parallel feeders in same raceway

[mull982]

I have an application which requires 2 sets of 250MCM parallel feeders at 480V. The contractor is wanting to run both of these feeders in the same conduit. I am trying to decide weather or not to let him run both in the same conduit but am not all that knowledgeable on what the effects of this will be.

Can someone explain if this is good practice and what some of the negative effects will be? Are there any code violations by doing this.

This circuit is also a circuit which is being controlled by an SCR. I dont know how this will effect this situation.

 

[iwire]

There is no problem and it is relatively common.

Of course you take a loss on the ampacity of each conductor due to the required derating for more then 3 current carrying conductors in a raceway.

 

[iwire]

I will assume copper THHN

Two sets of three 250s run in two conduits has an ampacity of 510 amps

Two sets of three 250s run in one conduit has an ampacity of 464 amps

 

[LarryFine]

And people wonder why Bob has such a high post count. :grin:

 

[SEO]

Bob Made A Good Point About The Derating, Another Possibility Is That There Might Not Be Enough Room To Run Two Conduits And If The Ampaicity After Detating Works You Still Can Get The Job Done. A Little More Labor To Pull Wire But The Savings In Labor To Install The Conduit Plus The Price Of Conduit And Fittings It Might Be Worth It.

 

[Cow]

One other thing, if you're pulling an EGC, you will only need one instead of two.

 

[mull982]

OK Thanks guys. I wasn't sure if there could possibly be inductance casued by two cables of the same phase adding together inside a conduit and inducing a magnetic field on the conduit or other cables.

I had heard once before that it wasn't good practice to put multiple cables of the same phase in a three phase circuit in the same conduit because the same phases would add together and cause excess heat, and induce a magnetic field on the metal conduit. Any truth to this, or any difference btwn single phase and three phase applications.

Besides De-Rating in there any other downfalls to pulling parallel feeders in the same conduit? How about for a VFD application?

 

[charlie b]

OK Thanks guys. I wasn't sure if there could possibly be inductance caused by two cables of the same phase adding together inside a conduit and inducing a magnetic field on the conduit or other cables.

Certainly the magnetic field of one Phase A conductor will add to that of the other Phase A conductor. But they will be nearly cancelled out by the magnetic fields of the pair of Phase B conductors and the pair of Phase C conductors.

I had heard once before that it wasn't good practice to put multiple cables of the same phase in a three phase circuit in the same conduit because the same phases would add together and cause excess heat, and induce a magnetic field on the metal conduit. Any truth to this, or any difference between single phase and three phase applications.

Definitely true. Incorrect installations have caused fires. That is the reason we put all three phases (plus the neutral and the EGC) in the same conduit, except under specific, limited circumstances. Start with 300.3(B), and follow the trail to the other paragraphs it cites.

Besides De-Rating in there any other downfalls to pulling parallel feeders in the same conduit? How about for a VFD application?

You have to deal with conduit fill limitations, and there may be issues with pulling tension. As to VFD applications, this brings harmonics into the picture, and you may wind up having to count the neutral wires as being current-carrying conductors (which in turn has an impact on derating).

 

[mull982]

Ok I see as long as there are other phases run with two conductors of the same phase the fields will cancel. The problem becomes when you run only multiple feeder on the same phase by themselves in a conduit.

Is there any reason why a single 500MCM cant be used as opposed to the 250MCM cable other than economics and conduit size? In other words is there any performance advantage from using parallel 250MCM's as opposed to (1) 500MCM?

 

[charlie b]

In other words is there any performance advantage from using parallel 250MCM's as opposed to (1) 500MCM?

A pair of 250's starts with a combined ampacity of 580 amps (90C rating). If they are in the same conduit, they get derated (from the 90C value) to 464 amps (80%). A single 500 has an ampacity of 380. So there is an advantage in the ampacity. I can't tell you anything about advantages, one way or the other, in the matter of labor effort to install or cost of materials.

 

[iwire]

My bet is the EC does not want to handle 500 Kcmil if he does not have to.

