grouping of phase conductors in cable tray, and into an enclosure

[arubenstein]

Hello,

I have googled and searched this forum quite a bit about my issue, and while I see some relevant references, I open the floor for more comment.

The application is the wiring of a 600 kw generator. As it stands now, the genset has a 800 amp breaker in an enclosure which is attached to the generator. We have wired to that breaker, using RHH/RHW type cable, two conductors per phase and a single neutral, for a total of seven cables. We exit the enclosure, having drilled one hole per conductor, and then bring the cables to a ventilated, uncovered cable tray. The conductors are arranged in the tray as AABBCCN. Each cable is 1" in diameter (262 mcm) and then has a 1" space between. The cable is cable tied to the cable tray at a frequency of about once per two feet.

At the 'other end' of the cable tray, we drop out of the cabletray into the top of a piece of switchgear, and again, having drilled one hole per cable, we come into that switchgear and then land on the bus bar within.

My questions are:


a) is the cable arrangement of AABBCCN in the single cable tray actually a problem? From what I have researched, I don't believe so. But I am happy to be corrected.

b) is the way that the enter the switchgear an issue (one cable per hole drilled in metal)? From what I have researched, this is the real issue. I believe the solution to be to rearrange the wires so that all seven conductors enter through the same hole.

I have attached a photo showing.




I am not trying to force a situation down an inspectors throat or slip one by; I want to know the real issues and what the proper technique is.

 

[Smart $]

... using RHH/RHW type cable... bring the cables to a ventilated, uncovered cable tray.

Are the cables rated for cable tray (Type TC)?

a) is the cable arrangement of AABBCCN in the single cable tray actually a problem? From what I have researched, I don't believe so. But I am happy to be corrected.

Yes, a problem by my interpretation...

(C) Connected in Parallel. Where single conductor cables
comprising each phase, neutral, or grounded conductor of
an alternating-current circuit are connected in parallel as
permitted in 310.10(H), the conductors shall be installed in
groups consisting of not more than one conductor per phase,
neutral, or grounded conductor to prevent current imbalance in
the paralleled conductors due to inductive reactance.
Single conductors shall be securely bound in circuit
groups to prevent excessive movement due to fault-current
magnetic forces unless single conductors are cabled together,
such as triplexed assemblies.

I'd install ABCNABC.

​

b) is the way that the enter the switchgear an issue (one cable per hole drilled in metal)? From what I have researched, this is the real issue. I believe the solution to be to rearrange the wires so that all seven conductors enter through the same hole.

Yes, real issue...

[300.20 Induced Currents in Ferrous Metal Enclosures
or Ferrous Metal Raceways.

(A) Conductors Grouped Together. Where conductors
carrying alternating current are installed in ferrous metal
enclosures or ferrous metal raceways, they shall be arranged
so as to avoid heating the surrounding ferrous metal
by induction. To accomplish this, all phase conductors and,
where used, the grounded conductor and all equipment
grounding conductors shall be grouped together.

Exception No. 1: Equipment grounding conductors for
certain existing installations shall be permitted to be installed
separate from their associated circuit conductors
where run in accordance with the provisions of 250.130(C).

Exception No. 2: A single conductor shall be permitted to
be installed in a ferromagnetic enclosure and used for skineffect
heating in accordance with the provisions of 426.42
and 427.47.

(B) Individual Conductors. Where a single conductor
carrying alternating current passes through metal with magnetic
properties, the inductive effect shall be minimized by
(1) cutting slots in the metal between the individual holes
through which the individual conductors pass or (2) passing
all the conductors in the circuit through an insulating wall
sufficiently large for all of the conductors of the circuit.

Exception: In the case of circuits supplying vacuum or
electric-discharge lighting systems or signs or X-ray apparatus,
the currents carried by the conductors are so small that
the inductive heating effect can be ignored where these conductors
are placed in metal enclosures or pass through metal.
.

Informational Note: Because aluminum is not a magnetic
metal, there will be no heating due to hysteresis; however,
induced currents will be present. They will not be of sufficient
magnitude to require grouping of conductors or special
treatment in passing conductors through aluminum
wall sections.
​

​

I have attached a photo showing.

Sure looks like more than two conductors per phase and one neutral...???

 

[arubenstein]

Are the cables rated for cable tray (Type TC)?

Yes, this cable is listed specifically for use in a cable tray.

Yes, a problem by my interpretation...

I'd install ABCNABC.

See, this is where I run into confusion. Since there is no conduit surrounding the cables, where would there be inductive reactance?

Yes, real issue...

​

Sure looks like more than two conductors per phase and one neutral...???

I should have mentioned that there are four generators wired in parallel, and I have described how each one is wired. The picture shows all four.

 

[Smart $]

...

See, this is where I run into confusion. Since there is no conduit surrounding the cables, where would there be inductive reactance?

...

Between conductors. Why do you think conductors move during fault conditions?

 

[brian john]

The second picture 4 post down is what is in store for your installation (IF I UNDERSTAND YOU CORRECTLY)

http://forums.mikeholt.com/showthread.php?t=148912&page=2&highlight=routing+conductors

 

[weressl]

See, this is where I run into confusion. Since there is no conduit surrounding the cables, where would there be inductive reactance?

The magnetizable wall of the enclosure will heat up. Either use an OZ Gedney 3/4 condutor cable gland or a non-magnetizable gland plate, such as stainless, bronze, mica.

 

[masterinbama]

Hello,




b) is the way that the enter the switchgear an issue (one cable per hole drilled in metal)? From what I have researched, this is the real issue. I believe the solution to be to rearrange the wires so that all seven conductors enter through the same hole.



I am not trying to force a situation down an inspectors throat or slip one by; I want to know the real issues and what the proper technique is.

You can rearrange them so that they enter the enclosure as sets of 4 conductors, I.E. A B C N per hole.

 

[Smart $]

The magnetizable wall of the enclosure will heat up. Either use an OZ Gedney 3/4 condutor cable gland ...

I wouldn't think a single-conductor cable glands would mitigate the hysteresis.

 

[kwired]

The second picture 4 post down is what is in store for your installation (IF I UNDERSTAND YOU CORRECTLY)

http://forums.mikeholt.com/showthread.php?t=148912&page=2&highlight=routing+conductors

I think second picture in post 16 of that thread is what you were referring to.

 

[weressl]

I wouldn't think a single-conductor cable glands would mitigate the hysteresis.

Correct and that is why my post said '3/4 conductor cable gland'. Three phases and a neutral where needed. In this case it would be advisable to have individual neutral for each set of three-phase conductors.

 

[Smart $]

Correct and that is why my post said '3/4 conductor cable gland'. Three phases and a neutral where needed. In this case it would be advisable to have individual neutral for each set of three-phase conductors.

Ahh... mama mia!!! :slaphead: I misread your post as 3/4" conductor cable gland as opposed to 3-4. :roll: