Rigid Conduit Sizing

[Hugh W]

I'm beginning a project that will require an 800A/480VAC feeder to a panel board and the conduits will be 500MCM paralleled feeds with 1/0 grounding conductors. The total area for (3) THHN 500MCM and (1) THHN 1/0 is 3.333 sq. in., which will require a 3-1/2" RMC per 40% fill in Chapter 9 Table 4 Article 344, but Annex C Table C.8 indicates (4) 500MCM's can be installed in 3" RMC.... so which is correct?

Thanks,
Hugh Wright

 

[petersonra]

I'm beginning a project that will require an 800A/480VAC feeder to a panel board and the conduits will be 500MCM paralleled feeds with 1/0 grounding conductors. The total area for (3) THHN 500MCM and (1) THHN 1/0 is 3.333 sq. in., which will require a 3-1/2" RMC per 40% fill in Chapter 9 Table 4 Article 344, but Annex C Table C.8 indicates (4) 500MCM's can be installed in 3" RMC.... so which is correct?

Thanks,
Hugh Wright

I looked in the 2011 version under table c.8 in the annex. it says you can get four 500 KCM conductors in a 3" conduit.

according to chapter 9, the internal CSA of 3" rmc is 7.499 in^2, and 40% of that is 3 in^2.

table 5 in chapter 9 claims the CSA of 1/0 THHN is .1855 in^2 and the CSA of 500 KCM THHN is .7073 in^2. So the total CSA would be 2.3074. I don't know how you came up with a total CSA of 3.333 in^2. The numbers suggest you used the OD of 750 kcm in your calculations.

 

[Hugh W]

Thanks Peter. I did look at the diameter instead of area.