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Thursday, May 4, 2017
Question:
Hi I've got a service feeder question. I'm doing a remodel of a commercial retail store. There is currently a 3 phase 120/208 400 amp service which is being replaced by a 500 amp service. The existing switchgear has a 4 inch conduit run to it with parallel feed conductors in it. The plans call for 2 - 2 1/2 inch conduits with 250 mcm feeders for the 500 amp service and new switchgear. I would much rather use the existing 4 inch conduit and pull the parallel feeders in that than spend the money and time installing new conduits. My question is can I parallel two sets of 250 mcm in the 4 inch conduit, or do I need to up the size to 300 mcm with a 1/0 ground because of derating factors to be code legal?
Rob Frees
A
Answer:
Hey Rob thanks for your question. Maybe. Service conductors must be sized as stated in 230.42 based on the load as calculated according to Article 220. Some of the load in a store such as the lighting would be considered a Continuous Load as defined in Article 100 so the 125% factor needs to be applied as stated in 230.42(A)(1). That value is compared with the value determined by applying any adjustment or correction factors as stated in 230.42(A)(1) [without the 125% factor] and the largest value is used.
If the neutral conductor is considered a current carrying conductor based on the conditions in 310.15(B)(5) then you would have 8 current carrying conductors in the conduit and a 70 % adjustment factor as stated in 310.15(B)(3). If the neutral conductor is not considered current carrying you would have 6 current carrying conductors with an 80% adjustment factor.
If you use conductors with a 90 degree C insulation rating, that is suitable for the likely wet location of the service conductors, the ampacity value is 290 for 250 kcmil coper conductors from Table 310.15(B)(16). If there is no ambient temperature correction factor necessary from 310.15(B)(2)(a), and if the neutral is not considered current carrying, then 290 X 80% = 232 amps X 2 conductors per phase = 464 amps. So if that is enough for the load, including the factors previously mentioned, you could use a 500 ampere overcurrent protection device as stated in 230.60(A) Exception No. 2.
That is a lot of "ifs" and so after you determine the load and before you buy the materials and do the installation check with the AHJ to make sure they agree with the plan. Another" if" is if the owner wants add more load at a later date you might need to come back and do it all over again!