Sanity Check - Prewire

[swilly43]

I have a 2016 AS Interstate Grand Tour Ext. I have previously upgraded with a 100W solar panel from AM Solar along with the factory flex panel. I wired these in parallel and it has been working fine.

I now want to add a third 100 W panel from Renogy and also will wire in parallel. The factory documentation indicates that the wire from the solar panel to the solar controller (Victron 100/30) is 10 AWG (stranded). The roof connection was an SAE connector and it has corroded to the point that I will eliminate it with a pass through and butt splice the solar wires to the existing 10 AWG cabling inside the coach.

I recognize the difference in parallel vs series approach but it is easier to add the third panel in parallel. My question is .... Is it safe to use the existing prewire 10 AWG stranded cable for the expected (max theoretical) current of <20 amps? The flex panel is old and likely not anywhere near 100w. The most I ever saw out of the current 200W set up is 10 amps so I am thinking that adding the third panel would yield ~ 17 amps maximum. I do not care about maximum efficiency. I tried to find the cable running from the roof to the solar controller but could not locate it easily from the roof.

Various sources indicate that max current in free air of 10 AWG solid wire is about 58 amps for insulation rated to 105C. I think all automotive wire insulation is supposed to be 105C and I noted that some of the wiring is marked. If I de-rate this for stranded wire by 30% and then de-rate again for a wire bundle by 20%, I end up with 32 amps. If by chance 12 AWG is used somewhere unbeknownst to me, the final de-rating is 25 amps. This appears to be a reasonable approach. The wire is fused at 30 amps at the controller.

All that being said, safety and property protection is important and many will not agree that this is a large enough safety factor but I am interested in your comments.

I guess I could rewire in series but the mismatch in panels may be too much to overcome.

Thanks in advance for constructive assessments.

[AirMiles]

Three panels in parallel is fine. Some put four 100 watt panels in parallel on the factory prewire. Parallel is the preferred method for three panels on the prewire.

[BayouBiker]

Your reasoning and logic looks very sound to me. I concur with you and AirMiles and also agree parallel is preferred.