DC wiring to MPPT PV side

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dirkme911 asked Dec 22, '23

DC wiring to MPPT PV side

Good day, I am currently checking if the wiring between my db board DC breakers from the PV side and MPPTs are correct after i recently had an MC4 connector melted on the PV side. The installer originally installed 4 wires per mppt (pv side), so that is 2 for positive side, and two for negative side wired to the MPPT, coming from 2 DC breakers on the DB board (multiple arrays). Some of the wires are 4mm2 and some are 6mm2. I have a 250/100 and two 150/100's. There are parallel arrays of 300w panels coming in.

My questions are:

1. Is it fine to have 2 wires connected to MPPT PV positive terminal, and two wires connected to the MPPT PV negative terminal, or should i rather connect the 2 DC breaker outputs together and run a single pair from the DC breaker to the PV side of the MPPT?

2. The MPPT spec states max thickness of the incoming wire of 2AWG. For the PV side, is there a minimum thickness the wire must be?

MPPT Controllers wiring

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Answer 1 · 2023-12-22T13:27:03Z

kevgermany answered · Dec 22, 2023 at 01:27 PM

MC4 limit is 30A. Use a pre made Y cable to join as needed. Size input cables according to current (and voltage drop if the cables are long).

Make sure connectors are crimped properly with the correct crimping pliers.

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dirkme911 commented · Dec 22, 2023 at 03:03 PM

100%, I bought connectors and a kit, will do them once I have sorted the DB board -> MPPT side.

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Answer 2 · 2023-12-22T13:27:22Z

Jason - UK answered · Dec 22, 2023 at 01:27 PM

@dirkme911 This doesn't sound like a standard PV array set up. Do you have a schematic of your system wiring arrangement? Also what is the Voc & Isc of the panels? Im thinking you need to go back to your installer and get them to verify the installation, or get an opinion of another installer.

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Answer 3 · 2023-12-22T15:03:22Z

dirkme911 answered · Dec 22, 2023 at 03:03 PM

The installer disappeared after installation, according to the previous owner of the house. And at the moment I want to do as much as I can before calling someone. The system has been working for some years, I just want to know whether the cabling is correct with 4 wires per MPPT on the PV side.

What I have learned so far, is that there are 3 MPPTs, connecting to various strings of 300W panels.

There are 7 strings in total connecting to the 3 MPPTS. The voltage is normally around 120V on the PV side of each MPPT. Amps peaks at around 35A per MPPT. Another reason I am asking, is I am deciding on replacing existing PVC trunking with larger one 100mmx40mm(to cater for the existing 12 wires and possible future expansion), or use the existing 40mmx40mm PVC trunking size, and only have 6 wires going to the MPPTs, in which case the existing trunking can stay.

20231221-185312.jpg (1.6 MiB)

20230427-191112.jpg (2.2 MiB)

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Craig Chamberlain commented · Dec 22, 2023 at 05:05 PM

I'd strongly recommend you get an experienced installer in to do a full health check on the system, especially since it has been running for a few years. It's hard to tell from the pictures above if the cables going into the MPPT are mechanically supported but it is important that they are. From the pictures I would say it's not the tidiest installation I've ever seen which is another reason to get someone to take a look.

I don't think there is anything inherently wrong with having two wires going into each MPPT terminal as long as the wire type is fine stranded and they are stripped, inserted and torqued up correctly. Obviously also important that the combined cable cross sectional area isn't too much for the terminal.

As someone else said, you could do with finding out the Voc and Isc for your panels and determine how they are wired up exactly. As you may already know, it's crucial that the MPPTs never see more than the rated voltage on the PV terminal under any circumstances. PV panels produce higher voltages in cold temperatures and the voltage will also be higher when open circuit compared to when under load.

Perhaps you have a suitable background to tackle this yourself but there are a lot of factors to consider so please take care. That said, it can be tricky to find a good installer so I totally get why you might prefer to do it yourself. You might want to invest in a torque screwdriver at the very least to ensure you tighten all the terminals correctly without over tightening them. HTH

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