poly phase harmonic imbalance????

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giddingselectric@monmouth

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just built a 1200a service. have three conduit runs and put one phase in each conduit. i asked inspectors and power company if this was ok and they had no problem. i know what code says to alternate phases in each conduit because of harmonics but i was undermaned at the time and it was simpler for them. now that they hooked up permenant power meter guy says this is a melt down hazard and needs fixed asap. i just orderd 3000.00 worth of material to fix it the way they want it but it wont be in for a few days. i need to know how much of a problem this is. could somebody please tell me(other than rephrasing the code book) if they have ever experianced a problem with this. because all i am getting around here is hear say. i took a infared thermometer out there and there is only a 10 degree difference between conduit and room.
 
You have violated 300.3(B) unless your installation qualifies under the exception of 300.3(B)(1). The reason for this is to reduce inductive heating that will occur when the circuit conductors are not grouped in the same raceway. I would add that this has nothing to do with harmonics.
 
First, how was it easier to do it that way? Three wires in each conduit either way, isn't it?

Infinity is right on, the problem is heating not harmonics. As far as how much of a problem, that depends on how much current you are actually pulling. I'm sure there's an equation somewhere that could be used to convert inductance to heat, but I don't have that kind of free time at this very moment. Maybe later. Suffice to say that more current = more inductance = more heating.

Also, it is hotter inside the conduit than it is outside the conduit. Go somewhere that you can actually hit the wire with your infrared thermometer and make sure it isn't exceeding its temperature rating.
 
Why would it be so expensive to fix? You have three conductors in one conduit, and they are presently wired as Phase A, Phase A, and Phase A. All you need do is to reconnect them (at both ends) as Phase A, Phase B, and Phase C.
 
Good for the meter guy to catch this.

The 'induction' that infinity describes is current flow induced in the conduit itself because of the magnetic fields produced by current flowing in the conductors. If all phases were grouped in each conduit, then the net current in the three phases would balance out and little magnetic field would be produced. With the phases separated, you get lots of magnetic field and thus lots of induced current flow. This induced current flow goes straight to heating up the conduit.

-Jon
 
What if all the conduits were plastic? Would isolated phase installation rules apply? See 300.5(I) exception 2 which takes you to 300.20(B). It might be permissible. Not enough facts presented. :smile:
 
they are in aluminum conduit 300.20 b fpn says this is permissible. there are to many factors involved to why this is done there is just one person that is having problem with it now dont know what to do
 
giddingselectric@monmouth said:
They are in aluminum conduit 300.20(B) FPN says this is permissible.
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300(B) tells you how to do it and the FPN explains why it is OK to do it that way. You need to contact the serving electric utility's Chief Electrical Engineer to get this resolved. Most of the Indians have no clue what you are doing.
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giddingselectric@monmouth said:
i need to know how much of a problem this is. could somebody please tell me(other than rephrasing the code book) if they have ever experianced a problem with this. because all i am getting around here is hear say. i took a infared thermometer out there and there is only a 10 degree difference between conduit and room.
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Conductors are installed like this are called segregated phase. As others have mentioned the problem comes from heating caused by the magnetic field around each phase. The more magnetic material you have around the conductors the worse the problem. Segregated phase construction is common on medium voltage systems.

I have seen 480V systems fail in a matter of hours. The last one involved PVC conduit with GRC elbows, the flames came out of the conduit like from a blowtorch and entered the padmount transformer secondary compartment.
 
the system is 120v three phase. there is no high leg. all coduits, lb are aluminum. all power company requires is this be done at the three mast weatherheads. the wires go from the mast thru a ct cabinet thru a 1200a outdoor breaker type disconnect then three aluminum parrellel runs 70 feet long to the mdp. this was done in the first place because of a bad storm that went thru and destroyed everything which in turn had a time frame effect on everything. the help didnt know better i was gone but they asked everybody involve and they said ok now some guy is involved from power company. meter guy said most that has ever drawn on this system is 300 amps at one time
 
giddingselectric@monmouth said:
the system is 120v three phase (I assume 208Y/120V?). there is no high leg. all conduits, lb are aluminum. all power company requires is this be done at the three mast weatherheads. the wires go from the mast thru a ct cabinet thru a 1200a outdoor breaker type disconnect (Isn't this your service disconnecting means?) then three aluminum parallel runs 70 feet long to the mdp. this was done in the first place because of a bad storm that went thru and destroyed everything which in turn had a time frame effect on everything. the help didn't know better i was gone but they asked everybody involve and they said OK now some guy is involved from power company (Do you know who?). meter guy said most that has ever drawn on this system is 300 amps at one time (You can't depend on the "meter guy", you must deal with a power company engineer.)
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Have you talked to anyone except field personnel from the serving electric utility? You need to be talking with and engineer as well as the AHJ. :-?
 
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