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Battery DC Disconnect Switch Sizing Question

moe007

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Mar 23, 2021
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Hello Everyone I'm new to this forum and not sure if this question has ever been answered before but I'm building a 48V battery bank consisting of 8 x 6V Trojan T105 225AH batteries and with DC disconnect switch. Now I purchased a 2 Pole DC MCCB Switch 100A for this function (See pic below but mine is 100A) and would like to know is the Amp rating adequate for my battery bank or would I need to go higher to maybe a 250A one ?
 

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What is your wire size from the battery? What is the peak load you expect to see?
 
Hey Bob thanks for replying. Cable size I will be using is 2/0 cables for connecting batteries in series as well connecting to DC MCCB Switch to Inverter (5kw inverter https://www.mustpower.com/pv1800-vhm-series-high-frequency-off-grid-solar-inverter-2-5-5kw/)
Since the batteries are Lead acid I estimate the most I can use is 5.4KW hours. Right now they will be powering 1 refrigerator, lights, 2 ceiling fans, and 1 mini standalone freezer. So not sure if I was able to answer that last part of your question?
 
Yeah ... I wasn't wasn't wondering about the watts ..... but the peak current draw. It's a common to mistake power for current.

If it's 2/0 copper, it should be able to handle 175 to 200 watts depending on ambient temperature .... not sure how much your battery pack can sustain.

If you have a switch capable of 100A, you would probably only want to operate it at 80A or so .... and be aware if you are using an inverter that 10 amps on the A/C side is approximately 100 amps on the DC side. Plus you will have some inverter efficiency losses.
If everything is DC it's a little easier to figure out the current draw.
 
What's the purpose of this 'disconnect'?

To me - there is a distinction between a breaker/fuse and a shunt-trip (emergency) disconnect.

I have properly sized circuit breakers for each charge (amps coming in) and discharge (amps going out) unit connected to the battery buss-bars - to protect the wire and to isolate individual units to work on them. As an example - there are 2 inverters and each one has it's own circuit breaker.

I also have an overall battery shunt-trip for 100% battery isolation that can be turned off manually to work on things and triggered by the BMS in the event the BMS reads something bad.
 
@Bob B thank you for getting back to me and what you say makes a lot of sense
@Pappion I suppose I should have looked at these and probably will make that purchase if I cant use what I already have.
@OffGridInTheCity I want to be able to isolate my battery banks whenever I need to work on them and I've seen these switches used on some solar builds on Youtube so thought it would be a good idea to buy a couple. And since I didnt specify the current draw so I got sent the default I suppose.
 
Update:
Watching one of my favorite Youtubers on this topic, Davidpoz (
) he stated that the way he calculated the sizing of his battery disconnect is as follows
" 5000W/44V=114A. I chose 44V because that's the lowest the battery can go. It just seemed to make sense to use 1/0 and a 150A breaker. But, keep in mind, I'm a DIYer, not a licensed electrician."
So by his calculations I fell short of those requirements. Now I think I will have to go with @Pappion suggestion...
 
Update:
Watching one of my favorite Youtubers on this topic, Davidpoz (
) he stated that the way he calculated the sizing of his battery disconnect is as follows
" 5000W/44V=114A. I chose 44V because that's the lowest the battery can go. It just seemed to make sense to use 1/0 and a 150A breaker. But, keep in mind, I'm a DIYer, not a licensed electrician."
So by his calculations I fell short of those requirements. Now I think I will have to go with @Pappion suggestion...
Remember, the main function of a breaker is to protect the wire! You need to size the wire for the application - and then the breaker will flow from the wire size. Its OK to have a breaker smaller than the wire can carry - but its not necessary.

The BMS protects the battery, not the breaker on the wire. If you don't have a BMS, then that's the issue there.
On the other end of the wire, the load (inverter) will protect itself from over-drawing amps. :)
 
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@OffGridInTheCity so what you are saying is that since Im using a 2/0 cable to connect the batteries to the inverter than the 100A circuit breaker I have will be sufficient ?
 
@OffGridInTheCity so what you are saying is that since Im using a 2/0 cable to connect the batteries to the inverter than the 100A circuit breaker I have will be sufficient ?
2/0 wire has a 283amp rating for chassis lengths and 190a for transmission lengths. https://www.powerstream.com/Wire_Size.htm

A 200a breaker will protect fine-strand, welding wire for shorter lengths (<20feet) - I use 250a breaker on my 2/0 welding cable wire from my battery to the main buss - about 10feet. This breaker has blown a couple of times with no damage to the wire - so I know for sure this is OK from actual experience. :)

If you only need 100a, then you could use a smaller wire if you wanted like 2AWG or 1AWG. A 100a breaker is perfectly OK (and cheaper) for 2/0 - but is overkill in terms of protecting the wire.
 
2/0 wire has a 283amp rating for chassis lengths and 190a for transmission lengths. https://www.powerstream.com/Wire_Size.htm

A 200a breaker will protect fine-strand, welding wire for shorter lengths (<20feet) - I use 250a breaker on my 2/0 welding cable wire from my battery to the main buss - about 10feet. This breaker has blown a couple of times with no damage to the wire - so I know for sure this is OK from actual experience. :)

If you only need 100a, then you could use a smaller wire if you wanted like 2AWG or 1AWG. A 100a breaker is perfectly OK (and cheaper) for 2/0 - but is overkill in terms of protecting the wire.
Thank you @OffGridInTheCity !!!
 
I was looking earlier at the specs for my Daly 24V 100A BMS and was reminded that it doesn't cut power until 300A, so a breaker/switch needs to be capable of breaking at least that, and I'm not sure how much more (it's conceivable that the BMS could fail with a short which allowed higher currents, or short could occur for other reasons), but, not too quickly. I intend to use 150A DC breakers, because I don't expect to draw more than 125A. I suppose I might go to 300, but, I don't want it too large, or the pretty girls won't be able to reach it (this reminded me of a Star Trek, TOS clip I made a while back):

 
I was looking earlier at the specs for my Daly 24V 100A BMS and was reminded that it doesn't cut power until 300A, so a breaker/switch needs to be capable of breaking at least that, and I'm not sure how much more (it's conceivable that the BMS could fail with a short which allowed higher currents, or short could occur for other reasons), but, not too quickly. I intend to use 150A DC breakers, because I don't expect to draw more than 125A. I suppose I might go to 300, but, I don't want it too large, or the pretty girls won't be able to reach it (this reminded me of a Star Trek, TOS clip I made a while back):

Well. After I tested that link, Google AI had a suggestion, and after I viewed that, another. I will say it's too large for this application:
 
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