In all the considerations for installing conductors in parallel I have a question regarding the requirement that the raceways have the same physical characteristics. Why would it be a problem to install one set of conductors in an EMT raceway and the other set in an RMC or IMC raceway? I could see it being a concern if one raceway were magnetic and the other non-magnetic in relation to fault current, but in figuring the fault current the "C" constant for the wire seems to only be concerned with the magnetic properties of the raceway and not the thickness of the raceway wall (if magnetic). I know that there is probably a simple answer, (maybe dissipation of heat?) but I can't seem to come up with the substantiation. Any thoughts would be appreciated.
310.4
- Thread starter pete m.
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: 310.4
The issue here is balance. Almost any difference in the installation of two or more parallel runs can result in a difference in impedance (notably inductance). But will that difference be significant enough to cause a voltage drop problem, or a fault current problem, or any other problem? There are two ways to approach an answer to this question. One way is to analyze, in detail, all possible combinations of conduit types, conductor materials, geometric configurations, and a host of other variables, and publish (in the code) a list of which combinations are ok and which are not. The other way is to simply prohibit the parallel runs from having different characteristics, thereby eliminating the expensive analysis. I think the second way is the reason for the existing code restriction.
I think it is exactly the same reasoning that prohibits the use of personal electronic devices during take off and landings of commercial aircraft. You may think (and justifiably so) that your PDA or your CD player could never influence the cockpit instrumentation. But the FAA is not going to perform an expensive (and probably inconclusive) analysis to determine which combinations of how many PDAs, CD players, game boxes, computers, and cell phones might adversely influence which types of aircraft instrumentation under what types of environmental conditions. Instead, they just prohibit the use of the personal electronics, and eliminate the concern. I think it?s the right approach.
The issue here is balance. Almost any difference in the installation of two or more parallel runs can result in a difference in impedance (notably inductance). But will that difference be significant enough to cause a voltage drop problem, or a fault current problem, or any other problem? There are two ways to approach an answer to this question. One way is to analyze, in detail, all possible combinations of conduit types, conductor materials, geometric configurations, and a host of other variables, and publish (in the code) a list of which combinations are ok and which are not. The other way is to simply prohibit the parallel runs from having different characteristics, thereby eliminating the expensive analysis. I think the second way is the reason for the existing code restriction.
I think it is exactly the same reasoning that prohibits the use of personal electronic devices during take off and landings of commercial aircraft. You may think (and justifiably so) that your PDA or your CD player could never influence the cockpit instrumentation. But the FAA is not going to perform an expensive (and probably inconclusive) analysis to determine which combinations of how many PDAs, CD players, game boxes, computers, and cell phones might adversely influence which types of aircraft instrumentation under what types of environmental conditions. Instead, they just prohibit the use of the personal electronics, and eliminate the concern. I think it?s the right approach.
Re: 310.4
It is a common belief that only the conductors in the individual phase are required to be of the same impedance.
This is not true, as Charlie says "it's the balance".
Three power sources, with three different impedances, will have a different voltage drop internally, which will create a load imbalance.
It is a common belief that only the conductors in the individual phase are required to be of the same impedance.
This is not true, as Charlie says "it's the balance".
Three power sources, with three different impedances, will have a different voltage drop internally, which will create a load imbalance.
- Location
- Illinois
- Occupation
- retired electrician
Re: 310.4
Bennie,
Bennie,
Are you saying that the impedance of the set of conductors for "A" phase must match the impedance of the condctors for "B" and "C" phases? What about the wording in 310.4 that says otherwise?
Don
Re: 310.4
You are correct according to the NEC. The section is wrong according to engineering standards. The section should be deleted and the FPN replace the existing wording.
There will always be some degree of impedance difference. The code section implies the difference can be unlimited.
You are correct according to the NEC. The section is wrong according to engineering standards. The section should be deleted and the FPN replace the existing wording.
There will always be some degree of impedance difference. The code section implies the difference can be unlimited.
- Location
- Illinois
- Occupation
- retired electrician
Re: 310.4
Bennie,
Assuming that each set of conductors is protected at their ampacities, how does it make any difference? Look at a 120/240, 3 phase, 4 wire system, with a very small 3 phase load. The B phase conductor will often be much smaller and also be supplied by a smaller transformer. Why should this set of conductors have to be the same size as A and C?
When the conductors of any single phase are not closely matched and the load is at or near the ampacity of the paralleled set of conductors, any miss match in the impedance of the individual conductors of the phase set could result in the conductor with the lowest impedance being overloaded. I don't see how a miss match of the impedances of the different phases can cause any problem.
Don
Bennie,
Assuming that each set of conductors is protected at their ampacities, how does it make any difference? Look at a 120/240, 3 phase, 4 wire system, with a very small 3 phase load. The B phase conductor will often be much smaller and also be supplied by a smaller transformer. Why should this set of conductors have to be the same size as A and C?
When the conductors of any single phase are not closely matched and the load is at or near the ampacity of the paralleled set of conductors, any miss match in the impedance of the individual conductors of the phase set could result in the conductor with the lowest impedance being overloaded. I don't see how a miss match of the impedances of the different phases can cause any problem.
Don
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