Different conduit type for a parallel feeder - NEC 310.10(G)(3)

[westgbazo]

We have an existing feeder with (2) sets of 3-1/2" RMC. The load increased and required a third set of conductors. The contractor added a third conduit run of 3-1/2" IMC.

2023 NEC 310.10(G)(3) states that: "Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics."

I'm interpreting "electrical characteristics" to mean the type of material the conduit is made of. For example, aluminum vs. steel or steel vs. pvc, would not be allowed.

For my situation, wouldn't IMC and RMC have the same "electrical characteristics" since they are both rigid steel conduits?

 

[infinity]

For my situation, wouldn't IMC and RMC have the same "electrical characteristics" since they are both rigid steel conduits?

IMO obviously they're not the same raceway but the electrical characteristics are close enough to use for parallel raceways.

 

[ron]

Is the raceway being used as the equipment grounding conductor, or is there a wire type EGC provided inside the raceway? If the raceway is the EGC, I would say the different raceway type is a violation of 310.10(G)(2)(2)

 

[westgbazo]

Is the raceway being used as the equipment grounding conductor, or is there a wire type EGC provided inside the raceway? If the raceway is the EGC, I would say the different raceway type is a violation of 310.10(G)(2)(2)

No, none of the conduits are being used as ECG.

 

[winnie]

I'm inclined to agree with post 2.

Electrical characteristics means the resistance and reactance of the conductors.

The surrounding conduit affects the temperature (thermal effects on resistance) and the reactance (magnetic effects on inductance).

Different wall thickness of ferrous will have an effect similar to a transformer with different core dimensions.

But my _guess_ is that the actual difference in impedance and the actual deviation from balanced current flow will be small.

You might want to make an actual measurement rather than trusting internet randos

Jon

 

[westgbazo]

I'm inclined to agree with post 2.

Electrical characteristics means the resistance and reactance of the conductors.

The surrounding conduit affects the temperature (thermal effects on resistance) and the reactance (magnetic effects on inductance).

Different wall thickness of ferrous will have an effect similar to a transformer with different core dimensions.

But my _guess_ is that the actual difference in impedance and the actual deviation from balanced current flow will be small.

You might want to make an actual measurement rather than trusting internet randos

Jon

I did check with tech support at Allied Tube & Conduit and they agree that the "electrical characteristics" of IMC and RMC are the same.

The bigger issue we have is the third conduit is much longer than the other two. That's definitely a violation!

 

[jap]

I did check with tech support at Allied Tube & Conduit and they agree that the "electrical characteristics" of IMC and RMC are the same.

The bigger issue we have is the third conduit is much longer than the other two. That's definitely a violation!

Oddly I see nothing about IMC and RMC that have the same characteristics other than they're both rigid raceways that can be threaded and bent.

JAP>

 

[busman]

I did check with tech support at Allied Tube & Conduit and they agree that the "electrical characteristics" of IMC and RMC are the same.

The bigger issue we have is the third conduit is much longer than the other two. That's definitely a violation!

I'm on a ship and don't have the BOOK with me, but I thought it was just the conductors that had to be the same lengths, not the raceways.

Mark

 

[westgbazo]

I'm on a ship and don't have the BOOK with me, but I thought it was just the conductors that had to be the same lengths, not the raceways.

Mark

You are correct but the conduit is 20-30ft longer than the other two sets, which means the conductors are that much longer.

 

[jap]

I did check with tech support at Allied Tube & Conduit and they agree that the "electrical characteristics" of IMC and RMC are the same.

The bigger issue we have is the third conduit is much longer than the other two. That's definitely a violation!

I'd bet if you pulled the conductors out of the 2 existing runs the conductors in each run would not be identical lengths either.

JAP>

 

[busman]

You are correct but the conduit is 20-30ft longer than the other two sets, which means the conductors are that much longer.

Yeah, that's a problem.

 

[winnie]

I'm on a ship and don't have the BOOK with me, but I thought it was just the conductors that had to be the same lengths, not the raceways.

Mark

The raceway definitely changes the electrical characteristics of the conductors.

I would not consider conductors of the same length but with radically different raceway lengths to be matched.

On the other hand, when raceways follow generally the same path, they almost certainly have different lengths because of bends. This is generally not a problem

Jon

 

[don_resqcapt19]

The raceway definitely changes the electrical characteristics of the conductors.

I would not consider conductors of the same length but with radically different raceway lengths to be matched.

On the other hand, when raceways follow generally the same path, they almost certainly have different lengths because of bends. This is generally not a problem

Jon

I would not see the change between RMC and IMC, assuming both are ferrous and of the same size, to be a significant change in the electrical characteristics of the parallel installation. If it was between PVC or other non-metallic raceway and a metal raceway I would, and also would if one metal raceway was ferrous and the other was not.

 

[jap]

I would not see the change between RMC and IMC, assuming both are ferrous and of the same size, to be a significant change in the electrical characteristics of the parallel installation. If it was between PVC or other non-metallic raceway and a metal raceway I would, and also would if one metal raceway was ferrous and the other was not.

I don't know why, but I was thinking aluminum rigid.

My bad.

JAP>