I am having a debat with an electrical engineer I am told that I can fill my condit to 100% capacity after I derate my wires. Example I have a 800 amp service 120/208 wire system I calculated my wire to be 8 legs of 600mcm thhn in two 4 inch conduits he is saying I can go with 3 inch because the conduit fill table allows 4 if you do the calculation with circular mills it doesn't fit
conduit fill
- Thread starter kuros
- Start date
- Status
- Location
- Massachusetts
Re: conduit fill
You where not specific about the type of raceway you are using, it makes a difference.
Four 600 Kcmil THHN.
Will fit in 3" EMT.
It will not fit in 3" RMC or RNC.
If you are using RMC or RNC the minimum size is 3.5"
I have installed four 600 Kcmil THHN in long runs of 3" EMT and it goes fine.
However whoever is on the feed end must make sure the conductors enter the EMT correctly, one 'cross' and you will jam up and be in big trouble.
[ October 13, 2005, 04:48 AM: Message edited by: iwire ]
You where not specific about the type of raceway you are using, it makes a difference.
Four 600 Kcmil THHN.
Will fit in 3" EMT.
It will not fit in 3" RMC or RNC.
If you are using RMC or RNC the minimum size is 3.5"
I have installed four 600 Kcmil THHN in long runs of 3" EMT and it goes fine.
However whoever is on the feed end must make sure the conductors enter the EMT correctly, one 'cross' and you will jam up and be in big trouble.
[ October 13, 2005, 04:48 AM: Message edited by: iwire ]
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Re: conduit fill
No you can't fill it to 100%. Ever.
No you can't fill it to 100%. Ever.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: conduit fill
Oh, you mean with conductors.
What about explosionproof, vapor-seal, etc?
Oh, you mean with conductors.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conduit fill
I think it more likely that the engineer was using the phrase "100% capacity" to mean 100% of the conduit fill that is allowed in the tables. That is, if the table says you can fit a set of conductors into a 3 inch conduit, then it is acceptable to use a 3 inch conduit. You don't have to use a bigger one, just to give yourself some extra room. Use 100% of the allowed interior area (i.e., use all of the 40% fill).
Of course, that type of calculation is good for an engineer working with a writing tablet and a slide rule (or am I dating myself). But I don't have to physically pull the wires. We generally don't have a feel for the ease or difficulty of pulling wires through conduit. Sometimes it is cheaper to use a larger conduit, for the simple reason that it takes less time to pull the wires.
There has to be a miscommunication going on here. What is meant by "100% capacity"? At first glance, it sounds like this means you can fill 100% of the interior cross-sectional area with conductors. That is nonsense. Forget about code rules, it is physically impossible to fill in 100% of a circular conduit with two or more circular conductors. Plus, it starts getting hard to pull wires, when they take up too much of the area.
I think it more likely that the engineer was using the phrase "100% capacity" to mean 100% of the conduit fill that is allowed in the tables. That is, if the table says you can fit a set of conductors into a 3 inch conduit, then it is acceptable to use a 3 inch conduit. You don't have to use a bigger one, just to give yourself some extra room. Use 100% of the allowed interior area (i.e., use all of the 40% fill).
It doesn't have to. If the table says it will fit, then it is permitted. I have use the method of adding up cross-sectional areas when there were different sizes of conductors (e.g., the EGC was smaller than the ungrounded conductors). The table said that only 4 wires would fit, but I proved that 4 of this size plus 1 of this smaller size added up to less than 40%.
Of course, that type of calculation is good for an engineer working with a writing tablet and a slide rule (or am I dating myself). But I don't have to physically pull the wires. We generally don't have a feel for the ease or difficulty of pulling wires through conduit. Sometimes it is cheaper to use a larger conduit, for the simple reason that it takes less time to pull the wires.
- Location
- Massachusetts
Re: conduit fill
Assuming the conductors fit in the conduit the size does not effect the pulling tension.
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