Multi-3phase circuits in one conduit

[fastfrank]

Want run multiple 480 3 phase circuits in one conduit. Each circuit consists of a 15 amp load. The length is about 200'. What is the correct way to derate them and size the conductors?

Thanks
FF

 

[roger]

Read 310.15(B) 1 through 5

Roger

 

[charlie b]

Step 1 is to figure out how many circuits you want to put in the same conduit. That gives you the number of current-carrying conductors. You use Table 310.15(B)(2)(a) to find a derating factor. Then I would do a "backwards calculation," to find the minimum wire size. Finally, I would do a VD calculation, to see if the selected wire size would have an acceptable VD.

Example: You wish to put four 3-phase circuits in the same conduit. That's 12 CCCs. The derating factor is 50%. Divide 15 by 50% and you conclude that the conductor's ampacity (before derating) must be 30 amps. That would be a #12 (you can use the 90C column for derating purposes). I calculate the VD at 2%, which is acceptable. So I would use #12 conductors protected with 15 amp circuit breakers.

 

[infinity]

How many circuits?

 

[LarryFine]

This strikes me as the perfect example of when a 200' feeder to a sub-panel near the loads might be a better option (not knowing the details, of course.)

 

[iwire]

200' is not a long run at all if your used to dealing with commercial spaces, many customers do not want panels all over the place.

Next time your in a Home Depot, Lowe's, Target, Walmart etc see if you can find the panels. They would rather bump up the wire sizes then to drop panels everywhere. (A few of those places actually use prefabbed electric rooms that are dropped in place and you just connect the field wiring.)

Even in office buildings 200' is not a long run by the time you get to the last device on the circuit. We use lots of 10 AWG for 20 amp circuits. I was even involved in a large warehouse that we had to use 4 AWG CU for the 20 amp roof top service receptacles.

I agree a panel can be less costly option in some applications but many times that is just not acceptable to the customer.

 

[captaincrab55]

fastfrank, Don't forget reliability... If you get a burn-out in one conduit, you could shut a commercial business down...

 

[benaround]

captian, How or why could you get a 'burn out' in a conduit??

 

[iwire]

captian, How or why could you get a 'burn out' in a conduit??

It happens and when it does it will usually take out some of the adjoining conductors.

Normally I find it in wet under slab runs but it can happen anywhere.

 

[peter d]

Next time your in a Home Depot, Lowe's, Target, Walmart etc see if you can find the panels.

Actually that's usually what I try to do when I shop at those stores. Just follow the the rack of conduits but try not to bump into anyone or their shopping cart in the process.

 

[iwire]

Actually that's usually what I try to do when I shop at those stores. Just follow the the rack of conduits but try not to bump into anyone or their shopping cart in the process.

As weird as it is I was thinking of this thread while driving to work and realized that at Lowe's they did run a few feeders out key areas. The lighting dept, paint area maybe the carpet racks.

Anyway the feeders supply a short (less then 8') section of bus duct, on the bus duct they placed a few breaker units and had cord drops from them.

Of course this means the breakers are 22' AFF but they have lifts.

 

[peter d]

Anyway the feeders supply a short (less then 8') section of bus duct, on the bus duct they placed a few breaker units and had cord drops from them.

Of course this means the breakers are 22' AFF but they have lifts.

True, Lowes seems to be the only big box retailer that does that. Also, it looks like all the bus duct stuff, cord drops, and all the wiring that goes on the store racks is all prefabbed and just plugs together.