Need advice on PV array wire sizing and fusing

[NelsonB]

I've gotten confused using the Renogy and other wire sizing calculators and seem to get conflicting answers, so I'm hoping someone here can possibly know this off the top of their head. Here's my setup: I've got the Renogy DCC50S (50A DC-DC charger/ MPPT charge controller). I'm installing two 200 watt mono 12V panels on the RV roof (flat) and doing so as parallel. Individual panel specs are: Vmp=20.31, Voc=24.35, Imp=9.85A, Isc=10.33A. At the VERY most, the wiring run from the farthest-most panel to the MPPT will be 30 feet.
Two questions: What size AWG solar/copper wire should I use from the MC4 (1x2) junctions to the MPPT and what should the fuse size be that I suppose will be installed inline on the positive leg after the two panels are joined and before the controller.

 

[Dzl]

I've gotten confused using the Renogy and other wire sizing calculators and seem to get conflicting answers, so I'm hoping someone here can possibly know this off the top of their head. Here's my setup: I've got the Renogy DCC50S (50A DC-DC charger/ MPPT charge controller). I'm installing two 200 watt mono 12V panels on the RV roof (flat) and doing so as parallel. Individual panel specs are: Vmp=20.31, Voc=24.35, Imp=9.85A, Isc=10.33A. At the VERY most, the wiring run from the farthest-most panel to the MPPT will be 30 feet.

Okay, I'm a bit shaky on the details, but I think I'm giving either accurate or overly conservative basic advice here (the electrical code will have caveats and multipliers etc for different conditions). Someone, maybe @FilterGuy @Forbisher @Hedges or @svetz can hopefully correct any mistakes I may make or give better advice.

In terms of wire size:

1. Ampacity: Take Isc (10.33) x 2, and then multiply by 1.56 = 32.25A, the wire (and possibly connectors?) should be able to handle at least this much current.
2. Voltage Drop: Then take Vmp and Imp x 2, and distance (most want round trip, some want one way distance and silently multiply by two), plug this into a wire size calculator like this one. A common target is <3% voltage drop.
3. Wire type: You want to buy wire meant for PV or for outdoor use exposed to the sun/elements.

In terms of the fuse:

1. While it wouldn't hurt, I don't think a fuse between the PV array and SCC would necessarily add any safety in your arrangement. Since PV panels cannot exceed their Isc, If your wire is sized to 1.5x the short circuit current of the panels a short should not endanger the wire. Follow the advice of your SCC manufacturer.
2. However a fuse or breaker between the controller and the battery bank is a good idea, as is a way to disconnect the PV array (switch or circuit breaker). In some cases this may be a requirement, in some cases this may just be convenient.

 

[Forbisher]

I've gotten confused using the Renogy and other wire sizing calculators and seem to get conflicting answers, so I'm hoping someone here can possibly know this off the top of their head. Here's my setup: I've got the Renogy DCC50S (50A DC-DC charger/ MPPT charge controller). I'm installing two 200 watt mono 12V panels on the RV roof (flat) and doing so as parallel. Individual panel specs are: Vmp=20.31, Voc=24.35, Imp=9.85A, Isc=10.33A. At the VERY most, the wiring run from the farthest-most panel to the MPPT will be 30 feet.
Two questions: What size AWG solar/copper wire should I use from the MC4 (1x2) junctions to the MPPT and what should the fuse size be that I suppose will be installed inline on the positive leg after the two panels are joined and before the controller.

6 AWG for under 3% drop
20Vmp
21A
30 feet
No fuse needed
You can play with the inputs in this calculator.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

 

[Dzl]

Note, the calculator I posted and the one Forbisher posted are different in that his uses one way distance and multiplies by 2 and mine just uses round trip. So long as you are aware of that and input the distance accordingly, results should be mostly the same.

 

[FilterGuy]

Ampacity: Take Isc (10.33) x 2, and then multiply by 1.56 = 32.25A, the wire (and possibly connectors?) should be able to handle at least this much current.

This is correct as far as it goes. However, if the wire is exposed to direct sunlight and/or is routed in conduit, there are some calculations needed to de-rate the wire for temperature. (At least if you are following NEC.) In practical terms however, If you keep the wiring out of the direct sunlight and don't put more than 3 current carrying wires in a conduit.... you are pretty much ok without the extra de-rating.

While it wouldn't hurt, I don't think a fuse between the PV array and SCC would necessarily add any safety in your arrangement. Since PV panels cannot exceed their Isc, If your wire is sized to 1.5x the short circuit current of the panels a short should not endanger the wire. Follow the advice of your SCC manufacturer.

