Rapid Shutdown on flat commercial roof


Senior Member
We do mostly residential around my parts but our recent job is on a small commercial building with tilted arrays spaced about 24 feet apart N-S. There are either 4 or 7 sub-arrays depending how you look at it (there is a small corridor down the middle for walking, but thats a little beside the point).

We are using a Fronius string inverter with a rapid shutdown box. But I am wondering if every sub array needs to be compliant with the rapid shutdown rule?

The way it is designed right now, if RS is initiated, there will still be DC power present in the EMT between subarrays located on the roof.
Since we don't do many commercial roofs I don't come across this much.

Do we need a controller at every sub array? If so, I guess that is a major case for going with Solaredge or AC modules in the future.



Senior Member
It can depend on the AHJ and which NEC version you are under. If you are talking about the 2014 NEC then you have a 10' boundary to play with. Some AHJs will allow you to space subarrays 20' apart, 10' from each array adding to 20', and some will only allow 10' between arrays. If you are under the 2017 NEC then basically each subarray has to have its own RSS becasue the boundary is only 1'.

Under the 2017 NEC after January 1st module level disconnect will pretty much be required to comply with RSS unless UL can pull a rabbit out of their hat. You can toss those remote string disconnects so don't stock up on them.