View Full Version : question on conduit fill limitations

09-13-2007, 09:37 PM
i've used your forum posts for answering every question but this one; so ive had to create a membership to ask this one. thank you so much for all the useful information. the professionalism displayed on this website is wonderful. its nice to see.
but now to the question... ive searched the nec charts and multiple websites but none seem to answer this question. i feel dumb by not being able to answer it myself but i dont want to make a mistake by assumption. im installing a 220 line to the back of my house and im running 6/3 w/g thhn cable through conduit until it runs under the house and when it comes back out. i was wondering what is the fill capacity for the conduit. the only reason its specifically important is because of the needs for a roto hammer bit that can make way for the conduit. the cable in its sheath is just under 3/4" making it acceptable--for the standards according to 3+ cables only being allowed to occupy 40% of the conduit--to then occupy a 1-1/4" conduit. i have only two 90 degree bends. the question i have though is because the cable is sheathed in its own, singular unit can i then follow the guidelines for one cable per conduit and let it fil a capacity of 53% and use the 1" conduit?
thank you very much for your attetion to this question.

09-13-2007, 09:57 PM
40% of 1 1/4" is 1/2" so that would make your 3/4" feed too big according to the standards you posted. Also making it more than 53% of 1" conduit. You would need 1 1/2" conduit to allow the 53% usage spec for 3/4" wire. I'm assuming that since it is all sheathed together it would be considered one wire, also assuming they reduce the fill capacity for multiple wires because they expect them to be spread out more within the conduit due to not being bundled together thus taking up more space even though they have the same mass as a single wire feed. I'm not an electrician though, maybe a licensed one can add a little more info.

Bob NH
09-14-2007, 06:35 AM
You can use the 53% for single wire but the area of the cable must be determined based on the maximum diameter.

Area of a 1" PVC conduit is 0.832, and 53%=0.441 sq in., while an 0.75" cable is also 0.441 sq in, so it just makes it.

You are going to have a hard time pulling a 3/4" diameter cable through 1" conduit. Just fastening a puller to it could make it too large to get through the conduit unless you strip a piece of the jacket and connect to the conductors in a staggered fashion. You better have some lubricant and a come-along.

A more significant question is, what kind of "cable" are you using?

Under ground (as in under a building) is considered a wet location, even if in conduit. Anything you put under ground must be suitable for a wet location. THHN is not rated for a wet location.

Wire is often rated for both THHN and THWN, and THWN is rated for a wet location. The designation must be marked on the insulation.

THHN is also use in NM cables, but NM is NOT rated for wet locations.

What are the markings on the cable?

If you don't have the cable yet, you might consider running individual conductors, or service entrance cable.

09-14-2007, 06:56 AM
the run will go under my house and attach on the joists by way of clamps. it is dry under my ouse but i spoke with the code inspector in my town and he said thhn/thwn should be fine. thank you bob for clarifying. i ran the math the same way you did and the number being dead on i thought i better ask all the questions i had; to be DAMN safe. i found the wire on **** for half of the cost of running individual terminals. i really couldnt turn that down so the chance of running the larger size of conduit was worth it. im glad to get such a quick response. i have lube and a fish to do the job. is a fish also whats meant by a come-along?
PS thank you again to all the patient and incredibly helpful professionals who help answer these posts, thus preventing ignorant mistakes, and thus saving lives. the more electricians i know the safer i feel.

09-14-2007, 07:02 AM
If it is a cable and under the house why are you installing it in pipe?

09-14-2007, 08:00 AM
it runs from the main breaker outside for four feet then under the house. it then comes out again for another four feet. as per code it regulates the need for a conduit while running any span 'outside' the house

09-14-2007, 08:59 AM
as per code it regulates the need for a conduit while running any span 'outside' the house

Bob NH
09-14-2007, 09:06 AM
A come-along is a cable device that you can use to pull 1000 pounds or more if necessary. You probably don't need that much but you probably need at least a lever arrangement on the pulling end or a couple of buddies.

A pusher helper where the cable goes in is a big help.

You are going to use the fish wire to run a rope through the conduit. With a large cable like you have you are going to need more than the fish wire to pull it.

Since you probably don't have professional tools for connecting to the pull cable you will have to connect the wires in a way that doesn't make an oversize lump. I usually strip the jacket off a length of conductors and cut the conductors to staggered length, strip the individual insulation off a length of conductor and remove about half the strands, put the remaining strands through the rope with a "marlinspike" (maybe a nice smooth nail to make a hole), and wrap the strands around the wire to make a good connection. I use a braided rope rather than twisted so it doesn't unravel or untwist from the wire through it.

Then put lots of lube on the head end and keep lubing the cable as required as it goes in.

The conduit will have to be well anchored at the ends and you want to be sure you don't burn through the PVC fittings at the ends from the friction of pulling. The rope is probably going to do the burning.

09-14-2007, 11:24 AM
im installing a 220 line to the back of my house and im running 6/3 w/g thhn cable through conduit until it runs under the house and when it comes back out.

it runs from the main breaker outside for four feet then under the house. it then comes out again for another four feet. as per code it regulates the need for a conduit while running any span 'outside' the house

Based on what is posted above you are not (it would be a violation) installing NM cable but instead you are installing either SE-R or a UF cable.

This cable if aluminum is only good for 50 amps in SE-R and 40 amps if UF and if it is copper UF 55 amps and 65 for SE-R.

You stated this is coming off the main so is this the main feeder for the house? If it is then you will need something bigger than #6.

Type SE-R can be installed exposed (without conduit) if it is not subject to damage.
The only part of the cable that would require protection would be the part exposed to the outside of the house so a full pipe installation would be unnecessary unless there are some rules adopted in your area that requires the cable be protected for its entire length.

338.10(B)(4) (b) Exterior Installations. In addition to the provisions of this article, service-entrance cable used for feeders or branch circuits, where installed as exterior wiring, shall be installed in accordance with Part I of Article 225. The cable shall be supported in accordance with 334.30, unless used as messenger-supported wiring as permitted in Part II of Article 396.

340.10(4) Installed as nonmetallic-sheathed cable. Where so installed, the installation and conductor requirements shall comply with Parts II and III of Article 334 and shall be of the multiconductor type.

09-14-2007, 03:07 PM
thanks jw and bob. jw, the part mentioned about needing the pipe outside the house--only--is exactly what i was trying to convey/say. the conduit will only span the 4ft lengths plus a little to feed through the concrete. im sorry for the error; thanks for the clarification. thank you to all for the tremendous help on this post.

09-20-2007, 11:10 AM
For some reason I thought that you were not supposed to run multiconductor sheathed cable through a conduit. It would appear from this discussion that it can be done. Correct?