Feeder Cables question from a "newbie"

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Hello all,

So I'm an electrical engineer, but have limited construction experience. I understand concepts and I'm handy with the NEC and NFPA codes, but that's a far cry from how things actually are constructed.

My question comes from working out conduit sizing calculations. I would like to provide power feeds for multiple equipment in the same conduit between the electrical room and the room with the equipment. I know for larger power requirements each phase would be run as a separate single core cable per phase, but I'm wondering about loads that can be fed from a multi-core cable.

I've got some data sheets from cable manufacturers to see what the overall area of the cables are.

So lets say I have a 3phase pump that requires 3#10 and a #10CPC (ground), would this be a 3 core cable plus a separate single core ground cable, or would it be a 4 core cable? or?
What if there is also a neutral cable required?

thanks!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Welcome to the Forum:

NEC 300.3(B) states:
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).
There are some exceptions, but basically in a situation like yours where you are using a multiconductor cable, the cable should consist of your phase conductors and your equipment grounding conductor.
(The grounded conductor would only be present if needed)
 

iwire

Moderator
Staff member
Location
Massachusetts
Hello all,

So I'm an electrical engineer, but have limited construction experience. I understand concepts and I'm handy with the NEC and NFPA codes, but that's a far cry from how things actually are constructed.

My question comes from working out conduit sizing calculations. I would like to provide power feeds for multiple equipment in the same conduit between the electrical room and the room with the equipment. I know for larger power requirements each phase would be run as a separate single core cable per phase, but I'm wondering about loads that can be fed from a multi-core cable.

I've got some data sheets from cable manufacturers to see what the overall area of the cables are.

So lets say I have a 3phase pump that requires 3#10 and a #10CPC (ground), would this be a 3 core cable plus a separate single core ground cable, or would it be a 4 core cable? or?
What if there is also a neutral cable required?

thanks!

Are you planing on having cables inside raceways?

In general that is an unusual and difficult installation method here in the US.

Typically we use individual conductors in whatever sizes and numbers required.
 
great, I wasn't sure if that meant it was ok to have a single cable carry multiple phases with ground when required.

300.3(B)(1) is for "paralleled installations", does that mean when two or more single core cables are used per phase to split the ampacity load amongst the cables?

In that type of case would each phase go in it's own conduit or would all the cables for the circuit run in the same conduit? ie. if I have a large three phase load and I opt to use two single core cables per phase, would I use one large conduit for all the cables or would I split the cables into three different conduits in some manner?

thanks!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You could do either, but each conduit would need to contain the same conductors (Phase A,B,C, N, and ground in each conduit).
You need to keep in mind that you have to derate the ampacity as the number of conductors grows and, generally, you can't parallel anything less than 1/0.
 
gotcha!

ampacity adjusted via 310.15(B)(2)(a), and 310.4(A) and it's exceptions discuss cable sizing for parallel runs?

thanks!
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
For uniformity, I would recommend you refer to NEC definitions; and that the conductors in question are branch circuits, not feeders. May seem like nit-picking, but when working within NEC it may help keep you on the straight and narrow as far as references from one article to the next.

Interesting reference to single core and 3-core, as that is usually outside US terminology.
 
Are you planing on having cables inside raceways?

In general that is an unusual and difficult installation method here in the US.

Typically we use individual conductors in whatever sizes and numbers required.

Yes, the cable will run in raceway (in this case conduit, most likely EMT). I will be feeding separate pieces of equipment in various locations and would like to run all the cables for each circuit in the same conduit to the room to be routed individually as needed when it arrives in the room. I know in general single phase house wiring the cable has multiple cores for phase/N/G, so I'm wondering if that translates to running three phase power as well.

thanks!
 
For uniformity, I would recommend you refer to NEC definitions; and that the conductors in question are branch circuits, not feeders. May seem like nit-picking, but when working within NEC it may help keep you on the straight and narrow as far as references from one article to the next.

Interesting reference to single core and 3-core, as that is usually outside US terminology.

Sorry, yes, I'm speaking of branch ciruits.

I wasn't sure how they're called in the US, I know overseas they use the term core to mean the individual insulated conductor, but not sure how the terminology works in the US.
 
follow up, is it more common to use individual cable per phase or to use a cable that combines all phases and ground in a single cable?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...In that type of case would each phase go in it's own conduit or would all the cables for the circuit run in the same conduit? ie. if I have a large three phase load and I opt to use two single core cables per phase, would I use one large conduit for all the cables or would I split the cables into three different conduits in some manner?

thanks!
You can put them in one conduit or two conduits. The use of 3 conduits would be an insolated phase installation and would, in many cases, be a NEC violation.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
I think your question about how common certain installations are can be answered more accurately if you narrow down the field to the type of work or installations you work with more often. Different applications different material.
Also I dont think anyone answered your question in the first post about a neutral. If you're talking about a 3-phase motor then a neutral is likely not required. Occasionally some control components may required line to neutral connections so you may need to drag around a neutral for that (i.e. 3-phase 208VAC motor with control components running on 1-phase 120VAC).
 

wiigelec

Member
Location
Red Desert
I would like to provide power feeds for multiple equipment in the same conduit between the electrical room and the room with the equipment.
Depending on the amperage of your feeder circuits running multiples in the same raceway can be a bad idea due to induced voltage in adjacent circuit conductors. You will most likely be better served by running each circuit in its own conduit...
 

wiigelec

Member
Location
Red Desert
Another thing to consider:

Let's say you want to run 10 cable fed circuits in a single conduit. The cables cannot be installed one at a time, all ten will need to be installed at once. This means you will need 10 individual cable spools and if they are each going a different spot this type of wire pull can be a real pain.
 
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