low voltage PV system & 690.12

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tortuga

tortuga

Code Historian
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Oregon
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Electrical Design
Hypothetical situation you have a dozen PV panels on a roof, panels are marked with a open circuit voltage of 23.5.
The panels are wired in parallel, system is ungrounded and runs at about 14.4 volts.
At some point after some fused disconnects the voltage is stepped up to 48 Volts with a DC/ DC converter.

I have the following questions about this scenario:
If a PV system is never over 30 Volts does it inherently meet the rapid shutdown 30 volt requirement ?

In a system with no inverter, but does have a DC to DC converter where do the 'PV system circuits' end and a DC feeder begin?
A first disconnect?
 
tortuga said:
Hypothetical situation you have a dozen PV panels on a roof, panels are marked with a open circuit voltage of 23.5.
The panels are wired in parallel, system is ungrounded and runs at about 14.4 volts.
At some point after some fused disconnects the voltage is stepped up to 48 Volts with a DC/ DC converter.

I have the following questions about this scenario:
If a PV system is never over 30 Volts does it inherently meet the rapid shutdown 30 volt requirement ?
Click to expand...
Yes.

tortuga said:
In a system with no inverter, but does have a DC to DC converter where do the 'PV system circuits' end and a DC feeder begin?
A first disconnect?
Click to expand...
Yes, at the disconnecting means for the PV system, where it supplies the feeder.
 
Thats very interesting, makes sence voltage is the safty issue, not amperage, so one could cover a roof with less than 30 volt panels and not need any rapid shut down.
Thank you.
 
tortuga said:
Thats very interesting, makes sence voltage is the safty issue, not amperage, so one could cover a roof with less than 30 volt panels and not need any rapid shut down.
Thank you.
Click to expand...
Yeah, in concept, you could cover a roof in panels that all are 30V or less, and only parallel them, and avoid the rapid shutdown requirement.

But economic factors are going to steer everyone away from such a design. Your trunk cables would end up the size of a tree trunk, by the time you parallel hundreds of panels, to get the ampacity and voltage drop mitigation you need. Plus, most modules already start over 30V, prior to any connection to any other modules.
 
Agreed. Unless you have a few dozen 15W 23.5 volt panels sitting around doing nothing.
 
tortuga said:
Agreed. Unless you have a few dozen 15W 23.5 volt panels sitting around doing nothing.
Click to expand...
You would still have to have rapid shutdown on the high voltage side of your DC to DC converter, and of course with all that parallel wiring the current rather than the voltage adds, so as Carultch points out, your DC conductors would be huge on the LV side, and every module would have to be fused. Also, a DC combiner that would accommodate what in essence would be "a few dozen" strings would be hideously expensive for a residential system; you would need a utility scale combiner. The cost of the modules would be a very small part of the cost of the project; a bunch of Tigo boxes and a Sunspec compliant inverter for the modules would be a whole lot cheaper.
 
ggunn said:
You would still have to have rapid shutdown on the high voltage side of your DC to DC converter,
Click to expand...
The DC/DC converter would be hypothetically used for a branch circuit after the PV system disconnect

ggunn said:
and of course with all that parallel wiring the current rather than the voltage adds, so as Carultch points out, your DC conductors would be huge on the LV side, and every module would have to be fused. Also, a DC combiner that would accommodate what in essence would be "a few dozen" strings would be hideously expensive for a residential system; you would need a utility scale combiner. The cost of the modules would be a very small part of the cost of the project; a bunch of Tigo boxes and a Sunspec compliant inverter for the modules would be a whole lot cheaper.
Click to expand...
Well this is just a thought experiment I had while out camping with a friend who has these 15 watt panels on his camper.

They are much heavier than the new generation of portable 'camping' panels so people are upgrading and selling them cheap.
He said they can be found cheap at flea markets and craigslist.
His panels cost him 33 cents per watt.
We were discussing a scenario if somebody put 24 or 48 of those on a roof.
 
Why would you boost the voltage? Just use 12V stuff, right?

I think this idea only makes sense for tiny systems with small loads, like electronics that want even less voltage DC. But it's still a worthy point that nothing extra is required for RSD.
 
tortuga said:
The DC/DC converter would be hypothetically used for a branch circuit after the PV system disconnect.
Click to expand...
Anything on the DC side of an inverter is subject to rapid shutdown rules. The PV system disconnect is typically in the AC conductors.
 
ggunn said:
Anything on the DC side of an inverter is subject to rapid shutdown rules. The PV system disconnect is typically in the AC conductors.
Click to expand...
And if there's no inverter?

Looking at the OP and the code again, I'd say the PV disconnect needs to be at or on the supply side of the DC-DC converter, and also outside if on a one or two-family dwelling, to comply with 690.12.

That said...If the wiring on the load side of the PV disco happens to remain energized at 48V by another source, such as a battery bank, that's irrelevant. There's no RSD requirement for batteries.
 
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