It can be a bear if your not used to it and don't have the tools for it.

That said I feel no sympathy for someone that bid the job one way then wants to change it for their own ease.

 

[bbaumer]

.......

I had heard once before that it wasn't good practice to put multiple cables of the same phase in a three phase circuit in the same conduit because the same phases would add together and cause excess heat, and induce a magnetic field on the metal conduit. Any truth to this, or any difference btwn single phase and three phase applications.

......

Definitely true.

Without looking it up I THINK it is allowed if the install is in PVC, but still not a good idea.

I saw an installation once on a building service, don't remember the size now but over 1000A, with several sets of parallel 500's from the transformer secondary into the main switchboard which was located in the basement. Secondary conductors were in 4" PVC. All the "A" phase in one conduit, all "B" in another etc.

All was hunky-dory until someone moved into the office above the switchboard. His computer screen was all screwed up. Couldn't even use the computer. He had replaced the screen, video card and some other things and discovered it was his location that was the problem.

I got a call on it and got the bright idea to shield the secondary conduits with sheet metal. I did not know at the time that the secondary conductors were paralleled in the manner they were. It did not cross my mind.

The shielding reduced but did not eliminate the interference. One of the electricians who installed the shield noticed it was very hot to the touch.

Did a little more investigating, discovered how it was wired and had to shut down the building one weekend to re-terminate everything. Fortunately the conductors all reached. Barely.

The problem went away instantly.

 

[iwire]

Definitely true.

I would say definitely one opinion. :smile:

Without looking it up I THINK it is allowed if the install is in PVC, but still not a good idea.

It is allowed, I have personally done it twice with 3000 amp services.

Kind of looks like this, although this picture is of someone else's job.

All was hunky-dory until someone moved into the office above the switchboard. His computer screen was all screwed up. Couldn't even use the computer. He had replaced the screen, video card and some other things and discovered it was his location that was the problem.

Can't comment on that, I never went in with a meter and these where electrical rooms in warehouse type space so no PCs to be effected.

I got a call on it and got the bright idea to shield the secondary conduits with sheet metal.

Thats does not sound like a good idea. :smile:

Did a little more investigating, discovered how it was wired and had to shut down the building one weekend to re-terminate everything. Fortunately the conductors all reached. Barely.

Seems like a lot of trouble.

The problem went away instantly.

I wonder how many other solutions could have also fixed the issue?

I also wonder if the original installer fooled all the rules that the NEC does require?

 

[bbaumer]

I would say definitely one opinion. :smile:



It is allowed, I have personally done it twice with 3000 amp services.

Kind of looks like this, although this picture is of someone else's job.

Can't comment on that, I never went in with a meter and these where electrical rooms in warehouse type space so no PCs to be effected.




Thats does not sound like a good idea. :smile:




Seems like a lot of trouble.



I wonder how many other solutions could have also fixed the issue?

I also wonder if the original installer fooled all the rules that the NEC does require?

Not sure if you are agreeing with or making fun of me on this. Doesn't matter, I can take it if so, even deserve it sometimes. :grin:

You're are right about the shielding plan. It was NOT a good idea. It was definitely a learning experience though. :grin: :grin:

Sure looks pretty though. Too bad it can cause problems.

 

[mull982]

A pair of 250's starts with a combined ampacity of 580 amps (90C rating). If they are in the same conduit, they get derated (from the 90C value) to 464 amps (80%). A single 500 has an ampacity of 380. So there is an advantage in the ampacity. I can't tell you anything about advantages, one way or the other, in the matter of labor effort to install or cost of materials.

What section of the NEC specifies the 80% deratting for parallel cables in the same conduit? I cant seem to find it.

 

[mayanees]

310.15

310.15

What section of the NEC specifies the 80% deratting for parallel cables in the same conduit? I cant seem to find it.

NEC 2005 table 310.15(B)(2)(a)

 

[LarryFine]

What section of the NEC specifies the 80% deratting for parallel cables in the same conduit?

I thing the rules about rats in conduit is in the building codes. :grin: