DC fuse sizing and placing example

[paulclinger]

I went through the discussions I could find on this forum about battery fuses including the fusing guidelines (https://diysolarforum.com/resources/dc-fuse-and-breaker-sizing-and-placement.129/), but am still a bit confused about what positions and devices should be used in my specific case.

Let's say I have a configuration with a 12kW inverter that has two EG4 batteries connected to it. The diagram on p9 of the fusing guidelines covers inverter/charger (which matches my configuration), but it only has one battery and it shows an external bus bar, which I don't have (as the EG4 Wallmount battery has an internal bus bar, so I plan to connect two of them directly). Given this I have the following questions:
- If a battery has two positive wires to the inverter, do both need to be protected with a fuse? Given that fuses protect wires, I assume the answer is "yes", but would like to confirm.
- I plan to daisy chain the batteries without using an external bus bar. The diagram on p9 shows OCPD1 "sized for total load" between the battery and the bus bar and OCPD2 "sized for inverter load" between the inverter and the bus bar, but given that the bus bar is internal in my case, do I just use a fuse sized for inverter load?
- Given the discussions using a MRBF terminal fuse or a class T, is having a class T fuse connected to each positive battery terminal sufficient or is it still advisable to add a terminal fuse? Or is it only considered in configurations where it can replace some of class T fuses, as it's smaller and cheaper (but with worse AIC protection)?
- Where do you normally place the fuse holder? All the images of batteries connected to inverters I've seen show a conduit box sitting on top of a battery and below the inverter, so does the fuse holder go inside the conduit box?
- What specific devices can be used as OCPD1 and OCPD2 shown on those diagrams? In my case, it's 2 280Ah batteries connected in parallel to one inverter. If having 250A or 300A class T fuse connected to each positive terminal is enough, what devices/holders can you recommend?

Thank you.

 

[AH Trimble]

- If a battery has two positive wires to the inverter, do both need to be protected with a fuse? Given that fuses protect wires, I assume the answer is "yes", but would like to confirm.

Yes. Class T fuses protect from dead shorts and other large over amperage draws. Each of your two positives should have a Class T for max protection.

- I plan to daisy chain the batteries without using an external bus bar. The diagram on p9 shows OCPD1 "sized for total load" between the battery and the bus bar and OCPD2 "sized for inverter load" between the inverter and the bus bar, but given that the bus bar is internal in my case, do I just use a fuse sized for inverter load?

Please explain "daisy chain"

- Given the discussions using a MRBF terminal fuse or a class T, is having a class T fuse connected to each positive battery terminal sufficient or is it still advisable to add a terminal fuse? Or is it only considered in configurations where it can replace some of class T fuses, as it's smaller and cheaper (but with worse AIC protection)?

I went through this ad nasium when upgrading my system. MRBF fuses normally have 1/2 the AIC rating of a Class T. I ruled out MRBF and use only Class T. In my next upgrade I will have 2 banks of 4 LifePo4 batteries each, each bank will have a Class T inside a Victron Lynx Class T unit. That unit is connected to a PowerIn that has MEGA fusing for the inverters and MPPTs. The batteries FWIW have 100a cutout on the BMS and a built in 125a circuit breaker. Lots of protection for the wire, etc.

- Where do you normally place the fuse holder? All the images of batteries connected to inverters I've seen show a conduit box sitting on top of a battery and below the inverter, so does the fuse holder go inside the conduit box?

Place the fusing as close to the power source as possible when talking about batteries...and in general. But, you have to be realistic as well. But closer is better.

In my case, it's 2 280Ah batteries connected in parallel to one inverter. If having 250A or 300A class T fuse connected to each positive terminal is enough, what devices/holders can you recommend?

I have gone to the Victron Lynx Class T unit for my Class T holder. In the past I have only used Blue Sea holders and fuses since they are a Tier 1 company.

 

[paulclinger]

> Please explain "daisy chain"

In my case the "main" battery is connected to the inverter and the second battery is connected to the "main" battery (what is shown on p8 in the EG4 quick start guide: https://eg4electronics.com/wp-conte...4®-Indoor-280Ah-Battery-Quick-Start-Guide.pdf). As I understand, both need to be independently protected, so I'd have one fuse on the second battery and two on the "main" battery (as it has two positive cables to the inverter).

Do you (or anyone else) have a picture of how this fuse may look on the battery connector (with or without the holder)? I've seen pictures of MRBF fuses or pictures of Class T fused in holder attached to a wall, but no pictures of Class T fuses next to a battery terminal. Thanks!

 

[AH Trimble]

You didn't mention the specific battery you have, only that it may be a 280ah EG4, if I understood you correctly.
Do you have the EG4® WallMount Indoor 280Ah Lithium Battery (MODEL #: WM-48-280-1-IN-LL-00) ?
If you do, notice the builtin circuit breaker?


That circuit breaker should be enough to protect your wire (if you install the right size wire). You would need to double check the breaker's amp rating vs the wire you intend to install...make sure the wire is protected.
If in-fact you have those two 280ah battery units that I referred to, then if it was my install I would connect them to a Victron Lynx Power In Class T unit with 400a Class T fuses (based on the batteries internal >300a @ 3sec protection). Then connect the Victron Class T unit to the inverter with the right sized cables.



Each battery unit would be connected into a side of the Victron Lynx Power In Class T unit. That would provide protection to each unit separately from each other and the system as a whole.

 

[paulclinger]

Yes, that's the correct battery, although I'm not yet sure of the exact model. Thank you for the suggestion, as it does look like a good solution for me, as I do plan to use 2 batteries. Given the size of this unit, it would need to be mounted outside of the conduit box. Or would you skip the conduit box altogether in this case?

 

[AH Trimble]

Yes, that's the correct battery, although I'm not yet sure of the exact model. Thank you for the suggestion, as it does look like a good solution for me, as I do plan to use 2 batteries. Given the size of this unit, it would need to be mounted outside of the conduit box. Or would you skip the conduit box altogether in this case?

For me, if I was doing it, my wiring would look like this...


And the install view on mine would look like this...


I would not install the Class T unit "in" anything. I would want easy access. Also, the blue color is really cool
Here is the link for the cabling tray/raceway if you are interested : https://amzn.to/41fjwNM
And yes, I used the exact same product on my system.

And lucky me...my Victron Lynx Class T was delivered today ! ! !