Parallel panels on tow vehicle?

[jameshowison]

I'm considering adding panels on the roof of a camper shell on my tow vehicle for a travel trailer RV. The set of panels would be identical to the set on the roof of the RV. I would have a female connector on the truck. When the truck is added in (while driving, when truck is at camp) that would double the capacity, but VoC etc would always be the same.

Two questions:

I don't think these would need to be fused. The combined amps of the parallel array would be below the "Maximum Series Fuse Rating" (15 amps) as the Imp of each panel is below 6amps. Do I have that right?

How to bring these together? Currently the rooftop RV panels have a shutoff using a DC rated DIN rail mounted breaker. I think that breaker is rated for two conductors in its bottom terminals. Can I just run the second string in there? Or would you cut an MC4 parallel connector into the existing line before the shutoff?

One concern is that the energy from the RV panels would always be present on the disconnected RV male end of the connection to the truck (since the shut off is upstream from the branching point). That doesn't seem right. So that suggests that each parallel string should have its own shut off, closer to the panels than the branching point. Don't usually see that in diagrams, but then again panels aren't being added and removed often!

 

[sunshine_eggo]

Two questions:

I don't think these would need to be fused. The combined amps of the parallel array would be below the "Maximum Series Fuse Rating" (15 amps) as the Imp of each panel is below 6amps. Do I have that right?

No. When they're in parallel with the other panels, they're part of the same array. Each panel should get its own fuse. A single in-line MC4 fuse is cheap and easy.

How to bring these together? Currently the rooftop RV panels have a shutoff using a DC rated DIN rail mounted breaker. I think that breaker is rated for two conductors in its bottom terminals. Can I just run the second string in there? Or would you cut an MC4 parallel connector into the existing line before the shutoff?

sketch please.

One concern is that the energy from the RV panels would always be present on the disconnected RV male end of the connection to the truck (since the shut off is upstream from the branching point). That doesn't seem right. So that suggests that each parallel string should have its own shut off, closer to the panels than the branching point. Don't usually see that in diagrams, but then again panels aren't being added and removed often!

Yep. It would be best to have a shutoff or breaker dedicated to the tow-array.

 

[justinm001]

I've been considering the same thing. The plan for mine is to have a separate MPPT in my RV for the trailer solar then have its panels wired up to an anderson/forklift type plug that I connect when it's hooked up. Having the mppt on the RV puts the higher voltage on the Anderson plug so thinner wiring between them.

 

[Hedges]

If tow vehicle has a single series string of panels 3s, and trailer has similar string 3s, you may connect those strings in parallel for 3s2p without fuses.

It is not that sum in parallel is less than fuse, it is sum of others is less than fuse. that is, if one string shorts to ground, others dump their current into it. The wires of a single string will only carry the current that string produces, or the current other parallel string(s), produce, not the sum of its current and the other (because flowing in opposite directions.)

As for output wire after combining, 6A Imp + 6A Imp = 12A. But size wires for Isc.

6.5A (?) Isc + 6.5A Isc = 13A Isc for array. 13A x 1.56 = 20.28A, which wire ampacity and any downstream fuses or breakers should be selected for.

You don't want current flowing in a connector when it is unplugged, need a way to interrupt.
You could make an open-circuit connector as weather seal. Want to avoid someone plugging male (PV+) into female (PV-) if single wire connectors are used. (One can also get confused between multiple sources and multiple loads, don't ask me how I know.)

 

[featherlite]

You could make an open-circuit connector as weather seal.

Hedges,

Thanks for the reply. I also am interested in adding panels to the top of my truck camper to charge the batteries in my trailer.

What is an "open circuit connector"?
How does it act as a weather seal?

Can you post a photo?

Thanks!

 

[Hedges]

I just meant if you use MC4 connectors, an MC for connector sealed with no hot wire coming out could cap the connector, prevent moisture or temptation to mate to the other. MC connectors are gasketed.

Some outlets or inlets have a gasketed lid. Some trailer connectors have a rubber plug.

 

[jameshowison]

I created a pic. I show a ? for the conductors coming from the bulkhead connector that the conductors would plug into from the Tow Vehicle. I'm trying to decide whether those should connect prior to the rooftop shutoff (segment A), or just before the MPPT (segment B). I think segment B. I guess I'd use an MC4 parallel connector (but it could be any sort of connection, really, right, like both landing on a stud or a small busbar.)

Either way, though, I think this means that when the truck is away, but the RV panels are producing, there would be current at the bulkhead connector.

So perhaps an additional shutoff is needed/preferred in the RV between the MPPT and the bulkhead connector (to avoid current at the disconnected bulkhead connector)? Either way will still need a shutoff on the tow vehicle, so make things safe when truck is separated from the RV.

So perhaps that additional disconnect (where the ? mark is) is actually not needed, since current at a female bulkhead connector is not unexpected or dangerous, and the disconnect on the tow vehicle would mean that we weren't connecting the panels under load (assuming it was off!).

 

[Hedges]

Voltage present, no current flowing when disconnected.
You need to interrupt current (switch, breaker, etc.) before unplugging so it doesn't pull an arc. "Exterior Shutoff" allows you to interrupt, but you have to remember.

I've been swapping MC3 cables inside a combiner box to test different inverters. Because those are GT PV, I shut off AC breaker first, and measure voltage at fuse holders to confirm Voc before unplugging. What I've been planning to do is feed PV through a suitable rated knife switch, and have the MC connectors inside switch box. The cover is interlocked, so can't be opened until switch is opened.

 

[featherlite]

Hedges,

What is your opinion about using one of these MCB's to interrupt a 100W 12v solar panel producing 8 amps?

WALFRONT 250V DC Miniature Air Circuit Breaker 63A, 2-Pole

 

[Hedges]

Should be no problem at all. "12V" panel is maybe 22Voc. Breaker shows 250V DC rating. It ought to work for several panels in series, or in parallel.

Unknown brands and retail channels I might worry about operating at higher voltage or current closer to its ratings, but your application is very low.

The printing mentions polarity, which is marked + and -, so follow that. It likely has a magnet to deflect an arc, one polarity only.

 

[featherlite]

The printing mentions polarity, which is marked + and -,

Thanks for the reply Hedges!

Would your answer be the same with 3 of those panels connected in parallel producing 24 amps and 24Voc?

The panels are otherwise properly fused. I use the MCB's soley as a disconnect for occasional maintenance.

 

[Hedges]

Yes, 12V to 48V, 8A to 32A, likely far enough below claimed 250V 63A ratings to be OK.

I used a 120V AC breaker for a 12V battery and motor on a bicycle. I forget if it did stick and then I added a capacitor, or not. The breaker would trip under heavier load and it seemed to work.

When disconnecting PV from an MPPT SCC, input capacitors would be at Vmp and panel would rise to Voc, so breaker is only opening to about 20% of PV array voltage, even easier. If there was a short which it then opened, it would be breaking full array voltage.

For my GT PV inverters, SMA recommends turning off AC breaker first. But you may not have a good way to shut off SCC in a DC coupled system.

 

[featherlite]

Thank you Hedges!