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NEC standard on max conductor volume in conduit

Vernon Tuck

Stainless
Joined
Oct 26, 2008
Location
Brenham, Texas
Gentlemen,

I am aware that as regards both bending conductors and running them through conduit, the NEC imposes limitations on both the bend radius (some multiple of the cross sectional area I think) AND the maximum "fill" of the conductors in the conduit, according to a formula (unknown to me) that is also based on the cross-sectional area.

At issue, is a plan to run the full 200 amp service entrance power supply about 20 - 25 feet to a new 200 amp sub-panel.

I happen to have enough 2/0 THHN copper wire to do the job. However, 2/0 is not quite good enough for 200 amps - never mind the overkill that I like to build into all my electrical projects.

I would like to run doubled 2/0 conductors from one box to the next. I've already received some excellent advice about this in here. One contributor explained the limitations and pointed me to some specialty double lugs that can be retrofitted into the existing boxes for this purpose.

Up to now I've run all conductors through thin wall 2" steel conduit. What I'm hoping somebody will help me with is this: Can I run six 2/0 conductors through a single conduit? The first part of this question is: Will six conductors even fit? The second part is: IF they will, IS THIS COMPLIANT WITH THE NEC code?

I suppose I could answer the first question for myself. However, I'd have to start chopping up some expensive copper cable into six strands. At present the cable is a single big coil. So, if you've been there and done that I will immensely appreciate the benefit of your experience.

If I can't run it all through a single conductor the fall-back plan will be to run double barrel conduit.

Thanks!
 
Single phase right?

I think you can up-rate the conductor to feed a sub panel. 2/0 copper is fine for a 200 amp panel according to the NEC. I can dig up the specifics if I pick up the 2008 book I have at the shop.

If you want to double all the conductors you will need 8 not 6 as you need a separate grounding conductor (ground) and grounded conductor (neutral).

I can look at the fill chart, but I think there is no way 8 2/0 conductors is kosher in 2" thin wall. I'd just take 4 runs of 2/0 and go with that.
 
I do not have the Code at my finger tips so I cannot cite the exact part, but as I recall its Appendix C of the Code, maximum number of conductors in trade sized conduit.

If you decide on separate conduits, make sure that there is a supply and return in the same conduit or you will get some nasty heating of the conduit.

Tom
 
Tom,

When you say "make sure there is a supply and return in the same conduit or you will get some nasty heating..." could you explain this?

Are you saying to put one hot and one neutral in each tube?

Thanks,

Vernon
 
The 2008 NEC Handbook (haven't picked up the 2011 edition yet) says you can run six 2/0 THHN conductors in a 2" EMT, 2" FMC, 2" IMC, 2" LFNC-A, 2" LFMC, 2" RMC, 2 1/2" PVC Sched 80, 2" PVC Sched 40 or HDPE, 2" PVC Type EB, but not in 2" ENT or 2" LFNC-B (the largest sizes for those conduit types in the tables).
 
If you pull the max in any conduit, you will definitely be pulling...... that I can assure you.

Derating hits you on parallel conductors in one conduit, too, you don't get double.

And, YES YES, pull one of every conductor in each conduit. else-wise you will maybe have some hot conduit from the induced currents.
 
I don't understand why you want to double up the conductors for a 25' run. I'd think it was a good idea for a 150' run, but you are so short why not go with what NEC Table 310.15(B)(6) will allow? It only applies if this is residential application. If this is commercial you are stuck with double 2/0 or moving up to 3/0. I can dam near guarantee you that the wire from the utility transformer to your service entrance is 2/0 aluminum if you have a 200 amp service. If that is the case using two runs of 2/0 copper to go 25' is like making an extension cord out of #6 SOO cord for a hairdryer. It isn't going to reduce flicker or voltage drop, that is going to happen elsewhere.

Also,
Were you able to find a 200 amp breaker for your panel? If not, and this is residential you might want to look at mobil home panels to replace your meter main, they are handy for applications like this since the bus has large lugs on it to feed the sub panel in the "mobil" home. It is a clean way to run a large sub, and they usually have an 8 position bus to allow you to run a smaller sub or branch circuits off from that point also. They look identical to a normal meter main, and are rated for this purpose.

