Parallel Feeders in conduit

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Cableman

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Hi
I have a 500A, 480V, 3 phase, 4 wire service. There are two parallel feeders of 4-250KCM. There are 3 conduits (one was intended to be spare).
The contractor used all three conduits, installing 3 cables each in 2 of the conduits and 2 cables in the third conduit.
Does the code require that each set of 4 be installed in its own conduit?
Your input frm a code standpoint and a practical standpoint is appreciated.
 
Each conduit must have a full set ...that is the three phases and the grounded conductor. See 310.10(H) in the 2011 code or 310.4 in older codes.
 
Thx. I looked at that section, and I cant find that it says that each set should be installed in a separate conduit. The older code stated that for sizes smaller than 1/0.
Do you have the capability of cut and pasting the verbage? or attaching a pdf?
thanks for the help.
 
Here you go...

(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.
Click to expand...
 
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Thx! I was looking at older code since that is what the municipality is going by and it did not have that verbage in there of

(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.
 
Dennis Alwon said:
I believe he may be up your way. What code is Chicago using?
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Chicago really has their own electrical code, but I think it may be loosely based on the 99 NEC.
The older codes are not as clear and used the words, "same physical characteristics" to require all of the phases and the grounded conductor, if used, to be in the same raceway.
 
correct. it is loosely defined. the code in play only states : "Where run in separate raceways or cables, the raceways or cables or raceways shall have the same electrical characteristics."
 
Cableman said:
correct. it is loosely defined. the code in play only states : "Where run in separate raceways or cables, the raceways or cables or raceways shall have the same electrical characteristics."
Click to expand...

You need to be looking at 300.3(B)(1) rather than 310.4 for the requirement to install each set in the same conduit.
 
Cableman, the reason you want to follow those requirements is that each individual conductor making up a parallel conductor set needs to have the same impedance as the other individuals of the set. If they are not the same impedance the one with the lowest impedance will end up carrying a higher amount of current, and could easily carry enough to actually overload that individual conductor. Just a .5 ohm difference between the two will make a big difference in how much is carried in each conductor, especially when near the designed ampacity of the paralleled set.

By not having same length and conductor type, you change resistance, by not placing every conductor of the set in the same circumstances (like one conductor of each phase in each raceway, each raceway be of same type, etc.) you possibly change inductive or capacitive reactance of each conductor of the set.
 
In this example it is not only the possible uneaqual current sharing, but also the inductive heating that will be a major problem if the raceways are ferrous.
 
Brian John has a great picture of a 1600 amp service, conductors in parallel when Phase A, B, and C are run in seperate conduits, the inductive heating is dramatic on the IR picture.
 
This is a 3000 amp where the conductors were not routed properly from cubicle to cubicle (The installing contractor installed the conduits in the wrong cubicle and then routed them through the switchboard to the correct cubicle.

InductiveheatingParallelFeedersIR.jpg


I think this is the one Tom mewntioned.

CTInductance-1.jpg
 
While we are discussing routing condustors avoid this. Inductive heating from seperating the phase conductors by the switchboards metal bracing.

INDUCTIVEHEATING2.jpg
 
brian john said:
While we are discussing routing condustors avoid this. Inductive heating from seperating the phase conductors by the switchboards metal bracing.

INDUCTIVEHEATING2.jpg
Click to expand...

Looks like something easy to do, many of us may have even done that and not even realize the consequences.

That picture alone is worth quite a few words.
 
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