Parallel Conductor Question

[jmp46205]

I'm trying to expand my knowledge of conductor selection for high amp systems and would like your assistance in clearly understanding how an equipment manufacture selected the recommended conductor and conduit sizes for the equipment.

Equipment Specs:
* Three Phase Input
* Nominal Input Current - 311 VAC
* Maximum Input Current - 389
* Recommended cable sizes are 167?F (75?C) (THW) wire at 86?F (30?C) ambient

Manufacturer Recommended input conductor and conduit:
* Conductor
- Phase: 3-250kcmil
- Ground: #1/0AWG
* Conduit
- (2) 2.5C

My questions are:
1. Based on the maximum input voltage, I would have expected the conduit size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil?

2. How is #1/0AWG ground calculated?

3. How is (2) 2.5C conduit calculated?

Note: This is not for a specific project that I'm working on, it's strictly for research and my own increased knowledge.

 

[jusme123]

...see sec. 310.4 of the '08 NEC (conductors in parallel). See 250.122 where parallel equipment grounding conductors are used.

 

[texie]

I'm trying to expand my knowledge of conductor selection for high amp systems and would like your assistance in clearly understanding how an equipment manufacture selected the recommended conductor and conduit sizes for the equipment.

Equipment Specs:
* Three Phase Input
* Nominal Input Current - 311 VAC
* Maximum Input Current - 389
* Recommended cable sizes are 167?F (75?C) (THW) wire at 86?F (30?C) ambient

Manufacturer Recommended input conductor and conduit:
* Conductor
- Phase: 3-250kcmil
- Ground: #1/0AWG
* Conduit
- (2) 2.5C

My questions are:
1. Based on the maximum input voltage, I would have expected the conduit size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil?

2. How is #1/0AWG ground calculated?

3. How is (2) 2.5C conduit calculated?

Note: This is not for a specific project that I'm working on, it's strictly for research and my own increased knowledge.

In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.

 

[Smart $]

...

My questions are:
1. Based on the maximum input voltage [current] I would have expected the conduit [conductor] size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil

2. How is #1/0AWG ground calculated?

3. How is (2) 2.5C conduit calculated?

Can't answer any of your questions based on the provided data. Spec's are ambiguous in that the current's are not provided in a manner consistant with the NEC. All the Recommendations just lead to more questions...!!!

 

[Smart $]

In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.

That's a good guess... but then I have to ask about the nominal current. I'd have to guess nominal current would be the continuous portion if energized for more than three hours. Max current should be something that only occurs occasionally, and thus non-continuous. You shouldn't have to use that value times 120%.

1/0 ground would be the minimum if you used aluminum conductor. Using copper, #2 is good for a 500A ocpd.

 

[texie]

In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.

I mispoke on the EGC. It would only need to be a #2 in each raceway in this case. Maybe that 1/0 is a neutral? But smart$ is right. We don't have enough info and this is all speculation.

 

[kwired]

1/0 ground would be the minimum if you used aluminum conductor. Using copper, #2 is good for a 500A ocpd.

But you still need 1/0 minimum if you have parallel conductors.

 

[Smart $]

But you still need 1/0 minimum if you have parallel conductors.

I'm aware of the 1/0 minimum for paralleled circuit conductors, but I cannot locate any such requirement for EGC's. Please enlighten me...

 

[kwired]

I'm aware of the 1/0 minimum for paralleled circuit conductors, but I cannot locate any such requirement for EGC's. Please enlighten me...

You are probably correct 300.3(B)(1):

(1) Paralleled Installations. Conductors shall be permitted to be run in parallel in accordance with the provisions of 310.10(H). The requirement to run all circuit conductors within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord shall apply separately to each portion of the paralleled installation, and the equipment grounding conductors shall comply with the provisions of 250.122.

 

[don_resqcapt19]

You are probably correct 300.3(B)(1):

But the wording of 310.10(H)(5) muddies the waters a bit.

(5) Equipment Grounding Conductors. Where parallel equipment grounding conductors are used, they shall be sized in accordance with 250.122. Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables in accordance with 310.104, provided the combined circular mil area of the sectioned equipment grounding conductors in each cable complies with 250.122.

It is my opinion that EGCs smaller than 1/0 are permitted to be installed in parallel, but this section casts some doubt, especially with a number of the CMPs saying that the words "shall be permitted" act to prohibit that that is not specifically permitted.

 

[Smart $]

But the wording of 310.10(H)(5) muddies the waters a bit.

The mere mention of "smaller than 1/0" does, as it infers that another requirement says they cannot be smaller than 1/0 AWG. Yet there is no such requirement for EGC's, sectioned or otherwise.

... especially with a number of the CMPs saying that the words "shall be permitted" act to prohibit that that is not specifically permitted.

Under that premise, sectioned EGC's in multiconductor cables which are 1/0 AWG and larger are not permitted :?

 

[steve66]

I can't get anywhere with the original post.

3-250's : Does that mean 1 conductor for each of 3 phases, or 3 conductors per phase?? One is too small, and 3 seems way too big. Even 2 would seem larger than necessary.

(2) 2.5" conduit: How do I divide one set of conductors into 2 conduits? If part 1 means 3 sets of conductors, again, I can't put three parallel sets into 2 conduits??

1/0 ground: Given the above confusion, all I can do is guess. Maybe they meant (2) sets of 250KCM for the conductors, and then sized the ground per table 250-66. But it seems like the ground for this should be sized per table 250-122.

Finally, do we need a neutral conductor for the equipment? Like most bad power specs, they leave you guessing at that one.


So all in all, I would call this a really bad power spec. to try and learn from.

 

[don_resqcapt19]

I can't get anywhere with the original post.

3-250's : Does that mean 1 conductor for each of 3 phases, or 3 conductors per phase?? One is too small, and 3 seems way too big. Even 2 would seem larger than necessary.

It means one 250 kcmil per phase per conduit

(2) 2.5" conduit: How do I divide one set of conductors into 2 conduits? If part 1 means 3 sets of conductors, again, I can't put three parallel sets into 2 conduits??

That means two 2.5" conduits, each with one 250 kcmil per phase and a 1/0 EGC.

1/0 ground: Given the above confusion, all I can do is guess. Maybe they meant (2) sets of 250KCM for the conductors, and then sized the ground per table 250-66. But it seems like the ground for this should be sized per table 250-122.

When you are running parallel conductors in more than one conduit or cable, the EGC in each conduit or cable must be full sized based on the size of the over current protective device and Table 250.122. See 250.122(F). The maximum OCPD for parallel 250's would be 600 (assuming this is not a motor load) and the code required EGC would be a #1 in each raceway. The spec writer may have increased that to a 1/0 based on the parallel conductor rules in Article 310, but in my opinion a #1 in each raceway is code compliant.

Finally, do we need a neutral conductor for the equipment? Like most bad power specs, they leave you guessing at that one.

That I cannot answer.

So all in all, I would call this a really bad power spec. to try and learn from.

Not really a bad spec.