Rapid shutdown vs DC disconnect

[paintbb8]

First off, thank you to all who contribute to this group. I've been reading previous posts and sucking up knowledge where I can.

I've got a client who wants a ground array 500 ft from his attached garage but insists on the inverters being inside the garage so they're "out of the weather". He won't be persuaded otherwise. So the plan was to run 4 strings of #10 uf to the garage, conduit up through the attic then down to the interior wall where he wants his inverter mounted.

I plan on having a set of disconnects at the array and an AC disconnect on the exterior between the inverter and tap, but do I need RSD on the DC side as well or would a set of disconnects on the exterior wall be sufficient? I know the code at times can be murky. I don't want to go overboard and waste money when it's not needed either. Your thoughts are appreciated.

 

[SparkyBirdman]

Your RSD and a DC disco are 2 separate things-if one can accomplish both tasks, then thats fine. However, typically (and depending on your AHJ/POCO) the RSD is located as close to the service as possible so all sources of power can be disconnected at one location. These things may influence where your RSD is.

The other thing to consider is your PV Inverter manufacturer and how their RSD is initiated. For example, Enphase brand has micro inverters that can initiate RSD via the loss of power by the MCB or its backfeed breaker. But they also have a model that requires a dedicated RSD switch, since even in a utility shutdown, they will form a microgrid. Solaredge will typically initiate if theres a loss of utility or its breaker. Some manufacturers require a separate component that has to communicate RSD via power line communication.

Something else to look at: your array is 500’ from the inverters. Depending on your manufacture and if its ac or dc for the feeder, you may have alot of voltage drop to consider and #10 isn’t going to be big enough.

Also, as a reminder: if it is dc all the way back to the house, any dc wires inside the attached garage will need to be in metal conduit.

 

[paintbb8]

Your RSD and a DC disco are 2 separate things-if one can accomplish both tasks, then thats fine. However, typically (and depending on your AHJ/POCO) the RSD is located as close to the service as possible so all sources of power can be disconnected at one location. These things may influence where your RSD is.

The other thing to consider is your PV Inverter manufacturer and how their RSD is initiated. For example, Enphase brand has micro inverters that can initiate RSD via the loss of power by the MCB or its backfeed breaker. But they also have a model that requires a dedicated RSD switch, since even in a utility shutdown, they will form a microgrid. Solaredge will typically initiate if theres a loss of utility or its breaker. Some manufacturers require a separate component that has to communicate RSD via power line communication.

Something else to look at: your array is 500’ from the inverters. Depending on your manufacture and if its ac or dc for the feeder, you may have alot of voltage drop to consider and #10 isn’t going to be big enough.

Also, as a reminder: if it is dc all the way back to the house, any dc wires inside the attached garage will need to be in metal conduit.

I think I'm picking up what you're putting down. I'm trying to give my client options on inverters and its in turn giving me headaches. But that's okay. I'll adapt and overcome (learning new things in the process). I've already got a call in to the local inspector to get direction. Ultimately, his opinion is the only one that matters, right? I'm thinking Tigo RSD might be my easiest general purpose RSD pathway to have ready. I need to find out more on how the TS4 module works in a RSD situation. Does it physically break the circuit? If so, would I only need one TS4 module per string since it would stop the flow of electricity within 1 ft of the array? My logic has to have a flaw here. It would be too cheap of a solution.

The math with my arrays shows 450ish volts per string at 11-14 amps. #10 UF gives me less than 3% voltage drop theoretically.

Metal conduit wasn't in my calculations but it makes good sense. I'll factor that in (~30 ft). Thank you for the tips! I'll go ahead and ask one more stupid question. Is there anything that says that I can't have that conduit go straight up into the soffit through the eave of the roof/attic space and go overhead to drop down to my inverter (fully enclosed the entire way)?

 

[Carultch]

Is there anything that says that I can't have that conduit go straight up into the soffit through the eave of the roof/attic space and go overhead to drop down to my inverter (fully enclosed the entire way)?

No issue with that. You can take an internal route, or an external route with the conduit, as you see and your end-user sees fit.

It needs to be in metal if it is DC, but it doesn't make a difference on whether rapid shutdown is needed or not.

