Parallel feeder lengths

[binney]

I'm getting ready to supply 10, 400amp subpanels on multiple floors. I'm using 2 pair of 4/0 alum. MC cable. The route is going to be very challenging and I'm wondering how to keep the feeders the same length. I'm also wondering how close in length your supposed to be. I only have two ways of doing it right now. I can pull them in and keep them as tight together as possible, which will be extremely challenging on the riser. The other option, I though of, was pulling them in and using a meter, I priced a TDR900 MEGGER. These meters you the speed of soud to measure wire length, how accurate are they? Is this just a waste of time and money.

All help is greatly appreciated.

 

[infinity]

They're supposed to be the same length. You could test them after they're pulled and leave slack on the shorter one at the terminations. One problem with parallel feeders and MC cable is that the EGC needs to be full sized in each parallel run of MC cable.

 

[hurk27]

While I agree they are required to be the same length, conductors 1/0 and above would have very little effect on the imbalance current between the conductors even if you have a difference of 6", look at the ohms per foot, then calculate the circuit out, and you will see that current between a pair of 1/0 conductors would not vary much in that short of length, now to me termination is very important and can and have caused a parallel conductors to overload, torquing your terminations is very important.

 

[iwire]

I'm getting ready to supply 10, 400amp subpanels on multiple floors. I'm using 2 pair of 4/0 alum. MC cable.

2 x 180 = 360, your load must stay below that.

Also how will you comply with 250.122(F)?

250.122(F) Conductors in Parallel. Where conductors are run in
parallel in multiple raceways or cables as permitted in
310.4, the equipment grounding conductors, where used,
shall be run in parallel in each raceway or cable.
Each parallel equipment grounding conductor shall be
sized on the basis of the ampere rating of the overcurrent
device protecting the circuit conductors in the raceway or
cable in accordance with Table 250.122.

 

[jimmac49]

Just keep in mind that not all conductors of the feeders have to be the same length, just the matched pairs of each phase and the neutral.

Jim

 

[maghazadeh]

I'm getting ready to supply 10, 400amp subpanels on multiple floors. I'm using 2 pair of 4/0 alum. MC cable. The route is going to be very challenging and I'm wondering how to keep the feeders the same length. I'm also wondering how close in length your supposed to be. I only have two ways of doing it right now. I can pull them in and keep them as tight together as possible, which will be extremely challenging on the riser. The other option, I though of, was pulling them in and using a meter, I priced a TDR900 MEGGER. These meters you the speed of soud to measure wire length, how accurate are they? Is this just a waste of time and money.

All help is greatly appreciated.

I agree with the points mentioned in replies so far.
You said (10) 400A panels on multiple floors, assuming (1) panel per floor = 10 floors, or (2) panels per floor = 5 floors. What are you going to do with vertical support of conductors?

For that many floors if I had a choice I would use bus duct sized to handle the total load including roof top mech. loads (if there is a straigh vertical shaft) and tap off at each floor.

 

[binney]

You were right with the 2 panels 5 floors. I recently started with a new company and the way they run things, our guy in the office designs everything, we just do the install. Which as you all know usually turns out to be a design as you go job.

We're using 4/0 MC, the main reason for MC is for it's flexibility, this is a large remodel and the MC will have to be fished between floors.

Until recently I never used MC bigger than #10, everything else I did was emt. I never questioned the use of 4/0 alum for 200 amps, without looking it up it sounded right, now I look and it's only good for 180amps.

Like I said I've never used MC larger than #10. By you guys questioning the ground size, I have to assume the larger wires come with a reduced ground. Otherwise my ground would be more than sufficient, for 400amps I need a #1.

Besides my MC problems, is it practical to try to use that TDR900 to gauge my wire lengths?

 

[maghazadeh]

Remember for avoiding problem with vertical support of conductors, use the riser type MC cable.
Why not use one feeder per panel instead of 2 parallel feeder since you are using Alum conductors which is way lighter than CU conductors considering easy install.

 

[don_resqcapt19]

... By you guys questioning the ground size, I have to assume the larger wires come with a reduced ground. Otherwise my ground would be more than sufficient, for 400amps I need a #1. ....

The issue with any cable installed in parallel is that the EGC is sized based on the ampacity of the conductors in that cable. The rule is 250.122(F) requires an EGC in each cable that is sized on the circuit ampacity. This results in a requirement for an EGC that is larger than what will be provided in the cable unless you get a custom made cable.

 

[kingpb]

If this is single phase feeder w/neutral, then you could use 4 conductor #4/0 AWG MC cable and utilize one of the full sized conductors for the ground, if it is 3 phase, then you will have to oversize your current carrying conductors to get the size ground you need so you would have to use a 250KCMIL to get the #1 AWG ground.

At least it won't be a custom cable.

 

[infinity]

If this is single phase feeder w/neutral, then you could use 4 conductor #4/0 AWG MC cable and utilize one of the full sized conductors for the ground, if it is 3 phase, then you will have to oversize your current carrying conductors to get the size ground you need so you would have to use a 250KCMIL to get the #1 AWG ground.

At least it won't be a custom cable.

Wouldn't 250.122(B) require you to increase the size of the EGC if you increased the ungrounded conductor size? Or is 250 kcmil Al only good for 200 amps?

 

[kingpb]

Wouldn't 250.122(B) require you to increase the size of the EGC if you increased the ungrounded conductor size? Or is 250 kcmil Al only good for 200 amps?

250KCMIL Al 3 conductor w/ground MC cable is rated for 205A at 75 deg C, and can be purchased with a 3/0 ground. Looks like the manufacturer already thought about this since you could parallel 4 x 250KCMIL in this case for 800A, and the #3/0 ground is still good.

 

[binney]

it's a three phase application, but they make a MC with an oversized ground. I imagine just for this reason. The wholesaler had some in stock even.