Derating for multiple CCCs in EMT vs. SMR

[paulcbrowne]

Article 310, para. 310.15(B)(2)(a) and its table designate limits on the number of current carrying conductors that may be installed in raceway. However, Article 386 describes an exception to Table 310.15(B)(2)(a) for surface metal raceway (SMR) where the cross section is over 4 sq. in. If the fill does not exceed 20%, a maximum of 30 CCCs is permitted.

My question is why would the same not apply if one used 2 1/2 in. EMT whose cross-section is greater than 4 sq. in. (actually 5.858 sq. in.). Both the SMR and EMT can be in a configuration where it is inspectable only at boxes and there is no length limitation for either one. In both, the wires can lie in any random fashion because either SMR like Wiremold or EMT can be installed at any angle.

 

[Volta]

Article 310, para. 310.15(B)(2)(a) and its table designate limits on the number of current carrying conductors that may be installed in raceway. However, Article 386 describes an exception to Table 310.15(B)(2)(a) for surface metal raceway (SMR) where the cross section is over 4 sq. in. If the fill does not exceed 20%, a maximum of 30 CCCs is permitted.

My question is why would the same not apply if one used 2 1/2 in. EMT whose cross-section is greater than 4 sq. in. (actually 5.858 sq. in.). Both the SMR and EMT can be in a configuration where it is inspectable only at boxes and there is no length limitation for either one. In both, the wires can lie in any random fashion because either SMR like Wiremold or EMT can be installed at any angle.

My guess- a tube or conduit, being round, will tend to force the contained conductors (if small in ratio to the ID) to the center low spot, and there will be less free air around those conductors, no matter that there is free space above them.

Not necessarily so for a rectangular raceway.

 

[paulcbrowne]

But not necessarily not so either.