Parallel Feeders

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sculler

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Need to install a 3000 amp feeder. Planning on using 500 MCM CU, (feeders spec'd CU). There is am exisitng 600 amp feeder along the exact same route made up of 350 MCM in 3" EMT. I plan on reusing those conduits and pulling out 350s and pulling in 500s and installing 6 more 3 1/2" conduits, larger size for ease of pulling in the wire.
Question is, conductors will be cut to same length to meet 310.4, but would the different sizes of EMT not meet the "raceway..same physical characterisitcs" requirements?
 
Question is, conductors will be cut to same length to meet 310.4, but would the different sizes of EMT not meet the "raceway..same physical characterisitcs" requirements?
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If the raceways are of different sizes, they won't have the same physical characteristicts as size is a physical characterisitc.
Don
 
OK, wasn't sure if they were refering to materials, ie: PVC, IMC or Rigid materials as "physical characteristics" and /or size.
 
sculler said:
OK, wasn't sure if they were refering to materials, ie: PVC, IMC or Rigid materials as "physical characteristics" and /or size.
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Physical characteristics includes all aspects.

Keep in mind this also means the new conduits will have to follow the exact same route as the existing.
 
don_resqcapt19 said:
If the raceways are of different sizes, they won't have the same physical characteristics as size is a physical characteristic.
Don
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I am questioning whether the phrase "physical characteristics " means the size has to be the same for each conduit. I do not see why a set of cables could not be installed in a 3" and a 4" EMT conduits and still have the currents balance properly between the two sets of cable. I believe the rule that the characteristics must be the same means the the conduits must be of the same material. You can not have one PVC and one GRC and meet the requirement. The following article by Mark C Ode seems confirm this conclusion.

http://www.ul.com/regulators/ode/0703.pdf
 
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bob said:
I am questioning whether the phrase "physical characteristics " means the size has to be the same for each conduit. I do not see why a set of cables could not be installed in a 3" and a 4" EMT conduits and still have the currents balance properly between the two sets of cable. I believe the rule that the characteristics must be the same means the the conduits must be of the same material. You can not have one PVC and one GRC and meet the requirement. The following article by Mark C Ode seems confirm this conclusion.

http://www.ul.com/regulators/ode/0703.pdf
Click to expand...


I have felt this way also that parallel runs of the same raceway could be different sizes providing they are the same type of raceway. But the words physical characteristics would also define something by size. So I don't see how two different size raceways can be considered have the same physical characteristics.
 
I don't think that Mark Ode's paper either confirms or rejects the conclusion about raceway physical size mattering. He simply does not mention it.

Different sizes of ferromagnetic conduit would have different magnetic path lengths, and presumably different reluctance and different inductive effects. It is entirely plausible that changing the size of the conduit would change the inductance of the cable run, and thus alter current balance. At the same time, I doubt that this would be a very large effect.

-Jon
 
winnie said:
I don't think that Mark Ode's paper either confirms or rejects the conclusion about raceway physical size mattering. He simply does not mention it.
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Maybe he does not mention it because it does not matter
winnie said:
Different sizes of ferromagnetic conduit would have different magnetic path lengths, and presumably different reluctance and different inductive effects. It is entirely plausible that changing the size of the conduit would change the inductance of the cable run, and thus alter current balance. At the same time, I doubt that this would be a very large effect.

-Jon
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We discuss this point regarding the conduit size, but the NEC ignores the most important point. 310.4 does not require the conductors of the parallel
phases to be the same size or same material. You could have 3 - 500 cu
in one conduit and 3 - 500 AL in the other. IF you need the impedance of the cables you would look in table 9 and you would look under the conductor size and whether the conduit was PVC, Steel or AL. No mention of the size of the conduit.
 
A quote from my favorite book:

"However, all raceways or cables must be of the same SIZE, material, and length."

(Please see signature below)
 
Just because a physical difference is ignored in a table doesn't mean that there is not a physical difference.

Look at the application of conductor ampacity and derating for number of conductors in a raceway. 6 12ga current carrying conductors in a 3/4in EMT conduit will certainly have different heat dissipation that 6 12ga CCC in a 2in EMT conduit, but code approximates them to the same value.

It would be cool to figure out a reasonable way to test this out in a small lab :)

-Jon
 
Bob,

I'm going to disagree on this point. "one conduit with 3 500's cu. the other

with 3 500's al."

All phase conductors must be the same!! you could have 2 500's cu. and

1 500 al. in all the conduits. Do you agree?
 
bob said:
...but the NEC ignores the most important point. 310.4 does not require the conductors of the parallel
phases to be the same size or same material. You could have 3 - 500 cu
in one conduit and 3 - 500 AL in the other.
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But section 310.4 does require the conductors to be of the same size and material. The requirement is in there after exception No. 4 and a FPN.
 
benaround said:
Bob,
I'm going to disagree on this point. "one conduit with 3 500's cu. the other
with 3 500's al."
All phase conductors must be the same!! you could have 2 500's cu. and
1 500 al. in all the conduits. Do you agree?
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I am quoting the code. On page 70-134 Just above the FPN, it states
"Conductors of one phase, polarity, neutral or grounded circuit conductor shall NOT be required to have the same physical characteristics as those of another phase, polarity, neutral, or grounded conductor to achieve balance."
This really does not make sense to me. If anything is required, it should be that the conductors should be the same size,
length, material, insulation etc. etc. in order to balance the load.
 
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Yes? The only thing that code requires to be balanced (same material, same physical characteristics, same length, etc.) are the conductors that are electrically in parallel, meaning joined at both ends to act as a single conductor. Different phases are _not_ in parallel.

-Jon
 
bob said:
You could have 3 - 500 cu in one conduit and 3 - 500 AL in the other.
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Wouldn't it be more accurate to say "3 - 500 cu's in one phase and 3 - 500 al's in another"?

In other words, each conductor in one paralleled group must match one another, but a second group's (phase, neutral, etc,) conductors could be different from the first group, as long as they match one another.

I also feel that, if we split phases and neutrals among the conduits as we usually do, the conduits must match in order for the paralleled conductors' characteristics to match, at least if the conduit affects the conductivity.
 
consider the following scenario:
parallel feeders are placed and the neutrals, for whatever reason, have different resistances (different length, conductors, whatever). now, for whatever reason, the loads become unbalanced and the neutral has substantial current flow (if unbalanced load not good enough, lets say a fuse blows and one phase drops out). Now, given that the neutrals have different resistances, the one with least resistance is now going to conduct an unsuitable amount of current flow with disastrous results.

(we always match our parallel feeders exactly.)
 
Here on the west coast pipes gotta be all the same size , make and model
Inspector would not sign off on it. Your gonna bury 6 sets anyhow why not just bury one more? I think the extra material would be offset by all the hassle from you save from the job Super , Project manager or Inspector
just my opinion
 
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