can you run #8 parallel conductors for 24vdc

[markthompson9591]

can you run #8 parallel conductors for 24vdc

 

[petersonra]

is it a NEC application? if so, in general the answer would be no.

 

[Besoeker]

is it a NEC application? if so, in general the answer would be no.

Just as a matter of interest, what would be the objection?

 

[iwire]

Just as a matter of interest, what would be the objection?

With few exceptions the NEC prohibits using conductors smaller than 1/0 in parallel.

310(H) Conductors in Parallel.

(1) General. Aluminum, copper-clad aluminum, or copper
conductors, for each phase, polarity, neutral, or grounded circuit
shall be permitted to be connected in parallel (electrically
joined at both ends) only in sizes 1/0 AWG and larger where
installed in accordance with 310.10(H)(2) through (H)(6).

Exception No. 1: Conductors in sizes smaller than 1/0
AWG shall be permitted to be run in parallel to supply
control power to indicating instruments, contactors, relays,
solenoids, and similar control devices, or for frequencies of
360 Hz and higher, provided all of the following apply:

(a) They are contained within the same raceway or
cable.

(b) The ampacity of each individual conductor is sufficient
to carry the entire load current shared by the parallel
conductors.

(c) The overcurrent protection is such that the ampacity
of each individual conductor will not be exceeded if one
or more of the parallel conductors become inadvertently
disconnected.

Exception No. 2: Under engineering supervision, 2 AWG
and 1 AWG grounded neutral conductors shall be permitted
to be installed in parallel for existing installations.
Informational Note to Exception No. 2: Exception No. 2 can
be used to alleviate overheating of neutral conductors in existing
installations due to high content of triplen harmonic
currents.

(2) Conductor Characteristics. The paralleled conductors
in each phase, polarity, neutral, grounded circuit conductor,
equipment grounding conductor, or equipment bonding
jumper shall comply with all of the following:

(1) Be the same length

(2) Consist of the same conductor material

(3) Be the same size in circular mil area

(4) Have the same insulation type

(5) Be terminated in the same manner

(3) Separate Cables or Raceways. Where run in separate
cables or raceways, the cables or raceways with conductors
shall have the same number of conductors and shall have the
same electrical characteristics. Conductors of one phase, polarity,
neutral, grounded circuit conductor, or equipment
grounding conductor shall not be required to have the same
physical characteristics as those of another phase, polarity,
neutral, grounded circuit conductor, or equipment grounding
conductor.

(4) Ampacity Adjustment. Conductors installed in parallel
shall comply with the provisions of 310.15(B)(3)(a).

(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.

(6) Equipment Bonding Jumpers. Where parallel equipment
bonding jumpers are installed in raceways, they shall
be sized and installed in accordance with 250.102

 

[markthompson9591]

can you run parallel two or three #8 thhn condutors for 24vdc that is the largest wire possible for power distribution board (92 amp bus rated 60 degree C) will take

 

[petersonra]

can you run parallel two or three #8 thhn condutors for 24vdc that is the largest wire possible for power distribution board (92 amp bus rated 60 degree C) will take

why don't you draw a sketch of what you want to do.

generally, you cannot parallel conductors under 1/0 to gain additional ampacity.

there is no reason you cannot run mulitple branch circuits though.

 

[markthompson9591]

see pdf attahed

see pdf attahed

see pdf attahed

MODERATOR's NOTE: The attached PDF has been removed. It included a copyright statement that forbad its duplication in any form without written permission. Forum rules do not allow posting of material in violation of the owner's copyrights. In the opinions of the moderators, the posting of this particular document exceeded the boundaries of any "fair use" application.

 

[Besoeker]

With few exceptions the NEC prohibits using conductors smaller than 1/0 in parallel.

Perhaps I could have phrased my question better.
When Petersonra commented about it being an NEC application, I took from that, that it would not be NEC compliant.
So, perhaps better worded, why does the NEC prohibit it?
Is there any technical basis for doing so?

In practice, we wouldn't parallel such small conductors either but, as far I can tell, it isn't prohibited.

 

[tom baker]

The reason smaller than 1/0 are not paralled is the wire size/amps is fairly proportional up to 1/0. IE if you need more copper then run a #6.
Above 1/0 its cheaper to run two 2/0's instead of one larger wire.
We often hear its bases on skin effect, but for the smaller sizes its based on the OD of the cable, to radiate heat, as doubling the size of the conductor does not double the surface area. so the parallel rule would apply to DC

 

[iceworm]

see pdf attahed

Interesting.
Three indepent 120V sources powering three 24VDC power supplies.
The three power supplies are connected to a power distribution block
Two outputs from the power distribution block to a 24V DC power distribution panel
Power distribution panel internally parallels the two incoming 24VDC feeders

Looks like the system was designed to allow for a power supply failure and still keep all the 24V loads energized.

