DC and AC in the Same Trench

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darunedefig

Member
Location
HV, New York
Occupation
Electrician
Searching this forum there were some old discussions about AC and DC next to each other. One thread talked about not having an issue with DC spaced 2x depth from AC because, but that was for medium voltage cables and concerning heat flow.
I don't believe there will be an issue with electric magnetic fields if the DC and AC are in separate conduits/cable in the same trench. I don't think there will be an issue if those conduits/cables are horizontal (not vertical) spaced 1" for the full length of the trench run from each other.
Seems the 2023 NEC allows for DC and AC in the same conduit together or at least for solar as per 690.31(B) Exception: Where all conductors or cables have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the same wiring method.

I have a job where there are two existing PVC SCH 40 conduits touching each other in a trench and a customer who is worried about EMF from AC messing up the DC or vice versa. The job is less than 200 amps from both DC 600V and AC 240V and 400' trench run. Does anyone know if there could be an issue?

Also let us say a 200 amps 350kcmil Al direct buried cable is right next to the same size DC cable, would there be an issue?
 
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I don't believe there are any rules about trenches and your customer is paranoid. I believe your conduits are not supposed to be touching each other, for ampacity reasons.

If you think about it, any rules about things being in the same or different trenches are stupid because the trench isn't a permanent part of the installation; it ceases to exist when you're done. So the only thing that matters is spacing, and as far as I've noticed that's reflected in how the rules are written.
 
D

darunedefig

Member
Location
HV, New York
Occupation
Electrician
tom baker said:
Some separation is nice in case of damage, it allows to cut and repair conduit

PVC would provide limited shielding. Provide your client a cost for RMC.
Or trench with 24” bucket and lay at opposite sides
Click to expand...

When I run multiple conduits in a trench I always keep them separated. More like 10" and not 24" for residential jobs if two conduits. I did give the customer a quote for a new 400' trench, but it was hard to swallow for them. Directional boring would not be deployed here. There are two existing trenched PVC SCH 80 conduits that customer said an "electrician" ran in the past. These conduits are touching each other coming out of the ground, so I dug down and found they are also touching each other horizontally in the ground.

My question about direct burial cable was hypothetical and doesn't apply to this job. I only asked it to figure out how close is too close.

If NEC 2017 310.15(B)(2)&(3) correction factors / conditions of use are taken into consideration and wire is sized appropriately
then the question remains about the EMF.
A sleeve could be installed with the wires to potentially solve EMF issues, but my question is how close is too close to be concerned with EMF?
This question is not about EMF affecting humans or health issues. This question is not about EMF affecting communication cable as there is none for this project. This is about the voltage/frequency/current from a DC wire and an AC wire affecting each other from their magnetic fields.
I would think that the only interference would be noise that could affect some electronic devices.
If DC and AC wires are wrapping around each other then that would create issues, but I don't believe there would be hardly any impact when in separate conduits.

I would be interested if anyone had any resources or research or other forums concerning this.
 
D

darunedefig

Member
Location
HV, New York
Occupation
Electrician
Yes Solar. Thank you for your response. This is resolved.

Correction to my earlier post where I said "If NEC 2017 310.15(B)(2)&(3)", it should be "310.15(C)" because "310.15(B)(3)(b)
(b) Raceway Spacing. Spacing between raceways shall be maintained." could be interpreted as conduits should not touch.
 
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