24Vdc from Rapid Shutdown and AC cables grouped

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IPS

Member
Location
California
Hi all!

I am wondering if one can run a 24Vdc Rapid Shutdown switch supply cable in the same conduit as AC cables (coming out of my inverters).

Here is my thinking process:
1) In general, we can make DC and AC work together code-wise per 303.3(C), it's not good field practice from what I've seen on this forum but it's code-compliant provided insulation is upgraded.
2) However, since this 24Vdc is not just a DC cable, but a DC power source for a switching device (Rapid Shutdown), it is a Class 2 cable. We then need to turn to NEC 725.136 which overrides 300.3(C).
3) 725.136(A) tells me that I can't mix Class 2, 3 with Class 1 in the same tray, enclosure...unless permitted by (B) through (I).
4) I turn to permission (D)(2) to reclassify my 24Vdc Class 2 cable into a Class 1 per 725.41.

Does it make sense ? Where do I go from there anyway ? How do I apply 725.41 to reclassify my Class 2 24Vdc cable into a Class 1 ?

Thanks!




 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
IPS said:
Hi all!

I am wondering if one can run a 24Vdc Rapid Shutdown switch supply cable in the same conduit as AC cables (coming out of my inverters).
Click to expand...
Okay, first question: are you doing this because your inverters are on the roof? If so, why do you need any rapid shutdown capability besides the AC disconnect(s)? Just wondering.
Here is my thinking process:
1) In general, we can make DC and AC work together code-wise per 303.3(C), it's not good field practice from what I've seen on this forum but it's code-compliant provided insulation is upgraded.
Click to expand...


Ok.
However, since this 24Vdc is not just a DC cable, but a DC power source for a switching device (Rapid Shutdown), it is a Class 2 cable.
Click to expand...
Well, that isn't what defines a Class 2 power source or Class 2 circuit. see 725.121 Does your rapid shutdown equipment actually say it's a Class 2 power source?


I'm not an expert on Class 1 2 and 3 wiring so I'll defer that advice to others, if you actually need it.
 
P

pv_n00b

Senior Member
Location
CA, USA
I've never looked up the code requirement because I would not run the two together anyway. That's because I want the RSD system to function if there is an AC fault that burns out the AC wiring and conduit. If the RSD signal conductor is in there it may cause a problem with RSD operation. It's more expensive but I feel it's a cheap investment in safety.

The code probably allows it if the RSD conductor is rated for the same voltage as the inverter output.
 
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Besoeker

Besoeker

Senior Member
Location
UK
IPS said:
OK. No, not PWM. What is the technical reason though?
Click to expand...
Coupling between the conductors. PWM is generally a few kHz switching between the DC legs which may be a few hundred volts.
We generally use a dedicated SWA cable.

But if your inverters are not PWM what topology do you use?
 
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