Can I use table 310-60(c)(78) to determine the ampacity of one medium voltage aluminum 500kCM, 15kV circuit that each conductor is housed in 1 PVC 4" duct bank? I'm thrown of by the title of the table that states "THREE CONDUCTORS PER ELECTRICAL DUCT" and there is no other table in the NEC that best states what I need. Thanks in advance.
TABLE 310-60(c)(78)
- Thread starter Jose I.
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Dale001289
Senior Member
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- Georgia
The main issue is heat. One conductor per PVC duct will give you better ampacity than what's shown in Table 310.60(C)(78) since you won't have heat contribution from the other 2 phases. You'll probably be closer to ampacities shown in Table 310.60(C)(82), but you may need to contact the vendor for assistance on this one.
Thanks Dale001289;1926321, I'll contact the vendor.
beanland
Senior Member
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- Vancouver, WA
3-phase?
3-phase?
Is this a 3-phase circuit with three 500kcmil, each in its own duct? Okonite Bulletin EHB has ratings for this configuration BUT this is for special engineering where the neutral/shield is not grounded on both ends. EHB shows this as 357A, with the caveat above.
If this is a single-phase circuit, where the return is the concentric neutral, you may have problems because 500kcmil normally comes with 1/3 neutral.
If this is 3-phases in 3 ducts with the neutrals grounded, there can be a lot of current induced on the neutral, leading to overheating.
Same issue if you use steel duct with a single-cable in it. Conduit will get hot.
3-phase?
Is this a 3-phase circuit with three 500kcmil, each in its own duct? Okonite Bulletin EHB has ratings for this configuration BUT this is for special engineering where the neutral/shield is not grounded on both ends. EHB shows this as 357A, with the caveat above.
If this is a single-phase circuit, where the return is the concentric neutral, you may have problems because 500kcmil normally comes with 1/3 neutral.
If this is 3-phases in 3 ducts with the neutrals grounded, there can be a lot of current induced on the neutral, leading to overheating.
Same issue if you use steel duct with a single-cable in it. Conduit will get hot.
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