Best practice for changing wire types in a J-box

[EagleElectron]

Hello everyone,

I'm new here, but I must say that it's great to see such a knowledgeable and helpful community. I am a commercial solar designer/installer and I'd like to have some input on a wiring issue. I recently re-wired a 200 kW roof installation and discovered that inside the junction boxes, AWG 10 PV-Wire "home runs" switch to THWN wire that exits via conduit. Is it acceptable for PV-Wire to connect to THWN (or really any high-voltage DC connection) via wire nuts? I've been looking into various terminal blocks and barrier strips, but it's hard to come across one that is rated for the voltage and current while also being reasonably priced. What is the best practice? Thanks in advance

-John

[Sunking]

Wire nuts are rated at 600 volts same as any building light and power cable.

[EagleElectron]

Thanks for your input, Sunking. Yes, the wire nuts currently in place are rated for 600 volts. My concern is that they won't last 25 years in the heat that builds up in a J-box exposed to sunlight.

[Mike90250]

You could use split bolt connecters, and tape them up.

[EagleElectron]

There's an idea

Thanks Mike. That would definitely provide a solid connection, but I would be limited to connecting/disconnecting only at night. What do you think of using MC4 connectors? I know that they are designed to seal on double-jacketed PV Wire, so they wouldn't be able to make a watertight seal on the THWN wire, but if they're in a junction box it shouldn't matter, right?

[Sunking]

You have to use a tap of some kind unless you use a utility box for term blocks. For taps you have a choice of Wire Nuts, H or C. Take your pick.

[Naptown]

Thanks Mike. That would definitely provide a solid connection, but I would be limited to connecting/disconnecting only at night. What do you think of using MC4 connectors? I know that they are designed to seal on double-jacketed PV Wire, so they wouldn't be able to make a watertight seal on the THWN wire, but if they're in a junction box it shouldn't matter, right?

Why only at night?
If you have a 100KW system there are definately fuses or circuit breakers and a DC disconnect.
Open any of the above and you have shed the load so there will be no arcing. It only arcs when under load.
Now it will still be alive if there is light but you would have the same problem with any connector. Then just go to the string and disconnect one end at the MC4 connector.

[EagleElectron]

It only arcs when under load. Now it will still be alive if there is light but you would have the same problem with any connector. Then just go to the string and disconnect one end at the MC4 connector.

It may not arc, but since a bare ground wire runs through the J-box, a ground fault or short circuit is possible once the the bare wire is removed from the connector. I've been buzzed more than once from this happening. Call me lazy, but even with a full map of every string's beginning and end, disconnecting them out in the middle of the array is pretty time consuming. If I do that again, it would be nice to turn the J-box into a safe central place to disconnect the strings. The MC4 might serve the purpose of shielding the contact points on the circuit.

[inetdog]

The MC4 might serve the purpose of shielding the contact points on the circuit.

Since that was one of the design requirements for that connector, I would hope so!
Just do not try to use an MC4 (or any other pluggable connector) to disconnect DC under load.