The OP may want to review this:

Fusing & Wire Sizing guidelines for solar panels

This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what size the OCPDs need to be. There is also guidelines for wire sizing for the...

diysolarforum.com

 

[blutow]

1. While it wouldn't hurt, I don't think a fuse between the PV array and SCC would necessarily add any safety in your arrangement. Since PV panels cannot exceed their Isc, If your wire is sized to 1.5x the short circuit current of the panels a short should not endanger the wire. Follow the advice of your SCC manufacturer.
2. However a fuse or breaker between the controller and the battery bank is a good idea, as is a way to disconnect the PV array (switch or circuit breaker). In some cases this may be a requirement, in some cases this may just be convenient.

Maybe a dumb question, but do you want/need a way to cut power from the PV array to the SCC? I'll have my system installed in a van, and it seems like there should be a way to kill power from the panels, if nothing else for when I might be working on the system. Do people generally just use the connectors to disconnect power or use a switch/breaker? I read the doc that was posted above and it sounds like there is nothing recommended from a safety standpoint, but it just seems there should be an easy way to cut power.

 

[FilterGuy]

I read the doc that was posted above and it sounds like there is nothing recommended from a safety standpoint, but it just seems there should be an easy way to cut power.

Actually, the doc posted above does not attempt to address PV disconnect requirements. I was going to try to add it in but it got to be too complicated so I pulled it out. I plan to do a paper about PV disconnect requirements and post it..... but it might not be for a while. (I am kinda burnt out on writing papers)

There are NEC requirements around disconnects, but I seriously doubt there are more than a handfull of people that has ever implemented them all in a van. From a practical standpoint....on a van conversion I would just put a switch or breaker near the SCC and call it good.

 

[Dzl]

Maybe a dumb question, but do you want/need a way to cut power from the PV array to the SCC?

Yes, it is desirable to have a way to disconnect the PV array from the rest of the system (or more broadly, to disconnect a power source from the rest of the system).

For stationary installations I'm pretty sure the code requires it. Not sure how that translates to hobby sized systems or vehicles.

For any system beyond maybe very small systems, I personally feel its prudent and convenient. A circuit breaker or marine style switch (or any type of switch rated for the appropriate amount of DC voltage and current would be an easy way to accomplish this.

I like the idea of being able to manually isolate the electrical system from basically every power source including the batteries.

 

[blutow]

Thanks. I'm planning to use one of those blue sea breaker/switches for my DC/DC charger, so will use one from the panel as well.

 

[Hedges]

Voltage drop from PV panels to SCC doesn't have to be limited to 3%. That just minimizes power loss.
30' isn't far, but for a long run, it is OK to allow larger voltage drop because copper (wire) is expensive and silicon (PV panels) are cheap.

32A ... We're not supposed to exceed 30A with 10 awg, although its ampacity is actually 40A. So 8 awg would be proper. I wouldn't bother with 6 awg. I'd be tempted to use 10 awg. It's an RV, after all. And it will generally carry 20A or less.

Flat on the roof; won't hit max current except when sun is directly overhead.
Would be nice if you could tilt the panels for much more power on clear winter days.

I want to connect panels in series, but ...

https://www.renogy.com/content/RBC3050D1S-G1/RBC3050D1S-Datasheet.pdf

"Maximum input solar voltage 25V"

"Individual panel specs are: Vmp=20.31, Voc=24.35"

I'll assume Renogy isn't strict about that 25V limit, since on a cold day when Voc rises about 16%, many "12V" panels will exceed 25 Voc.
Yours will reach about 28V.

 

[HRTKD]

Maybe a dumb question, but do you want/need a way to cut power from the PV array to the SCC? I'll have my system installed in a van, and it seems like there should be a way to kill power from the panels, if nothing else for when I might be working on the system. Do people generally just use the connectors to disconnect power or use a switch/breaker? I read the doc that was posted above and it sounds like there is nothing recommended from a safety standpoint, but it just seems there should be an easy way to cut power.

I had to redo some wiring in my RV yesterday that required the feed from the battery to be turned off. The 2-pole breaker between the solar panels and the solar charge controller made it easy to first turn off the solar feed, before turning off the battery feed. I chose to put the breaker in the same compartment as all the other components (controller, battery, etc).

 

[Hedges]

You could open breaker between SCC and battery, stopping current flow. Then MC could be disconnected.

(unless a fault in SCC has PV panels shorted, in which case shading them would stop current.)

All mine have switch or breaker on PV side.

