DC conduit

[bpark]

Hey everyone. Just a quick disclaimer, I'm new to the electrical construction world and don't know all the correct lingo so try to bear with me. I may not have all my facts straight too.

Anyways

So for a conduit containing 3 phase conductors (A/B/C along with your ground and neutral) the NEC requires that you put the return path in the same piece of conduit otherwise the conduit will give off too much heat.
===== conduit top
A/B/C ->
G/N
<- A/B/C
===== conduit bottom

Here's the question
If I have 2 dc conductors i want to pull, does the return path have to be in the same piece of conduit? (like this)
=====conduit top
DC-/DC-
G/N
DC+/DC+
=====conduit bottom

forgive my lack of more information but thats the best i could explain it to my knowlege

 

[charlie b]

Might I ask what the nature of this circuit might be (i.e., for what type of facility, and what is the load)?

 

[bpark]

My supervisor said the load/facility doesn't matter. They'll size the conductors later based on the load. But most of the facilities we work with are mostly Data centers or Industrial buildings/factories.

 

[don_resqcapt19]

While there is not a technical reason that would require the all of the conductors of a DC circuit to be in the same cable or raceway, the NEC does not have an exception to 300.3(B) that would permit this.

 

[bpark]

We actually saw 300.3(B) when we were researching. Some of the guys over here say it applies to AC not to DC.

Also in a little box underneath 300.3(B), it says
"This general rule remains consistent with electrical theory; that is, to reduce inductive heating and to avoid increases in overall circuit impedence, all circuit conductors of an individual circuit must be grouped..."
So because there is no threat of heating/circuit impedence, could you say that it might be a valid exception to seperate the polarities?

Sorry about the persistence, I feel like I'm trying to interpret the Bible or something so I want to know every little thing i can

 

[charlie b]

My supervisor said the load/facility doesn't matter.

That is correct. But the fact that your supervisor has an opinion on the topic does matter. I was probing to learn whether this was a work-related question (discussion of which is encouraged on this forum) or a home-related DIY project (discussion of which is forbidden on this forum). Now I know. Welcome to the forum.

 

[charlie b]

We actually saw 300.3(B) when we were researching. Some of the guys over here say it applies to AC not to DC.

I don?t have a copy of the book handy, but I don?t think there is anything in the text that would back up that opinion.

 

[bpark]

Yea looks like we weren't able to find anything other than 300.3(B) even though it seems like it should only apply to AC circuits. Darn. Thanks for the input though

 

[hurk27]

Not sure why the NEC seems to not address this issue, but I know 250.134(B) exception 2 will allow the grounding conductor to be ran in a different raceway? DC doesn't cancel out the effect so why would this be required?

 

[winnie]

Keep in mind that any time the load is changing you have an AC component to the current flow. This means that inductance is still an issue even with DC circuits.

For example, if you suddenly need to open the circuit, the inductance will cause a voltage spike that tries to maintain the current flow. Additionally, having the + and - conductors separated sets up a large 'loop area', meaning large external magnetic field. I doubt that this sort of DC magnetic field would cause lots of trouble, but I wouldn't ignore this at the design phase.

-Jon

 

[Volta]

... Sorry about the persistence, I feel like I'm trying to interpret the Bible or something so I want to know every little thing i can

Then you'll fit in just fine!

Welcome to the forum .

 

[bpark]

Keep in mind that any time the load is changing you have an AC component to the current flow. This means that inductance is still an issue even with DC circuits.

For example, if you suddenly need to open the circuit, the inductance will cause a voltage spike that tries to maintain the current flow. Additionally, having the + and - conductors separated sets up a large 'loop area', meaning large external magnetic field. I doubt that this sort of DC magnetic field would cause lots of trouble, but I wouldn't ignore this at the design phase.

-Jon

would the loop area become stronger at higher voltages?
because I know that telco plants do separate the DC+ and DC- parts but they run it at 48V, whereas we are trying to run it at roughly 400V.