The panel by itself was designed for two separate inputs. Redundant inputs where the outputs can be kept on line if one of the inoputs fail. Any chance this panel is listed?

Are you HIMA and selling this?

Or are you buying this from HIMA and this is their installation drawing?

If the latter, I'd put it in exactly the way the drawing shows.

If the former - I'd have to think about that a bit.

ice

 

[GoldDigger]

Looking at the PDF, I do not see the problem.
The #8s originating at PS 1-3 are not parallel since they have different sources.
The #8s terminating at the three XG terminal strips are not in parallel under the NEC because they are landed at different terminals and the fact that they are paralleled inside the black box is not, IMHO, an NEC concern.
Now the "bus" shown in between the two sets of wires cannot be made up of parallel #8s, but I do not see that as necessarily being the OP's intention.
Also, of course, there is the question of whether the whole thing is the internal wiring of a machine or panel which is not subject to NEC in the first place.
If the actual design is to run separate #8s directly from each PS to the appropriate XG terminal, then this is not parallel wiring at all.

Tapatalk!

 

[iceworm]

One additional question;
Who is questioning the installation?

One comment:
I work on/install plenty of equipment that has paralleled power supplies. If this one was all in one box - nobody would question it.

If in two boxes split, between the Power Distribution Block and Power Distribution Panel, with an NEC conduit between them - someone could. I'd still tend to install per the drawing - unless I had an AHJ rep standing on me.

ice

 

[iwire]

Perhaps I could have phrased my question better.
When Petersonra commented about it being an NEC application, I took from that, that it would not be NEC compliant.
So, perhaps better worded, why does the NEC prohibit it?
Is there any technical basis for doing so?

I do not know the answer, I have read that one problem with paralleling small conductors is that small differences in length effect the splitting of the current flow possibly resulting in an overload on one of the conductors.

If you look at one of the exceptions you can see that the OCPD for those installations must be in line with the ampacity of a single conductor. It seems this exception is in place to allow small conductors to be paralleled for voltage drop reason and not additional ampacity.

 

[Smart $]

.... If this one was all in one box - nobody would question it.

That's what I'm thinking...

If in two boxes split, between the Power Distribution Block and Power Distribution Panel, with an NEC conduit between them - someone could. I'd still tend to install per the drawing - unless I had an AHJ rep standing on me.

Agree.

 

[petersonra]

I think it is non-compliant.

The drawing says it is for approval. I would not approve it.

 

[Besoeker]

I do not know the answer, I have read that one problem with paralleling small conductors is that small differences in length effect the splitting of the current flow possibly resulting in an overload on one of the conductors.

If you look at one of the exceptions you can see that the OCPD for those installations must be in line with the ampacity of a single conductor. It seems this exception is in place to allow small conductors to be paralleled for voltage drop reason and not additional ampacity.

Or maybe the possibility of one of the paralleled conductors developing a fault, say open circuit, and the other having to handle the full current. Our "regs" make a reference to protection for this eventuality.

As I mentioned earlier, we don't parallel small conductors like #8. It's just quicker, cheaper and simpler to just run a larger conductor.
Above about 120mm², (about 4/0) we then look at paralleling. In general two 120s will carry more current than a single 240. And easier to manhandle.

 

[Smart $]

I think it is non-compliant.

The drawing says it is for approval. I would not approve it.

FWIW, this appears to be the template for specific install and could be revised for use. Additonally, one title block reads...

HIMA CABINET
POWER DISTRIBUTION #1
WIRING DIAGRAM

This leads me to believe all the parallel wiring is within one cabinet and not subject to NEC purview.

 

[lapseofmind]

Maybe I'm missing something here and this actually doesn't pertain to the question but why does the drawing call for #8's to begin with? #10 thhn is rated for up to 40amps at 90C. #8 is rated at 55amps. Those power supplies are 40amps. I got to be missing something...

 

[iceworm]

Still interesting.

see pdf attahed

One additional question.
One comment.

Okay - One More
Why the Power Distribution Block?
There are three power supplies, and the Power Distribution Board has three inputs. Why not run each power supply to it's own input on the panel? That should get rid of any comments about paralleling.

Maybe it's a maintenance issue with R&R a defective Power Supply.

I realize I am getting into design issues. An answer of, "Cause that's the way we want to do it." is good.

ice

 

[lapseofmind]

I know that when I build panels, we use PDB's in case things change. Ex: I'm halfway through building my panel but the customer decides they want to add 5 more motors. So, I'll install a safety contactor that feeds 5 MSPs. That safety contactor will be fed from the PDB. That's just what I do, not saying that's why for this particular case.