Rooftop DC Disconnect

[jayzbond007]

Hello,

I have a job where the solar array is mounted on the roof, the USE-2 is running under the array and want to transition it into THWN-2 in emt conduit. Since this AHJ requires a rooftop DC disconnect, can I make the transition in there or does it need to be in a junction box first? Is there a specific code in the NEC that says specific enclosure used for transitioning?

Thanks!

 

[ggunn]

Hello,

I have a job where the solar array is mounted on the roof, the USE-2 is running under the array and want to transition it into THWN-2 in emt conduit. Since this AHJ requires a rooftop DC disconnect, can I make the transition in there or does it need to be in a junction box first? Is there a specific code in the NEC that says specific enclosure used for transitioning?

Thanks!

As far as I know there is no problem with running your USE-2 into the disco.

 

[Zee]

You just need adequate space in the DC Disco. to transition. The typical Square D HU361 dc disco (30A, 3 pole, unfused) has adequate space.

I take it this is a grounded system?

(SIDENOTE: I would strongly recommend using PV wire - in general - since it is now available. It is not an NEC requirement for a grounded system, yet.)

 

[Tech1]

You just need adequate space in the DC Disco. to transition. The typical Square D HU361 dc disco (30A, 3 pole, unfused) has adequate space.

I take it this is a grounded system?

(SIDENOTE: I would strongly recommend using PV wire - in general - since it is now available. It is not an NEC requirement for a grounded system, yet.)

To the best of my knowledge, you only have to install a DC Disconnect on the roof if your solar system wiring is going through the attic space. If not, I would install it next to the main panel.

 

[c_picard]

Totally fine to hit the line side with USE-2, and come off the load side with THWN-2.

Guessing you're in one of my favorite towns in CA Feel free to apply pressure on the local AHJ, at least until the 2014 NEC is adopted. With rapid shutdown, the disco can be skipped.

 

[electro7]

What AHJ is this, if you don't mind me asking?

 

[MasterTheNEC]

Totally fine to hit the line side with USE-2, and come off the load side with THWN-2.

Guessing you're in one of my favorite towns in CA Feel free to apply pressure on the local AHJ, at least until the 2014 NEC is adopted. With rapid shutdown, the disco can be skipped.

Not sure I agree with that statement 100% ( I agree with the USE-2 to THWN-2 part) as the Rapid Shut Down in Section 690.12 is entirely different than specifications in Part III of Article 690- Disconnection Means. The Rapid Shut Down is just to allow DC Capacitors to discharge down to safe (safer) levels. In the OP's post I don't believe the AHJ in his (or her) area will allow any disconnect to be skipped IMHO.

 

[jaggedben]

Not sure I agree with that statement 100% ( I agree with the USE-2 to THWN-2 part) as the Rapid Shut Down in Section 690.12 is entirely different than specifications in Part III of Article 690- Disconnection Means. The Rapid Shut Down is just to allow DC Capacitors to discharge down to safe (safer) levels. In the OP's post I don't believe the AHJ in his (or her) area will allow any disconnect to be skipped IMHO.

I would imagine that most AHJs that have this requirement are completely unaware of 690.12 and that it would be a conversation to have with them.

The DC disconnect on the roof is one way of complying with 690.12. If you can convince them that you have another way that doesn't require the disconnect, they may go for that instead.

There are more than a handful of California AHJs who require the roof DC disco, so I'm sure it won't play out the same with all of them.

 

[SolarPro]

In and of itself, a roof-top dc disconnect will not meet 690.12, unless the inverter also happens to be located alongside the disconnect (in which case you would just open an ac disco to initiate rapid shutdown). 690.12(2) says that the control conductors must drop below 30 V and 240 VA within 10 seconds of initiation of rapid shutdown. The problem with trying to use a rooftop disconnect to meet 690.12 is that the capacitors on the dc-input bus at the inverter remain energized for longer than 10 seconds. So unless the inverter at ground-level has a capacitor bleeder circuit, simply opening the PV source or output circuits within 10' of the array does not meet 690.12.