

jumper, charlieb--you are correct--even using the correct numbers for 4/0 the 75C column limits me to 230A,,which is what I thought but got myself going in circles and just confused the daylights outta myself about de/uprating

This is a service--the 200A OCPD is the first disconnect after the meter base and POCO xformer

Was just trying to find a reason to stay with 4/0 that the Eng's would buy--other than the added work to pull in 250s and that all the contractors are asking me why especially in light of the fact that everyone else(city, county PUDs etc) is pulling in 4/0

uprating based on ambient---isn't that the same as derating based on ambient--the way I read that section and the tables it would seem that that is allowed--hence the multipliers for a lower ambient and yes i realize that it is not a standard practice and would be difficult to convince people of.

Thanks guys, always a pleasure to learn from you!!

If your overcurrent device is 200 amps you can have a 176 amp conductor as long as calculated load is not over 176 amps. You could have a 2000 amp bus and all that matters is calculated load and the 200 amp overcurrent device to comply with NEC.