Here is a link to 310.15
http://www.chinoaz.net/dev_services/pdf/CondutorsforGeneralWiring.pdf

Peter
 
Peter and everybody.

What we're doing is converting a dumpy little frame house into a dumpy little machine shop. The house is barely more than a shack. but it's on a slab and the roof is tight. The house sits on 13 acres of land on a busy Texas state highway. Being as the gummint, from the top to the bottom, is broker than the ten commandments, they're overturning every rock and looking behind every tree for additional tax revenues. Hence the property taxes are going north and fast as they deem this to be "commercial property".

Never mind that it has no three phase, no city water, no cable TV, no internet, no city services or amenities of any type, whatsoever. It doesn't matter. Because, by golly this is PRIME COMMERCIAL PROPERTY and by golly we're gonna pay our fair share of property taxes!

So, I'm taking what is indeed, a 200 amp, single phase residential service, and attempting to max out the service to a commercial standard. As to your question about breakers: All my boxes are Square D QO panels. To the best of my knowledge the biggest QO circuit breaker in the galaxy or beyond is either 125 or possibly 150 amps - but not 200.

So, just as you suggested, I'm using mobile home type "feed thru" 8 space boxes. The service entrance is a QO 200 amp feed thru box. It feeds the 200 amps through the wall (using single 3/0 or possibly 4/0 conductors I don't remember for sure) to an identical panel on the INSIDE of the house directly behind the service entrance.

The plan is to pass the 200 amps out the second box in 2" EMT conduit - through the attic - and down to the front porch on the OUTSIDE of the house to a 200 amp disconnect. This disconnect will feed the 200 amps through the front wall of the house to a 40 space (but not feed-thru) QO 200 amp main breaker box. That box will be the end of the line for the 200 amps. From there any additional circuits will be not more than 100 amps.

The reason for this is that I have a Syncrowave 500 square wave tig welder. Should I ever need to run the welder near maximum power the entire 200 amp service will STILL be insufficient but it will get me two-thirds of the way there. So by pulling the wires from the 40 space panel I will be able to suck the entire 200 amps out to the welder on the front porch or in the front room.

In summary, I'm trying to build in as much overkill as possible. I try to compensate that way for my lack of experience. Probably wasting some money but that's ok.

It would be impossible for me to exaggerate how much I admire your knowledge of this stuff and appreciate your generosity with it. I've been hanging out in forums for 20 years. This is the best one I've ever seen! There is some serious braininess in here!

Best and thanks

Vernon
 
I forgot to mention that, beyond a doubt, but for y'all, I would have run one hot leg and a common through one tube and the other hot leg and a common through the other. Sounds like I would have created exactly what I'm trying to guard against - heat.

V
 
Here is a copy & paste from the 2008 NEC of art 310.4 (A-E)


310.4 Conductors in Parallel.
(A) General. Aluminum, copper-clad aluminum, or copper
conductors of size 1/0 AWG and larger, comprising each
phase, polarity, neutral, or grounded circuit conductor shall
be permitted to be connected in parallel (electrically joined
at both ends).
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

Exception No. 2: Under engineering supervision,
grounded neutral conductors in sizes 2 AWG and larger
shall be permitted to be run in parallel for existing
installations.
FPN 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.
(B) Conductor Characteristics. The paralleled conductors
in each phase, polarity, neutral, grounded circuit conductor,
or equipment grounding conductor shall comply
with all of the following:
(1) Be the same length
(2) Have 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
(C) 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 to achieve balance.
(D) Ampacity Adjustment. Conductors installed in parallel
shall comply with the provisions of 310.15(B)(2)(a).
(E) 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 310.13,
provided the combined circular mil area in each cable complies
with 250.122.

Here is 250.122 (B)

(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.


It used to be only in cases where the conductors were increased for voltage drop,but that was deleted several code cycles back, now it applies to everything where conductors have been increased, only way to get around it is to use a metallic conduit as the grounding conductor.

Instead of going through the trouble of paralleling the 2/0 conductors, how 'bout using a 175A enclosed circuit breaker to feed panel #2? You are still limited to 200A for the entire service.



BTW to other posters: 2/0 is not permitted for 200A residential feeders, subpanels, it is allowed only for service entrance conductors & feeders which carry the entire load of the dwelling unit, and it in no case applies to any non-residential use.
 








 
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