Parrell Feeders with different amperages

[sdv]

Impedance2

Impedance2

Beanland is correct...it's impedance not resistance. The best method for running the phase cables is in a triplex (triangle) configuration with the neutral conductor running adjacent. Also, transposing or twisting the phase conductors every so often ensures conductor lengths remain about the same. If one feeder is run as described above, and the other is not, then you could see this variance in current flow. However, for a short run this current delta in parallel feeders seems high. Perhaps the phase conductor runs are wrong. For example one conduit has A-B-C-C while the other has A-B-N-N.

 

[rcwilson]

Reading amps in parallel conductors.

Reading amps in parallel conductors.

There may also be some errors in the clamp-on CT readings. Theoretically, the CT ignores the magnetic field from conductors outside the CT, but theory and practice are different. For the best accuracy, the conductor being measured should be centered in the CT with no other conductors near by.

It is hard to push the parallel conductors around enough to get the clamp-on in place, much less meet the criteria for "accurate" measurements.

My guess is you might have 2-3% error in just the meter readings. That leaves 2-3% for impedance differences.

 

[dtiller]

Years ago our power company been installing A phase in one conduit, B in another and C in another making the wires virtually identical in length.

Wouldn't that result in eddy currents being induced in the conduit, assuming they're metallic?

 

[drbond24]

Years ago our power company been installing A phase in one conduit, B in another and C in another making the wires virtually identical in length.

I didn't even notice this statement until you quoted it, dtiller. I agree with you, though. This is a bad idea electrically, besides all of the extra conduit you would need. Also, I don't see how it helps at all with acheiving the same conductor lengths anyway.

 

[benaround]

dtiller, drbond24,

Check out 300.5(I)ex.#2 for isolated phase installations, which are permitted

in nonmetallic raceways.

 

[LarryFine]

It is hard to push the parallel conductors around enough to get the clamp-on in place, much less meet the criteria for "accurate" measurements.

That's not a big problem. You can buy a 'flexible-jaw' ammeter with a cable CT loop for exactly that purpose.

Here's an example:

 

[jameselectric]

brother,

Cable limiters don't provide overcurrent protection...they only provide short circuit protection, not overload.
Don

Sometimes i get those confused, the difference between 'overload' , overcurrent protection and short circuit protection especially when dealing with motors. Whats the legal 'NEC' difinition of it?

 

[brian john]

Larry I own numerous flexible CTs (in excess of 20) I find their accuracy to be off when compared to a properly utilized CT. Centered, 90 degrees to the conductor...

Still use them all the time, their ease of installation is hard to beat.

 

[don_resqcapt19]

Sometimes i get those confused, the difference between 'overload' , overcurrent protection and short circuit protection especially when dealing with motors. Whats the legal 'NEC' difinition of it?

The term "overcurrent" includes all of the others.

Overcurrent. Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

Overload. Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.

The terms short circuit and ground fault are not defined in the NEC.
Don

edited to fix quote

 

[Flagstaff]

It is feeding a UPS only. The UPS then feeds its own distribution.

 

[Flagstaff]

The phases are ABCN and ABCN. I agree on the twist issue. If they have slightly different twists in each run, that (I think) would effect the impedance. Thanks for your input.

 

[Flagstaff]

Wouldn't that result in eddy currents being induced in the conduit, assuming they're metallic?

The power companies here require plastic conduits only. DB120 and PVC. No metal allowed. I'd guess that any kind metal would be a big problem. I must assume they did their research on this, you know, make damn sure they were not screwing themselves with method. I know it sure makes pulling and terminating their wire a snap.

 

[beanland]

Utilities and Steel Conduit

Utilities and Steel Conduit

I do a lot of underground cable installation design for utilities. We use plastic (PVC, HDPE, fiberglass) exclusively. Steel will eventually rust out. Also, RGS is rough on wires in sweeps. It used to be that RGS was liked in sweeps but most use PVC and some utilities are going to fiberglass sweeps.

You cannot pull separate phases in separate conduits if the conduits are metal. Eddy currents will cause heating. Utilities pull seperate cables in PVC because the pulling distances are longer and the pulling tensions are lower.