Cable size into Multiplus? Oops?

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texascharlie asked Sep 26, '20

Cable size into Multiplus? Oops?

I'm wondering if I bought undersized cables to link my batteries to the Multiplus (12 | 3000 | 120 - 50 | 120V.)

Before I ordered cables, I looked at an AWG to mm2 table online and assumed it was accurate. But no. It says AWG 1 == 50 mm2.

Per tables I looked at after ordering cables, (e.g. this one) AWG 1 is actually 42.4 mm2. The Multiplus manual recommends 50 mm2 (x2) and I've got 42.4 mm2 (x2).

Each cable is 24" from Lynx distributor to the Multi.

The cable vendor's web page describes the cable I bought as such:

105C temperature rating
Outside Diameter of stranding: 0.3540 inches (nominal)
Rated to handle 245 amps continuous outside of engine space
836 strands of copper (30 AWG ea strand, rope lay Class K)

I'm starting with 2 Battle Born 100AH batteries (plan to add a 3rd if needed) and I've got a 200A mega fuse on each AWG 1 cable to the inverter. Am I okay? If okay with 2 batteries, still okay with 3 batteries?

This is a 30A RV and I won't be trying to run the air conditioner off grid. (Unless I attach my small Honda generator to shore power inlet, and then maybe I'll try air con if it's really hot.) The Honda spec says: 120V 2200W max. (18.3A), 1800W rated (15A).

@Kevin Windrem

Multiplus-II wiring inverter current draw

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2 |3000

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Matthias Lange - DE ♦ commented · Sep 26, 2020 at 03:36 PM

What Multi do you have? How long is the cable from Multi to battery?

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texascharlie Matthias Lange - DE ♦ commented · Sep 26, 2020 at 03:38 PM

Multi 12 | 3000 | 120 - 50 | 120V.

Each cable is 24" from Lynx distributor to the Multi.

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Kevin Windrem commented · Sep 26, 2020 at 04:34 PM

I'll let Mathias respond with Victron's official position. My gut says you will probably be fine.

My calculations suggest the voltage drop for 24" of 2x1 AWG is 0.00144 and 50mm2 is 0.00288 considering both the positive and negative runs.

You'll have other voltage drops in your system: battery internal resistance, wires from battery to shunt/Lynx, fuses, disconnect switches, ... These all add up to a total voltage drop as a function of inverter current.

Also, "ripple" voltage on the Multi DC input can be a factor if system resistance is too high. The inverter does not draw a constant DC current. Rather, there is a variation that follows the AC current provided (at twice the system frequency). Capacitors inside the Multi filter some of this out but not all of it.

I don't think there's a spec for maximum voltage drop in the system or for maximum ripple voltage at the inverter DC inputs.

A side note: Two Battle Born batteries can not supply the current necessary for the full output of the 3000 VA Multi. The battery internal BMS will shut them down. 3 batteries would ba a minimum. I don't think there is a way to limit the inverter output power to prevent a battery overload. If you have the room, weight and $$$ I'd definitely add that 3rd battery now/soon.

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Answer 1 · 2020-09-26T16:35:41Z

Matthias Lange - DE answered · Sep 26, 2020 at 04:35 PM

If you run 4 wires (2 for + and 2 for -) AWG 1 should be fine for this short distance.

The number of batteries doesn't matter for that, but you have to look into the specs of the batteries if they can deliver the current for the Multi.

@Kevin Windrem
I'm close to some Victron Devs/Staff and I work for a Victron distributor in Germany but I'm not official Victron :)

2 |3000

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