Conduit Fill Charts

[fifty60]

When I reference "conduit fill charts NEC" on google, the first 10 charts that populate all differ from the informative annex C in NFPA 70 2014. For example, 1/2 EMT per NFPA70 annex C table C.1 I can fit 2 #8AWG conductor in a 1/2" EMT.

When I type "Conduit fill chart NEC" into google, the first 10 charts that come up say I can fit 3 #8AWG in a 1/2" EMT. When did this change and why?

Also, do the conductors in the fill charts include the ground conductor, or only current carrying conductors?

 

[Carultch]

When I reference "conduit fill charts NEC" on google, the first 10 charts that populate all differ from the informative annex C in NFPA 70 2014. For example, 1/2 EMT per NFPA70 annex C table C.1 I can fit 2 #8AWG conductor in a 1/2" EMT.

When I type "Conduit fill chart NEC" into google, the first 10 charts that come up say I can fit 3 #8AWG in a 1/2" EMT. When did this change and why?

Also, do the conductors in the fill charts include the ground conductor, or only current carrying conductors?

Conduit fill is a mechanical/geometric rule, not thermal. Therefore you have to count the ground. These charts assume you have all identical wires, including an identical ground.

 

[Smart $]

...
Also, do the conductors in the fill charts include the ground conductor, or only current carrying conductors?

Fill includes all conductors: circuit, grounding, and unused.

Can't answer earlier question? Nothing changed AFAIK.

 

[Carultch]

When I reference "conduit fill charts NEC" on google, the first 10 charts that populate all differ from the informative annex C in NFPA 70 2014. For example, 1/2 EMT per NFPA70 annex C table C.1 I can fit 2 #8AWG conductor in a 1/2" EMT.

When I type "Conduit fill chart NEC" into google, the first 10 charts that come up say I can fit 3 #8AWG in a 1/2" EMT. When did this change and why?

Also, do the conductors in the fill charts include the ground conductor, or only current carrying conductors?

When I use my calculation spreadsheet, 3 qty #8 THWN-2 can fit in 1/2" EMT with a fill of 36.16%, and my limit is 40%. One possible reason why the charts might be inconsistent, is that jamming might be anticipated for the 3 conductors. Jamming occurs when the three conductors don't align in a triangular formation, but instead in a straight line. And that straight line of wires stacked, is so close to the conduit ID, that the conductors can get stuck during the pull. It often happens when you are very close to the 40% limit.


The jam factor J is J=1.05*ID/OD, where OD is the wire diameter and ID is the conduit interior. If J is between 2.4 and 2.6, there is a high probability of jamming, and it is best to upsize.

 

[Smart $]

When I use my calculation spreadsheet, 3 qty #8 THWN-2 can fit in 1/2" EMT with a fill of 36.16%, and my limit is 40%. One possible reason why the charts might be inconsistent, is that jamming might be anticipated for the 3 conductors. Jamming occurs when the three conductors don't align in a triangular formation, but instead in a straight line. And that straight line of wires stacked, is so close to the conduit ID, that the conductors can get stuck during the pull. It often happens when you are very close to the 40% limit.


The jam factor J is J=1.05*ID/OD, where OD is the wire diameter and ID is the conduit interior. If J is between 2.4 and 2.6, there is a high probability of jamming, and it is best to upsize.

Shouldn't be too awfully hard to factor that into your spreadsheet, right? :angel:

 

[cpinetree]

Shouldn't be too awfully hard to factor that into your spreadsheet, right? :angel:

Or just use this one on the web: http://www.electrician2.com/calculators/rf_calculator.html

It will ask you if you want to upsize the conduit if it is in the jam ratio.

The only thing that frustrates me is that you need to enter the quantity of wires before picking the size of the conductor, or a pop up box says "Please enter number of conductors!" :rant:

 

[fifty60]

Very helpful, thanks everyone. I will not trust any charts unless I do my own calculations.

The 40% is not for thermal considerations, but spatial. Is this only because problems arise when you are pulling the conductors through the conduit? If it is a small run (like 4 or 5 feet) this is not a major concern then right? But, the calculations are for long runs and short runs...

 

[Carultch]

Very helpful, thanks everyone. I will not trust any charts unless I do my own calculations.

The 40% is not for thermal considerations, but spatial. Is this only because problems arise when you are pulling the conductors through the conduit? If it is a small run (like 4 or 5 feet) this is not a major concern then right? But, the calculations are for long runs and short runs...

For 2 ft and less with no bends, you can fill up to 60%. For all other applications, it is 53% / 31% / 40%.

 

[ggunn]

For 2 ft and less with no bends, you can fill up to 60%. For all other applications, it is 53% / 31% / 40%.

Clarification: those numbers are for one conductor / two conductors / three or more conductors.

 

[don_resqcapt19]

When I use my calculation spreadsheet, 3 qty #8 THWN-2 can fit in 1/2" EMT with a fill of 36.16%, and my limit is 40%. One possible reason why the charts might be inconsistent, is that jamming might be anticipated for the 3 conductors. Jamming occurs when the three conductors don't align in a triangular formation, but instead in a straight line. And that straight line of wires stacked, is so close to the conduit ID, that the conductors can get stuck during the pull. It often happens when you are very close to the 40% limit.


The jam factor J is J=1.05*ID/OD, where OD is the wire diameter and ID is the conduit interior. If J is between 2.4 and 2.6, there is a high probability of jamming, and it is best to upsize.

The tables in Annex C just look at percentage fill...they do not look at the possibility of "jamming".

The 40% fill for 1/2" EMT is 0.122 square inches....a a #8 THWN is 0.0366 square inches....3.33 #8s fit in a 1/2" EMT.

 

[Smart $]

The tables in Annex C just look at percentage fill...they do not look at the possibility of "jamming".

The 40% fill for 1/2" EMT is 0.122 square inches....a a #8 THWN is 0.0366 square inches....3.33 #8s fit in a 1/2" EMT.

Which goes back to the OP saying NEC Table C.1 shows only 2 permitted. I don't know where he is looking... perhaps a type other than THWN ...but Table C.1 in my NEC shows 3 for THWN.