Cable Ampacity Adjustment

[Mike01]

When installing multi-conductor [MC] type cable in tray using the higher ampacities based on proper cable spacing etc. How is it affected when you transition to conduit for a short run? I thought it was based on a percentage basis [for example if it is less than 10% of the overall run] then you can continue to use the free-air higher ampacity rating but cannot locate this reference in the code and hoping someone could point me in the right direction…Thanks

 

[ron]

I think you are referring to an un-enforceable section in Informative Annex B of the code book B.310.15(B)(3).

The annex is not technically part of the code.

FWIW, I think you mean MC (metal clad) cable? Yes? and you are getting your free air ampacity from 392.80(A)(1)(c)? or do you mean multi-conductor and not MC, like a TC non-armored cable?

 

[Mike01]

Multi-Conductor

Multi-Conductor

For this application a TC consisting of 3-1/C+Grd with an overall PVC jacket, however the 392.80[C] references multi-conductor cables I assume this includes TC/MC type cable.

 

[Carultch]

When installing multi-conductor [MC] type cable in tray using the higher ampacities based on proper cable spacing etc. How is it affected when you transition to conduit for a short run? I thought it was based on a percentage basis [for example if it is less than 10% of the overall run] then you can continue to use the free-air higher ampacity rating but cannot locate this reference in the code and hoping someone could point me in the right direction…Thanks

You can take advantage of the 10 ft / 10% "stub rule", which states that as long as a region of reduced ampacity is less than 10 ft or 10% of the remaining length (whichever is less), the higher ampacity of the majority length can govern the entire circuit.

I do not believe that this applies for the use of splices to connect otherwise identical wires together. Since not specified, it probably has to be factory continuous lengths of wire.

 

[ron]

You can take advantage of the 10 ft / 10% "stub rule", which states that as long as a region of reduced ampacity is less than 10 ft or 10% of the remaining length (whichever is less), the higher ampacity of the majority length can govern the entire circuit.

I do not believe that this applies for the use of splices to connect otherwise identical wires together. Since not specified, it probably has to be factory continuous lengths of wire.

What code section allows for that?

 

[don_resqcapt19]

What code section allows for that?

310.15(A)(2) Exception

 

[don_resqcapt19]

When installing multi-conductor [MC] type cable in tray using the higher ampacities based on proper cable spacing etc. How is it affected when you transition to conduit for a short run? I thought it was based on a percentage basis [for example if it is less than 10% of the overall run] then you can continue to use the free-air higher ampacity rating but cannot locate this reference in the code and hoping someone could point me in the right direction…Thanks

Make sure you take a look at 110.14(C)(1) before you size your conductors.

 

[ron]

310.15(A)(2) Exception

:slaphead:I have that highlighted in my book Thanks for pointing it out

 

[don_resqcapt19]

:slaphead:I have that highlighted in my book Thanks for pointing it out

That exception comes in handy at times...things like wireways at panels or short home runs to a j-box above the ceiling.