DC in metal conduit

[augie47]

Is there a problem with samr polarity DC conductors routed through metal raceways ? I have an install with muliple runs of 4/0 from a DC rectifer.
All the + cables are routed in one metallic raceway, all the - cables in another.

 

[S'mise]

I don't see a problem with it. Far as I know, Hysteresis heating is only with AC

 

[don_resqcapt19]

While I don't see a technical issue with that, I see it as a 300.3(B) violation.

 

[G._S._Ohm]

Is there a problem with samr polarity DC conductors routed through metal raceways ? I have an install with muliple runs of 4/0 from a DC rectifer.
All the + cables are routed in one metallic raceway, all the - cables in another.

Since the current flow is not smooth DC like from a battery, I think this is not so different from separating the hots and neutrals carrying AC in different raceways.

At present I can't prove or disprove this idea but I think the induction heating of the raceway will be similar.

The DC component of the rectified wave does not radiate like the AC component so it's like you have an non-sinusoidal AC waveform with a peak to peak value half that of its equivalent sine wave.

This is one I'll have to search on.

 

[mivey]

Since the current flow is not smooth DC like from a battery, I think this is not so different from separating the hots and neutrals carrying AC in different raceways.

At present I can't prove or disprove this idea but I think the induction heating of the raceway will be similar.

That is correct. We often use the term DC to mean a steady current but it can mean a waveform that is even sinusoidal in shape but just does not cross the zero axis. Think about the half or full-wave rectifier.

The filtering gets us closer to steady but can have significant ripple for some loads. Anyway, after adequate filtering the heating will be a lot less but still won't be the same as a light on a battery.

 

[dbuckley]

If the rectifier is being fed from a single phase source, then the current in the DC cables will be like AC but in one direction, so you can still get heating effects.

The issue is much less pronounced when the source is three phase, and if you've got 12 pulse rectification it'll be an even smaller effect.

 

[masterinbama]

Do it all the time on large UPS projects. (500 KVA and above) Have some systems that have been in operation since the early 90's with no known problems.

 

[masterinbama]

Do it all the time on large UPS projects. (500 KVA and above) Have some systems that have been in operation since the early 90's with no known problems.

Need to add this is only done on the feeds from the battery racks to the UPS.

 

[G._S._Ohm]

The ill effects of induction heating may take years or decades to show up. I guess the main effect is degradation of insulation.

 

[Electric-Light]

The magnetic field won't cancel each other out, so it will have the same problem as an AC wiring without both path going together. I had a three way switch with just one wire behind the wall. Whenever that light was on, I would see waving effect on my CRT monitor and we're talking about 3A here.

If you have hundreds of amps, it can create problems with equipment sensitive to magnetic field. if it's steady, it will just be a long piece of magnet, but ripple current will cause nearby wires to act like a transformer and induce noise or data error.