 

[Carultch]

I'm thinking Tigo RSD might be my easiest general purpose RSD pathway to have ready. I need to find out more on how the TS4 module works in a RSD situation. Does it physically break the circuit? If so, would I only need one TS4 module per string since it would stop the flow of electricity within 1 ft of the array? My logic has to have a flaw here. It would be too cheap of a solution.

Tigo has two different kinds of communication setups. All of their devices are meant for the module-level and not for the string level. It is a device with internal switching that toggles between connecting the module and bypassing the module, with only contributing a tiny voltage to the string as a whole. It doesn't shut off the whole string, just the individual device's contribution.

One is power line carrier, for -F and -2F, where F stands for fire safety. You either need an inverter with a built-in transmitter, or a separate transmitter. It works similar to a CT in reverse, where it overlays a power-line-carrier signal on the DC circuit, as a "keep alive" signal for the RSD devices. Cut the power to this, the signal turns off, and the module-level devices turn off.

The other is wireless transmission, independent of the power line, for their products ending in -S, -O, and -L. This one uses their CCA and TAP setup, to transmit the "keep alive" signal, as well as monitoring. -S stands for safety (as in rapid shutdown), -O stands for optimizers + safety, and -L stands for longer strings + the other functions.

 

[paintbb8]

Tigo has two different kinds of communication setups. All of their devices are meant for the module-level and not for the string level. It is a device with internal switching that toggles between connecting the module and bypassing the module, with only contributing a tiny voltage to the string as a whole. It doesn't shut off the whole string, just the individual device's contribution.

One is power line carrier, for -F and -2F, where F stands for fire safety. You either need an inverter with a built-in transmitter, or a separate transmitter. It works similar to a CT in reverse, where it overlays a power-line-carrier signal on the DC circuit, as a "keep alive" signal for the RSD devices. Cut the power to this, the signal turns off, and the module-level devices turn off.

The other is wireless transmission, independent of the power line, for their products ending in -S, -O, and -L. This one uses their CCA and TAP setup, to transmit the "keep alive" signal, as well as monitoring. -S stands for safety (as in rapid shutdown), -O stands for optimizers + safety, and -L stands for longer strings + the other functions.

Ah that makes sense now. I was thinking why can't i just use one module instead of having 13 modules on the string (what a cost savings!). Knowing that it just bypasses the individual module makes a huge difference.
My goal is a safe and cost effective solution, but if I have to pick one, I'm going with "Safe". Who's got the best deal on the tigo TS4-F's or -2Fs?

 

[Carultch]

I was thinking why can't i just use one module instead of having 13 modules on the string (what a cost savings!). Knowing that it just bypasses the individual module makes a huge difference.

As of NEC2017, module-level rapid shutdown is required, or another method that meets 690.12, for systems on buildings. They didn't directly specify module-level shutdown, but it is a logical consequence of the standard 80V limit given in this section.

 

[NAjxBUEEdA7PoKoX7vD2]

First off, thank you to all who contribute to this group. I've been reading previous posts and sucking up knowledge where I can.

I've got a client who wants a ground array 500 ft from his attached garage but insists on the inverters being inside the garage so they're "out of the weather". He won't be persuaded otherwise. So the plan was to run 4 strings of #10 uf to the garage, conduit up through the attic then down to the interior wall where he wants his inverter mounted.

I plan on having a set of disconnects at the array and an AC disconnect on the exterior between the inverter and tap, but do I need RSD on the DC side as well or would a set of disconnects on the exterior wall be sufficient? I know the code at times can be murky. I don't want to go overboard and waste money when it's not needed either. Your thoughts are appreciated.

Would the opening of DC disconnects placed on the outside of the house reduce the voltage in the DC conductors inside the house to not more than 30 volts within 30 seconds of rapid shutdown initiation? 690.12(B)(1)

 

[paintbb8]

Would the opening of DC disconnects placed on the outside of the house reduce the voltage in the DC conductors inside the house to not more than 30 volts within 30 seconds of rapid shutdown initiation? 690.12(B)(1)

Yes. A DC disconnect would fulfill that requirement. I just hope an inspector has the same view as you. Of course, I'll try and guide the conversation in that direction but I know enough not to argue with them. They'll only dig in their heels and you lose every time.

