Parallel conductors question, kind of

sii

Senior Member
Location
Nebraska
We are installing an industrial machine that I have a question about. I’ve been asked to review the contractor’s proposal which is this:

Currently there is a 600 amp 2-wire, 480 volt panel fed from a 600 amp breaker in a 1200 amp switchgear. The conductors are parallel 350’s in two conduits (I am unsure if there is a neutral and I don’t know the EGC size but let’s assume it’s compliant). The panel is very lightly loaded, probably less than 50 amps at any time. I’ve not been able to find out yet if the panel is an MLO or main breaker. The contractor is proposing to remove one set of conductors from the 600 amp panel and reroute them to a 400 amp fused disconnect with 350 amp fuses, then feed the new equipment from the disconnect.

I haven’t opened a code book in a few years so this is a ways out of my wheelhouse. I don’t see this as code compliant unless there is a 350 amp or smaller main breaker in the 600 amp panel.

This will not likely be inspected but I need it to be a safe installation regardless. I’d welcome any help on this.
 

retirede

Senior Member
Location
Illinois
I’d think that both runs of 350 originating from a 600A OCPD would be subject to tap rules. Limited length and terminating on a suitably sized OCPD.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I’d think that both runs of 350 originating from a 600A OCPD would be subject to tap rules. Limited length and terminating on a suitably sized OCPD.
Agree
They would no longer be parallel conductors and 240.21 would be the prime point to take into account
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
To be compliant you might have to add a fuse disconnect in front of the 600 amp panel board too. On the other hand it is hard to know exactly what is there from your description but potentially it could be compliant depending on how far the taps run and the rating of the over current protection device that feeds the tap.

Incidentally you don't actually need a fused disconnect. It can just be a junction box with some fuse blocks in it that the tap terminates on.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The tap rules almost always require the conductors to terminate in a single protective device. I agree with your thought it needing new protection at the panel.
 

sii

Senior Member
Location
Nebraska
Sorry for the delay in coming back to this.

Thank you for the input which has led me to some conclusions:

Disconnecting one set of the parallel feeders makes them no longer parallel conductors and I don't see any way that modification as I described would be allowed by code.

Therefore both sets would have to be tapped at the same location which would maintain them as parallel. Now I think if we comply with 240.21(B)(4) I think we should be OK.

Taps over 7.5 m (25 ft) Long. Where the feeder is in a
high bay manufacturing building over 11 m (35 ft) high at
walls and the installation complies with all the following
conditions:
(1) Conditions of maintenance and supervision ensure that
only qualified persons service the systems.
(2) The tap conductors are not over 7.5 m (25 ft) long
horizontally and not over 30 m.
(3) The ampacity of the tap conductors is not less than
one-third the rating of the overcurrent device protecting
the feeder conductors.
(4) The tap conductors terminate at a single circuit breaker
or a single set of fuses that limit the load to the ampacity
of the tap conductors. This single overcurrent device
shall be permitted to supply any number of additional
overcurrent devices on its load side.
(5) The tap conductors are protected from physical damage
by being enclosed in an approved raceway or by other
approved means.
(6) The tap conductors are continuous from end-to-end and
contain no splices.
(7) The tap conductors are sized 6 AWG copper or 4 AWG
aluminum or larger.
(8) The tap conductors do not penetrate walls, floors, or
ceilings.
(9) The tap is made no less than 9 m (30 ft) from the floor.100 ft) total length.


This installation complies with everything except condition 8. Can anyone explain further the reason for this condition?
 

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
if we comply with 240.21(B)(4) .../...
(8) The tap conductors do not penetrate walls, floors, or
ceilings.

....

This installation complies with everything except condition 8. Can anyone explain further the reason for this condition?
That tap rule was first proposed without that wording for the 1981 NEC. The CMP 4 members were dubious and apparently that item (8) was one of three provisions added to the compromise for the CMP to accept it.
My guess is if this building is already big (35 FT tall) the qualified persons that are to service this system cant be expected to ensure electrical safety if the taps penetrate walls, floors, or ceilings and travel thru areas that are not easily monitored.

From the TCRA:
 

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