Max conduit fill for short runs

[mull982]

Is there an NEC provision for that allows greater than 40% fill for short conduit runs (say less than 10ft). I have a very short run consisting of (3) 600MCM, (1) 3/0 Neutral, and (1) 1/0 GND. This fill comes out to 61.2% for existing 2.5" conduit that we are trying to reuse instead of having to run a new 3" conduit or larger.

 

[mull982]

Also in a case like this is there an allowance to use a DLO cable or any cable with ampacity higher than what is shown in 310.16? In this case instead of using 600MCM using a smaller cable with a higher ampacity?

 

[charlie b]

My answers are "no" and "I don't know," in that order. Chapter 9, Table 1, Note (4) says you can go up to 60% fill for runs no longer than 24 inches. That tells me that your ten foot question has to be a "no."

 

[Coppersmith]

If you cut in a junction box every two feet, you can use 60% fill.

 

[electrofelon]

Regarding the "higher ampacity" part, you do have a few options:

1. Use the 90 degree ampacity and splice over the 75 degree ampacity for termination on equipment (might not be super practical because of the short length).

2. Use conductors in parallel instead of a single per phase will typically get you more amps/less size even after derating.

 

[tom baker]

You can use DLO if its dual rated as RHW (not sure on what the dual rating is). Also DLO is fine stranded and will require the appropriate lug.

 

[infinity]

Is there an NEC provision for that allows greater than 40% fill for short conduit runs (say less than 10ft). I have a very short run consisting of (3) 600MCM, (1) 3/0 Neutral, and (1) 1/0 GND. This fill comes out to 61.2% for existing 2.5" conduit that we are trying to reuse instead of having to run a new 3" conduit or larger.

The answer is no you'll need to run a larger conduit. Even at 24" or less you're over the 60% fill that is permitted for nipples.

 

[jim dungar]

You use DLO if its dual rated as RHW (not sure on what the dual rating is). Also DLO is fine stranded and will require the appropriate lug.

DLO has usually has higher ratings because of its temperature rating. It must still be applied per the maximum 75°C ampacity.

 

[don_resqcapt19]

Also in a case like this is there an allowance to use a DLO cable or any cable with ampacity higher than what is shown in 310.16? In this case instead of using 600MCM using a smaller cable with a higher ampacity?

DLO really doesn't have a higher ampacity...it is rated at 90°C and in free air. You cannot use those ampacities in conduit. Also 110.14 limits you to using the Table 310.16 ampacities, unless the equipment is specifically marked for use with a different table.

 

[acin]

In mikes dvd he said dlo was not in 310 104 A so it cant be used. Is that right?

 

[Carultch]

In mikes dvd he said dlo was not in 310 104 A so it cant be used. Is that right?

DLO "only" rated wire cannot be used in applications governed by the NEC. However, I've never seen DLO wire that exclusively carries the DLO rating. It is common that DLO is dual-rated for RHW-2 and similar ratings, which enables use within applications governed by the NEC.

In sizes over 4/0, DLO wire comes in atypical sizes, so you will see ampacities on the datasheet that won't coincide with Table 310.16. By default, expect to round down to the previous standard kcmil size, and use ampacities from Table 310.16. E.g. treat 535 kcmil DLO/RHW-2 as if it were 500 kcmil RHW-2, unless your AHJ agrees to allow you to use ampacity ratings from the manufacturer's datasheet.

 

[Carultch]

Is there an NEC provision for that allows greater than 40% fill for short conduit runs (say less than 10ft). I have a very short run consisting of (3) 600MCM, (1) 3/0 Neutral, and (1) 1/0 GND. This fill comes out to 61.2% for existing 2.5" conduit that we are trying to reuse instead of having to run a new 3" conduit or larger.

If you draw it to scale, 60% fill is going to look about as full as you can get it, so even if the NEC were not going to be enforced, it will be impractical to build it any tighter than the 60% limit for 24" and less. Draw your wires as if you packed them wall-to-wall, and encircle them. The virtual circle will be about 90% of the diameter of the conduit inner diameter. Two wires is a special case, where you can't fit greater than 50% fill by area, so if you do encounter that situation, limit yourself to about 40%.

Do the same thing with the typical fill percentages of 40% (3+ wires), 31% (2-wires), and 53% (1-wire), and you will see that the virtual circle is about 75% of the conduit inner diameter. This allows some wiggle-room to make it practical to pull it any significant distance.

 

[suemarkp]

The code book fill tables mention "compact conductors". I've never seen those or know if they exist. Didn't think that was DLO but something else. They are smaller, but not sure if they are enough smaller.

 

[Carultch]

The code book fill tables mention "compact conductors". I've never seen those or know if they exist. Didn't think that was DLO but something else. They are smaller, but not sure if they are enough smaller.

Compact conductors are conductors with strands that are non-circular, so that they can fill the voids that inevitable occur when packing circles. The shape of the typical strand is like a pie wedge, after you eat the center. This allows the conductor to be slightly smaller in overall diameter.

DLO wire is not made with compact conductors. It is usually circular strands in an extremely fine strand arrangement. Typically hundreds of strands. The overall diameter of DLO is generally larger than that of its THWN-2 and XHHW-2 counterparts, because it has much thicker insulation.

 

[winnie]

If you can change your 3x 600 kcmil conductors to 9x3/0 AWG conductors, and get the entire set of conductors custom formed into a cable assembly which you then treat as a single 'thing' for conduit fill then you might just be kosher.

If you can get compact stranded conductors, even better, though these are more common in aluminium (which won't help you) then copper.

I've never purchased from these guys, but they claim to make MC with compact strand copper conductors and also to custom make cable assemblies: https://northerncables.com/wp-content/uploads/2016/11/mc-compact-strands.pdf

Now the question is: how much is it worth to reuse the existing conduit. I think you can do it if you specify custom conductors on the back of a large enough check, but I bet it will be cheaper and faster to replace the conduit.

-Jon

 

[electrofelon]

If you draw it to scale, 60% fill is going to look about as full as you can get it, so even if the NEC were not going to be enforced, it will be impractical to build it any tighter than the 60% limit for 24" and less. Draw your wires as if you packed them wall-to-wall, and encircle them. The virtual circle will be about 90% of the diameter of the conduit inner diameter. Two wires is a special case, where you can't fit greater than 50% fill by area, so if you do encounter that situation, limit yourself to about 40%.

Do the same thing with the typical fill percentages of 40% (3+ wires), 31% (2-wires), and 53% (1-wire), and you will see that the virtual circle is about 75% of the conduit inner diameter. This allows some wiggle-room to make it practical to pull it any significant distance.

For some related math fun:

Circle packing in a circle - Wikipedia

en.wikipedia.org

I would have to think about it, but I think as the conduit circle becomes arbitrarily large (or the wires arbitrarily small) then the packing density would approach the circle packing of the plane density which is about .9 or (Pi/sqrt3)/6

 

[Rock86]

The code book fill tables mention "compact conductors". I've never seen those or know if they exist. Didn't think that was DLO but something else. They are smaller, but not sure if they are enough smaller.

They exist. I've seen them in rare cases, but they wont help the OP.

 

[kwired]

The code book fill tables mention "compact conductors". I've never seen those or know if they exist. Didn't think that was DLO but something else. They are smaller, but not sure if they are enough smaller.

They exist. I've seen them in rare cases, but they wont help the OP.

They are pretty common here in aluminum XHHW conductors.

 

[Carultch]

I would have to think about it, but I think as the conduit circle becomes arbitrarily large (or the wires arbitrarily small) then the packing density would approach the circle packing of the plane density which is about .9 or (Pi/sqrt3)/6

You would be correct, although the formula is really pi*sqrt(3)/6 for circle packing on an infinite plane. It takes an three digit number to even get to a density that rounds to 0.9 instead of 0.8. For purposes in the context of conduit fill, the density is generally 70%-80% fill, with Fodor's 19 circles being the densest you can expect to see. Other than the trivial case of just one circle, of course.

 

[winnie]

Side question on this topic: When a _cable_ is inserted into a conduit, you take the largest diameter of the cable and use that to calculate the cross section as if it were round, then you treat it as a single wire. This can be a big benefit because if you only have a _single_ wire in a conduit (or a single cable assembly) then you can use 53% as your maximum fill.

For purpose of this section, what is an acceptable 'cable assembly'? If the OP were to take the requisite wires, bundle them together neatly, and then use 'lacing' to form them into an assembly, would that count?

(To the OP, I think you are stuck anyway. If you assemble 10 #3/0 and 1#1/0 you exceed the 53% fill number...I think you could do it with custom 'sector' conductors', but now you are talking big $$)