Multi-runs of multicore cables

[Power0020]

I wonder if the code permits wiring power equipment (LV or MV) with mutli-runs of multicore cables per phase? I am familiar with ,multi-core single core cables per phase which looks reasonable for ampacity purpose however running multiruns of multicore cable may be just easier during installation, but a bit of conductor waste!

any ideas?

 

[waross]

It is good practice and also a code requirement of the codes that I work to that each cable or raceway include an equal number of phase and neutral conductors.
The advantage of multiple multicore cables is that a proper installation avoids the sheath and armour current issues that multiple single core cables may present.

with mutli-runs of multicore cables per phase?

This sounds as if a multicore cable is proposed to be used for each phase. That would negate most of the advantages of a multicore cable installation. Additionally, this may result in greater cable inductive reactance with greater reactive voltage drop and a lower voltage at the receiving end.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Power0020]

Well, the example here to explain, this is to put two multicore cables (R1,Y1,B1 & BK1) and (R2,Y2,B2 & BK2) and to connect -in the JB- the following:

R1 with R2 with R terminal of equipment
Y1 with Y2 with Y terminal of equipment
B1 with B2 with B terminal of equipment

would this be favorable with multicore cables?

 

[7anoter4]

The way you connected the cable is correct. NEC also permits to use multiconductor cable in parallel. See art.300.3 Conductors.: (1) Paralleled Installations. "Conductors shall be permitted to be run in parallel in accordance with the provisions of 310.10(H)
310.10(H) Conductors in Parallel:
“(1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).”
Art.310.10(H) (3):
“Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics".
If the cable it is a NEC listed type then the ampacity is recommended also.
In a multi-core cable the cores are twisted and change their location in space so the current unbalance between parallel cores is concealed. So- as waross explained-this connection is the best.
The single-core cable advantages are:
a)Individual cores= less weight easier installation.
b)More space between cores =better cooling= increased ampacity.
Disadvantages are:
c) More distance more reactance
d) More unbalance between parallel cables
e) More expensive [total assemble of cables].

 

[squeeky]

I seem to be the odd one out that has read this slightly differently to the answers supplied. Purhaps as I have seen this many times in Africa.

Yes you can run more than one cable with multicore cables with a phase connected to each conductor in the cable i.e., where it has one of each phase contained within it. This is normally done where the bending radius on the cable ways is very tight and you cannot run large conductors due to the limitations of bending. Smaller cables allow tighter bends.

You can run single core conductors but you must run them in trefoil. This is groups of conductors of each phase strapped together and seperated from other groups.

These two examples are to negate (neutralize) the emf and the resultant heating from imbalances.

Here in Africa I have seen the following. Four cables connected to a transformer and the other ends connected to the Motor Control Assembly. They use all four conductors within one cable as one phase. They then earth both ends of the cable armouring and the cable cooks and fails. The armouring acts as a single turn of a transformer and results in circulating currents in the armouring.

They then disconnect one earth on one end and the problem is solved as there is no circuit ....until the protective insualtion fails. The breakdown of the outer sheath completes the circuit to earth again and ....the cable cooks.

You can purchase single core conductors with aluminium armouring. If you bring single cores into the transformer connection box or the Motor Control Assembly, these must be through Aluminium gland plates or some other non inductive material. Steel / iron products are not permitted. Again circualting currents.