Am I overthinking this warning on a Victron Design Example?

DIY
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1

I am studying other designs to learn best practices and came across this warning on the bottom of a file that I found somewhere, (apologies for not being able to reference the location) “Split-Phase-Quattro-system-with-Cerbo-GX-Touch-50-Discover-42-48-6650-Smar-solar-MPPT’s”.

The design is using two split-phase configured 5K/48V Quattros attached to a generator and an RV 50A input power source.

With Power assist each Quattro can add
5KW to the output load per phase when
needed. Together with the adjustable 100A
input this all adds up to the max sum of
input and output current of 100+42=142A
per phase.

An Earth leakage device with
breaker or a combination MCB/RCD must
be installed on the output. Cable size must
be adjusted accordingly.

When reading these two statements (Max current of 142A, put a breaker on your output) individually, I’m following the logic: Lots of current here, check, and put a breaker on your output.

However, if I read these two statements as related, I’m confusing myself. I think that the output breaker in this situation should be 50A per phase, but then I’m wondering if I’m missing something, and the first statement in the warning (142A) is telling me that I should be setting an output breaker size of 142A.

Can someone help un-confuse me? Is the output breaker size set to anticipated load (50A), or the total system capacity (142A)?

Thanks very much.

2

You have breakers for individual inverters.

And a breaker for the whole system.

3

Thank you very much for taking the time to respond. Unfortunately, I don’t understand your answer. May I ask for a more verbose response?

Is the breaker between the inverter outputs (ACOut 1) and the electrical panel 50 A, or 142 A?

Thank you in advance.

4 
graph TD
    A[100A Input Breaker] --> B[50A Input Breaker 1]
    A --> C[50A Input Breaker 2]
    B --> D[Inverter 1]
    C --> E[Inverter 2]
    D --> F[Output Breaker 1]
    E --> G[Output Breaker 2]
    F --> H[Single Output Breaker]
    G --> H

Sizing particular to your design so here are just a vague suggestion. (100a and 50 on input are just an example.)

This way single inverters can be isolated if need be.

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5

Thank you very much for taking the time to draw a diagram.

If the inverter is a Quatro 5K/120 , it can pass through 50 A, and source an additional 42 A as power assist (worse case), causing a maximum output in theory of 92 A / leg @ 120V.

This means I need to size the wire from the to the breaker box to carry 92A. Then, Output Breaker 1 & 2 are sized to protect the wiring from the wiring closet to the panel in the coach.
This is going into an RV where the panel is (right now) set for 50Amps, so nominally, Output Breaker 1,2 would be 60 A (50A derated), and the wire going to the sub-panel would be sized to carry 75 A. (For example, I need to go check the NEC for actual requirements on wire and breaker sizing)

Thank you so very much for your patience. I think I’ve got this now.

6

Ok. So one small note for parrallel set up if this is your first time.
Size the cable either a slightly smaller gauge than you would normally or make longer runs, it makes load balance easier between inverters.

The input current can be turned down on the inverters so you can go smaller on the input.

7

Roger that. Important safety tip. Thanks again.

8

@lxonline Can you expand on the reasoning behind your comment of

it makes load balance easier between inverters.

9

Balancing parrallel set ups is notoriously difficult.
I think in all the posts i have read it is the biggest issue. And for some an absolute nightmare.

In the basic training for it from Victron and in the manuals it mentions it. So it is common enough mistake made in installs to be noted.

The manual i linked explains more. The key is really - cable resistance is the secret.

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