Diff. in conductors sizes in conduit

Status
Not open for further replies.

raider1

Senior Member
Staff member
Location
Logan, Utah
QES said:
Anyone know the Art. where nec mention about limits on diff. conductor sizes in same conduit.

Other than conduit fill, there is no limit what size wires can installed within the same conduit.

Chris
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
There is no such requirement. You can put a 500 MCM and a #18 in the same conduit if you wish. I wouldn't recommend it, but I know of is no rule against it.

There are rules about derating the ampacity of each conductor in a conduit, if there are more than three that carry current. There are rules about the required voltage rating of conductors, when there are conductors in the same conduit that serve different nominal voltages. Are you thinking along these lines?
 

iwire

Moderator
Staff member
Location
Massachusetts
I agree with Chris (raider1) as far as the code.

In the real world derating rules hit you hard if want to sneak a few extra small conductors in with a feeder.

You can usually take 70% derate on a 12 AWG with out any code issue, a 70% derate on 3/0 feeder is going to be costly.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
A few years back in a school job I was on the state inspector let us put 108 #8s in a 200 foot long run of 3" EMT because we were de-rating them to 20 amps apiece.

Ever see back to back tuggers break a good size chain? The scale only went to 2900 lbs and it was maxed out when the chain broke. We got a bigger chain and the head broke off the pull. Luckily for us it was inside the last nipple which was 12 inches long and we were able to pop it off and save the pull.

That was an interesting week....
 

kpepin

Senior Member
charlie b said:
There is no such requirement. You can put a 500 MCM and a #18 in the same conduit if you wish. I wouldn't recommend it, but I know of is no rule against it.

I've seen it done a million times. You have a large motor that has an internal thermal switch which is intended to be wired into the control circuit. Are you going to run seperate conduits into the ph of the motor (Which normally only has one knockout and is shaped in such a way to make adding another KO difficult) or just pull it all in one? As long as the conduit fill is correct and any derating issues are addressed, you are good
 

kpepin

Senior Member
K8MHZ said:
A few years back in a school job I was on the state inspector let us put 108 #8s in a 200 foot long run of 3" EMT because we were de-rating them to 20 amps apiece.

3" EMT only 96 #8 THHN allowed
41+ #8 only good for 19.25 Amps based on 90 degree chart

I hope this was a typo
 

peter d

Senior Member
Location
New England
K8MHZ said:
A few years back in a school job I was on the state inspector let us put 108 #8s in a 200 foot long run of 3" EMT because we were de-rating them to 20 amps apiece.

Ever see back to back tuggers break a good size chain? The scale only went to 2900 lbs and it was maxed out when the chain broke. We got a bigger chain and the head broke off the pull. Luckily for us it was inside the last nipple which was 12 inches long and we were able to pop it off and save the pull.

That was an interesting week....

I'm sure there are many more details you can add to that story, but that sounds pretty horrific!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
kpepin said:
I've seen it done a million times. You have a large motor that has an internal thermal switch which is intended to be wired into the control circuit. Are you going to run seperate conduits into the ph of the motor (Which normally only has one knockout and is shaped in such a way to make adding another KO difficult) or just pull it all in one? As long as the conduit fill is correct and any derating issues are addressed, you are good
Often when you are pulling large and small conductors in the same raceway, the smaller conductors will break if you have bends in the pull. I would never even think about trying to pull #18 in with 500kcmil.
The installation standard for the plant that I normally work in prohits putting control conductors in with power conductors when the power conductors are are #4 or larger.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
K8MHZ said:
A few years back in a school job I was on the state inspector let us put 108 #8s in a 200 foot long run of 3" EMT because we were de-rating them to 20 amps apiece.

Ever see back to back tuggers break a good size chain? The scale only went to 2900 lbs and it was maxed out when the chain broke. We got a bigger chain and the head broke off the pull. Luckily for us it was inside the last nipple which was 12 inches long and we were able to pop it off and save the pull.

The conductors were all OK?
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
peter d said:
I'm sure there are many more details you can add to that story, but that sounds pretty horrific!

I was just starting my second year of my NJATC apprenticeship. The tugger was up in 'the clouds', a catwalk about 45 feet above the floor of an auditorium. I was right next to the tuggers when the chains broke. That was our first test of the acoustics of the auditorium.

Please note that I counted the grounding conductors with my figure. 36 circuits with three wires each.

By the time the job was done we pulled 35.4 miles of #8 into the auditorium and it's supporting rooms to wire a total of 174 separate circuits.

I still have some of the rabbit from that job in my wire stash.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
don_resqcapt19 said:
Often when you are pulling large and small conductors in the same raceway, the smaller conductors will break if you have bends in the pull.
That is exactly what I had in mind, when I said that I wouldn't recommend doing this.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
peter d said:
A separate ground wire for every circuit????? Job spec?

You got it!

All of these ran to a dimmer room with *huge* remotely controlled dimmer panels. The real fun thing about the pull is that it went straight up for 50 feet and then across the 100 foot wide auditorium before it got to the square duct that passed through the wall into the dimmer room.

We had to pull up because the wire came on huge reels that had to be set up on the floor of the auditorium. I used some of the reels for tables and still have two of them in my basement.

Some of the circuits were 'stroked off' and some got pulled right off the reels. I have pulled in 750's that were easier to do than that job. That was, by far, the hardest pull I have ever done. I'll bet Ideal's stock rose a few points just from the amount of Yellow 77 we used on that pull. 3M probably did well too as there was nothing but Super 33+ on the job and we used it for everything, including making the heads up on our pulls. That high school had a bigger auditorium (1050 seats) than many colleges in the area. Tim (Tool Time) Allen's brother is a teacher at that school. I had the displeasure of meeting him there. He is a real Dick. No kidding, that is his (and Tim's) real last name.

Edit: Here is some info about the auditorium: http://rptheatrearts101.tripod.com/id10.html

The stage is 81 feet wide and 35 feet deep. The 100 foot figure I used is an estimate from wall to wall which counts the stairwells. There is also a moving orchestra pit.
 
Last edited:
Status
Not open for further replies.
Top