 

[UMWEE]

I've gotten confused using the Renogy and other wire sizing calculators and seem to get conflicting answers, so I'm hoping someone here can possibly know this off the top of their head. Here's my setup: I've got the Renogy DCC50S (50A DC-DC charger/ MPPT charge controller). I'm installing two 200 watt mono 12V panels on the RV roof (flat) and doing so as parallel. Individual panel specs are: Vmp=20.31, Voc=24.35, Imp=9.85A, Isc=10.33A. At the VERY most, the wiring run from the farthest-most panel to the MPPT will be 30 feet.
Two questions: What size AWG solar/copper wire should I use from the MC4 (1x2) junctions to the MPPT and what should the fuse size be that I suppose will be installed inline on the positive leg after the two panels are joined and before the controller.

I used a 30 amp circuit breaker before the mppt charger, mainly just to disconnect. The mc4's after the branch connectors going into the wire to the mppt should be changed out to an Anderson or better connection than the mc4. Mc4 doesn't handle amperage well at all above 25 amps. This is not only my conclusion from knowledge but Will's also. Here is a direct picture of what Will says if you don't believe me. Good luck and happy Camping.

 

[HRTKD]

TEMCo MC4 connectors are rated for 30 amps. Maybe there is some context missing from what Will said. All the MC4 connectors I considered were rated for 30 amps. Anderson is not waterproof, MC4 is.

Solar Panel Connector - 1 pair Pair - Male Female Set

Solar Connectors rated for 1000VDC. Features Lifetime warranty. Don't choose a product that will degrade or fail before your panels do. Precision snug mating fit. High tolerance dies are used in the molding process. High UV Resistance. Won't degrade in sunlight. Ultra Low Contact...

temcoindustrial.com

 

[UMWEE]

TEMCo MC4 connectors are rated for 30 amps. Maybe there is some context missing from what Will said. All the MC4 connectors I considered were rated for 30 amps. Anderson is not waterproof, MC4 is.

Solar Panel Connector - 1 pair Pair - Male Female Set

Solar Connectors rated for 1000VDC. Features Lifetime warranty. Don't choose a product that will degrade or fail before your panels do. Precision snug mating fit. High tolerance dies are used in the molding process. High UV Resistance. Won't degrade in sunlight. Ultra Low Contact...

temcoindustrial.com

Yes but you really dont have to have waterproof connections, unless you have high voltage. And many people run parallel configuration that exceeds 30 amps as is mine. Anderson plugs have been used for years in 12 volt systems in the outside environment. For his 2-200 watt panels Anderson or mc4 would be okay.

 

[UMWEE]

TEMCo MC4 connectors are rated for 30 amps. Maybe there is some context missing from what Will said. All the MC4 connectors I considered were rated for 30 amps. Anderson is not waterproof, MC4 is.

Solar Panel Connector - 1 pair Pair - Male Female Set

Solar Connectors rated for 1000VDC. Features Lifetime warranty. Don't choose a product that will degrade or fail before your panels do. Precision snug mating fit. High tolerance dies are used in the molding process. High UV Resistance. Won't degrade in sunlight. Ultra Low Contact...

temcoindustrial.com

Plus only original mc4 connectors are ul listed and can handle 43 amps, those cheap knock offs are not certified 30 amp by anyone.

 

[Hedges]

Amphenol says its products didn't cause Tesla’s Walmart fire woes

Amphenol, a maker of electrical connectors, rejected any suggestion that its products caused the solar-equipment fires that prompted Walmart to sue Tesla.

www.latimes.com

These would have been UL listed connectors, from the original OEM.
Methinks the manufacturer doth protest too much.

 

[HRTKD]

Plus only original mc4 connectors are ul listed and can handle 43 amps, those cheap knock offs are not certified 30 amp by anyone.

Are you saying the TEMCo MC4 connectors are cheap knockoffs?

 

[UMWEE]

Are you saying the TEMCo MC4 connectors are cheap knockoffs?

Of course not, TUV is probably better than UL. Any mc4 connections that are not certified are cheap knockoffs and should not be used. I still wouldn't use a mc4 connections between branch connectors and charger, especially with a 4-6 panel 600-800 watt parallel panel configuration pushing anything over 30 amps. Just my choice is all, and just because the mc4 connectors are certified, things change, companies change, certification is grab a handfull of manufacturers product and test it. No reliability on re certification.

 

[UMWEE]

Are you saying the TEMCo MC4 connectors are cheap knockoffs?

I do use mc4 and will always use them, especially when my warranty is no good if I cut off my panels manufacturers mc4...LOL