parallel conductors in the same conduit

Status
Not open for further replies.
mshields

mshields

Senior Member
Location
Boston, MA
I was recently asked by a contractor if they could run parallel 4/0 instead of a single set of 500kcmil (for a 400A feeder). And he wants to put them all in the same pipe.

Assuming we can do that within the limitations of the 40% rule, how many ground conductors are required. Do you just need the one in accordance with 250.122.


Also, as we will have a neutral, do I have to derate for 8 current carrying conductors?


Thanks,

Mike
 
One equipment grounding conductor and assuming 3 phase the neutrals probably don't count for de-rating. I assume the loads isn't mainly lighting


310.15(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).

(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye-connected
system, a common conductor carries approximately the same
current as the line-to-neutral load currents of the other
conductors and shall be counted when applying the provisions
of 310.15(B)(3)(a).

(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor
Click to expand...
 
mshields said:
I was recently asked by a contractor if they could run parallel 4/0 instead of a single set of 500kcmil (for a 400A feeder). And he wants to put them all in the same pipe.

Assuming we can do that within the limitations of the 40% rule, how many ground conductors are required. Do you just need the one in accordance with 250.122.


Also, as we will have a neutral, do I have to derate for 8 current carrying conductors?


Thanks,

Mike
Click to expand...

ground conductors are selected according to the size of the OCPD protecting the circuit conductors. so yes, 250.122 is the appropriate criteria.

having said that, if you have upsized the conductors from the bare minimum required you have to upsize the EGC by a proportional amount. 4/0 isd about 212 kcm so (2) 4/0 is less than 500 kcm so no upsizing is needed.

if the neutral is considered a CCC, then you have to derate accordingly. see 310.15 (B) (5). It might not be considered a CCC.
 
mshields said:
I was recently asked by a contractor if they could run parallel 4/0 instead of a single set of 500kcmil (for a 400A feeder). And he wants to put them all in the same pipe.

Assuming we can do that within the limitations of the 40% rule, how many ground conductors are required. Do you just need the one in accordance with 250.122.


Also, as we will have a neutral, do I have to derate for 8 current carrying conductors?


Thanks,

Mike
Click to expand...

Out of curiosity, why does he want to do that?
 
kingpb said:
Out of curiosity, why does he want to do that?
Click to expand...

perhaps because (2) 4/0 conductors are less copper than a single 500 kcm conductor and thus might be cheaper.

or maybe he just has 4/0 laying around he wants to use.

or maybe he already did it in 4/0 for some reason.

is it metal conduit? if so skip the EGC wire altogether.
 
Thank you gentlemen

Thank you gentlemen

As to why! As it turns out, he's just running through a couple of nipples from main to ATS to panel. He said the 4/0 will be easier to terminate.
 
mshields said:
As to why! As it turns out, he's just running through a couple of nipples from main to ATS to panel. He said the 4/0 will be easier to terminate.
Click to expand...

as good a reason as any. however, nipples are not conduit and IIRC, the so called derating for number of CCC is not required anyway.
 
mshields said:
As to why! As it turns out, he's just running through a couple of nipples from main to ATS to panel. He said the 4/0 will be easier to terminate.
Click to expand...

If they're 24" or less there is no derating required and the fill goes up to 60% so a smaller raceway may be used.
 
mshields said:
As to why! As it turns out, he's just running through a couple of nipples from main to ATS to panel. He said the 4/0 will be easier to terminate.
Click to expand...

The no hassle for engineer answer is then I would say have him install it as designed.

The be the nice guy answer is; "sure, just don't ask for a CO".
 
petersonra said:
perhaps because (2) 4/0 conductors are less copper than a single 500 kcm conductor and thus might be cheaper.

or maybe he just has 4/0 laying around he wants to use.

or maybe he already did it in 4/0 for some reason.

is it metal conduit? if so skip the EGC wire altogether.
Click to expand...

You forgot to mention 4/0 is way easier to bend than 500's.
 
Status
Not open for further replies.
Top