 

[NAjxBUEEdA7PoKoX7vD2]

Yes. A DC disconnect would fulfill that requirement. I just hope an inspector has the same view as you. Of course, I'll try and guide the conversation in that direction but I know enough not to argue with them. They'll only dig in their heels and you lose every time.

You don't need rapid shutdown for the ground mounted array because it is not on a building. Firefighters are not going to climb on it. It is not a firefighter safety hazard. Neither is underground wiring a firefighter safety hazard. All that is left is the DC inside the house.

Someone else mentioned grouping of the rapid shutdown initiation device with the outdoor service disconnect. The Code does not require it but the AHJ might insist.

If you had to, you could use equipment that was compliant under the 2014 NEC for array level shutdown such as:

https://files.sma.de/downloads/RSDBOX-DUS172426W.pdf

It may not be sold in the United States but it could still be legal in other countries.

 

[BackCountry]

First off, thank you to all who contribute to this group. I've been reading previous posts and sucking up knowledge where I can.

I've got a client who wants a ground array 500 ft from his attached garage but insists on the inverters being inside the garage so they're "out of the weather". He won't be persuaded otherwise. So the plan was to run 4 strings of #10 uf to the garage, conduit up through the attic then down to the interior wall where he wants his inverter mounted.

I plan on having a set of disconnects at the array and an AC disconnect on the exterior between the inverter and tap, but do I need RSD on the DC side as well or would a set of disconnects on the exterior wall be sufficient? I know the code at times can be murky. I don't want to go overboard and waste money when it's not needed either. Your thoughts are appreciated.

You’d absolutely want to take advantage of the DC voltage drop at 600v versus running AC at 240v. 500’ would probably be #8 but you can calc that out. Rapid shutdown location is required at a readily accessible location outside the building, aka by the main disconnect in my world.

I’d use SMA 7.7kw US41’s, no RSD required, and very reliable.

 

[paintbb8]

You’d absolutely want to take advantage of the DC voltage drop at 600v versus running AC at 240v. 500’ would probably be #8 but you can calc that out. Rapid shutdown location is required at a readily accessible location outside the building, aka by the main disconnect in my world.

I’d use SMA 7.7kw US41’s, no RSD required, and very reliable.

I was hoping you would reply. I was stalking your previous forum responses and wanted to pick your brain on this. Why isn't rsd required for these sma's?

 

[BackCountry]

I was hoping you would reply. I was stalking your previous forum responses and wanted to pick your brain on this. Why isn't rsd required for these sma's?

If it’s a ground mount, RSD isn’t required at the module level like rooftop solar requires. The UL spec and grid profile settings will dictate the reconnect speed and whatnot, but RSD at the module level under the new code is to prevent voltage from leaving the array for firefighter safety during rooftop operations only. There is some discussion as to whether or not RSD is required if your inverters are located inside the house, that has been clarified in 690.12 exception 1 which exempts PV equipment located indoors that’s served by a ground mounted array.

So on a ground mount, no RSD is required. That means that SolarEdge is out, who wants to buy a bunch of optimizers — not necessary, and you don’t need module level RSD.

Your next options would be SMA, Fronius (not allowed in CA because they’re not rule 21 compatible, but they may work where you’re at), that’s about it from my distributors.

I wouldn’t consider Enphase an option because of the distance.

So, I’d be doing SMA 7.7’s — they’re reliable and reasonably priced. Voltage drop over distance is more forgiving the higher the voltage. So for a residence, that’s the move especially with single phase service.

On a three phase service your options get bigger and you can do some more tricky design.

 

[wwhitney]

There is some discussion as to whether or not RSD is required if your inverters are located inside the house, that has been clarified in 690.12 exception 1 which exempts PV equipment located indoors that’s served by a ground mounted array.

690.12 Exception 1 reads "Ground-mounted PV system circuits that enter buildings, of which the sole purpose is to house PV system equipment, shall not be required to comply with 690.12."

So if you have a detached shed dedicated to PV system equipment, the ground mount PV circuits can enter that building without needing RSD. But for any other building, if the inverter is indoors, you don't get a blanket exception from 690.12.

Cheers, Wayne