Installing 1 Cable per Metal Conduit - Problem? 1

[joozu6]

I've heard talk that pulling urd cable into metal conduit with one phase per conduit is a bad idea due to induction. We currently do so at our risers and then the cable runs into pvc conduit underground.

Does anyone know where I can find documentation on this subject or can explain to me the details?

This hasn't been an issue for us until we started getting larger loads and worried that the induction will cause us future problems.

Thanks

 

[wareagle]

If you carry the neutral with the phase conductor you won't have the problem. What knid of problems have you had?

 

[joozu6]

None yet. However, we keep hearing that same comment everytime we mention our method of one phase per metal conduit. However, no one seems to know any details about the specifics of why it is a problem, i.e., what happens electrically, how much load before it becomes a problem, will shielded cable help, etc.

 

[jghrist]

Basically, if you run a single cable through a ferrous metal conduit, you create a transformer. The current induces magnetic flux and currents in the conduit which cause it to heat up. If the return current (the other phases for a balanced 3Ø load or the neutral for a 1Ø load) is in the same conduit, then the net current in the conduit is zero and you don't have the problem. Aluminum or stainless steel is non-magnetic and does not have the problem.

 

[joozu6]

That helps jghrist. Thanks.

Would a shielded cable help?

 

[BJC]

If your going to run single conduits ( a bad ideal in my opinion) watch the clamps and the rebar.
If you circle any conduit ( PVC, PE, aluminium or stainless steel ) with a magnetic clamp or rebar in a duct bank your going to heat it up.
I have engineered jobs with single conductor 15 KKV URD cable in PE and PVC conduit and it works. We grouped the conduits close together and made sure there was no magnetic loops around any one conduit.

 

[jghrist]

The shield won't help unless it is a 1Ø circuit and the shield is the only return path.

 

[tulum]

joozu6 sorry can't help you here, but here are some related observations...

A solution I have seen in switchgear is to only make 1 side of the box out of a non-ferrous material - The others can still be metal - elimantes path for inductive heating. I don't know if conduit can be purchased in this manner...

Another thing I have seen is when single conducters are run through a metal wall 4 slits are placed around the cutout- once again to eliminate inductive heating...

 

[wareagle]

Some URD cable has a stranded shield that is also use as the neutral. Is this what you are describing?

 

[joozu6]

We use a stranded concentric neutral that comes on the parameters of the urd cable.

Sounds logical in theory, but I still haven't found any documentation on the subject and haven't experienced it on any of our metal conduit.

 

[resqcapt19]

If this installation is covered by the NEC, look at 300.3(B).
Don

 

[joozu6]

Thanks resqcapt. Seems like the NEC is supporting putting all phases in one conduit if using metal conduit.

I'm thinking of going to a high scheduled pvc, though, instead of putting all the phases in one conduit. That way if we fault a phase we won't loose all three.

 

[dpc]

Yes, it is a problem. If this is a three-phase circuit, the shield won't solve the problem, since the phase current will not be equal to the neutral current.

You need to correct these installations.

Talk with any cable supplier - they can provide you with documentation.

 

[cuky2000]

Single cable in a magnetic raceway could generate significant losses and increase substantially the cable impedance. The increase in reactance may be estimated as 30% higher than in individual conductor in non-magnetic raceway. The following are quotes from the Okonite Engineering Guide:

o “Cables in ac circuits should not be installed with each phase in separate magnetic conduits under any circumstances due to the high inductance under such conditions."
o "Cables in a-c circuits should not be installed with each phase in separate metallic non-magnetic conduit when their size exceeds 4/0 unless the conduit is insulated to prevent circulating currents."
o "Three conductors in magnetic conduit will have a somewhat higher reactance than cables in non-magnetic conduit."
o "Single conductors should not be installed in individual magnetic conduit because of the excessive reactance.”