MULTI CORE CABLE USED AS SINGLE CORE CABLE

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Anwar Pasha

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In one of the project, due to site constraints contractor used 3 core cable instead of single core cable and connected it to the each lugs of the panel board in order to achieve 3 cables/Phase. My question is technically what are the implications when three core cables is used instead of single core cables??
 
3-core cable used as one conductor is better than having three single-conductors run parallel with each other.
 
petersonra said:
Except that might not meet code. Which might not be a problem if you don't have to follow the national electrical code
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If only those conductors were 1/0 and bigger and stripped into single cables, they will be complying with 300.10(H)(2), IMHO.
 
petersonra said:
Except that might not meet code. Which might not be a problem if you don't have to follow the national electrical code
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I did not understand,why code will forbid use of multi-core cable as a single core cable installation if there are no serious implication associated with it ??
 
Anwar Pasha said:
I did not understand,why code will forbid use of multi-core cable as a single core cable installation if there are no serious implication associated with it ??
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Let me give you one hypothetical situation in which a paralleled cable will behave differently than a single cable of the same total cross section:
Take a cable with three insulated cores and a single cable with the same cross section.
Cut one third of the strands in the single cable. This will create a localized spot of higher resistance which will create more heat but that heat will be conducted away by the full cross section of the cable
Cut one core of the multi-core cable. This will force the entire current to travel through the other two cores for the entire length of the cable, producing far more heat and causing a higher temperature over the entire cable that the temperature at the cut spot in the single cable.

I am not saying this is the justification for the code rule, but it is one "serious implication" of the difference between the two situations.
 
The biggest issue with using conductors in parallel is overheating. Code gives explicit permission for conductors in parallel, with limitations to prevent overheating.

When conductors are in parallel, the current will divide between the conductors in inverse proportional to the impedance of the individual conductors. Thus code requires conductors to have the same characteristics (same material, same conduit material, same length, etc.)

Code also specifies a minimum allowed conductor size for conductors in parallel, my _guess_ is that this is to ensure that normal manufacturing and installation tolerances (no two conductors can have the _exact_ same characteristics) do not have a material impact on the 'same characteristics' requirement.

Code specifies a minimum allowed EGC size. Multicore cables generally have an EGC sized for the conductors used individually; it is very likely that the EGC will be too small if conductors are used in parallel for increased ampacity.

Under NEC code, conductors in parallel used per the requirements above may be used at their full ampacity. This might be part of the reason that the NEC code is so strict about making sure the current divides evenly. I recall at least one non-NEC situation where smaller conductors were permitted in parallel, but the parallel sets could not be used at full ampacity (eg. 3 conductors in parallel at 2.25x the ampacity of a single conductor).

-Jon
 
petersonra said:
Sometimes there are rules that don't really make any sense but they are just rules that we have to follow.
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I beg to disagree. These rules are being debated over by the CMP, not just them but by other peers who feel they have more knowledge on the subject being tackled. These are consensus rules. I believe those people before us have thought these things over before these provisions get into print.
 
topgone said:
I beg to disagree. These rules are being debated over by the CMP, not just them but by other peers who feel they have more knowledge on the subject being tackled. These are consensus rules. I believe those people before us have thought these things over before these provisions get into print.
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One would like to thank so but based on a lot of the rules I see in the national electrical code that isn't always the case
 
petersonra said:
Sometimes there are rules that don't really make any sense but they are just rules that we have to follow.
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Not for this section. The reason that 1/0 and below can't be paralleled is very sound and well established. To answer the why, please look at table 310.16 and what is the difference in the 75 deg C column for conductor ampacity 1 and smaller, and then 1/0 and larger. It is easy to see if you make a graph....
 
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