Please advise a true NEWB on 24V system

[John Frum]

Makes no sense to have 0 awg with a 170 amp fuse on the inverter and 1 awg with a 125 amp fuse on the battery.
Suggest you either swap them or make them the same.

The buck converter has a common negative.
That means both the 12 volt and 24 volt negatives connect to the common negative terminal.

What is the continuous rating in amps for the bms?
How far between the busbars and the 12 volt fuseblock roundtrip?

 

[2manyquestions]

Makes no sense to have 0 awg with a 170 amp fuse on the inverter and 1 awg with a 125 amp fuse on the battery.
Suggest you either swap them or make them the same.

The buck converter has a common negative.
That means both the 12 volt and 24 volt negatives connect to the common negative terminal.

What is the continuous rating in amps for the bms?
How far between the busbars and the 12 volt fuseblock roundtrip?

Swap the wires? I don't understand. Looking at a wire gauge chart linked earlier in this thread, 1 AWG should be good for up to 150A, so the 125A fuse on the battery will trip and protect the wires if necessary, no? And, from what I can tell on that chart, 0 awg should be good for somewhere between 150A and 200A. Or, am I reading things incorrectly?

Regarding the common negative, thanks for that...I wondered if that was permissible. So, wrong advice from the vendor.

According to the PDF manual (one single page) on the BigBattery site, BMS specs are as follows:

Max Charge Current: 175A and Max Continuous Discharge: 175A. On the BMS label, it simply says "200A".

I don't have anything connected/laid out yet, but I hope roundtrip distance between either busbar and 12V fuseblock would be 30-36"

 

[John Frum]

Swap the wires? I don't understand. Looking at a wire gauge chart linked earlier in this thread, 1 AWG should be good for up to 150A, so the 125A fuse on the battery will trip and protect the wires if necessary, no? And, from what I can tell on that chart, 0 awg should be good for somewhere between 150A and 200A. Or, am I reading things incorrectly?

The battery needs to supply the load to the both the inverter and the buck converter.
Yet the inverter has a thicker wire and bigger fuse.

Regarding the common negative, thanks for that...I wondered if that was permissible. So, wrong advice from the vendor.

Wrong advice from vendors... never seen that happen.

According to the PDF manual (one single page) on the BigBattery site, BMS specs are as follows:

Max Charge Current: 175A and Max Continuous Discharge: 175A. On the BMS label, it simply says "200A".

Good, at least on paper that battery has a reasonably sized bms.

I don't have anything connected/laid out yet, but I hope roundtrip distance between either busbar and 12V fuseblock would be 30-36"

Ok so 18" one way.
At that distance voltage drop is not an issue.
BTW you can use 6 awg on the 12 volt side of the buck converter.
Save a line item on the bill of materials.

 

[12VoltInstalls]

edited away

 

[Stepandwolf]

The battery needs to supply the load to the both the inverter and the buck converter.
Yet the inverter has a thicker wire and bigger fuse.


Wrong advice from vendors... never seen that happen.

Good, at least on paper that battery has a reasonably sized bms.


Ok so 18" one way.
At that distance voltage drop is not an issue.
BTW you can use 6 awg on the 12 volt side of the buck converter.
Save a line item on the bill of materials.

I think he chose 4 ga on the converter is because it is 70A continuous but 85a peak and the wire chart says to use 4 ga with 85A. That also means there is more than 70A input if there is 70A (or 85A) out, so having 6 ga on input and 4a on output is almost backwards. You are probably fine with 6, but 85A potential should be on 4ga.

 

[John Frum]

I think he chose 4 ga on the converter is because it is 70A continuous but 85a peak and the wire chart says to use 4 ga with 85A. That also means there is more than 70A input if there is 70A (or 85A) out, so having 6 ga on input and 4a on output is almost backwards. You are probably fine with 6, but 85A potential should be on 4ga.

6 awg is good for 105 amps at 90c -- https://www.omnicable.com/technical-resources/nec-ampacity-data
Also the voltage drop over 36" is trivial.
Also wires and fuses should be sized for continuos load not peaks.
Unless of course the peaks can actually flirt with the trip curve.

 

[Stepandwolf]

6 awg is good for 105 amps at 90c -- https://www.omnicable.com/technical-resources/nec-ampacity-data
Also the voltage drop over 36" is trivial.
Also wires and fuses should be sized for continuos load not peaks.
Unless of course the peaks can actually flirt with the trip curve.

I am sure you are better able to read that chart properly, but when I look at table 310.15(B)(16), it says 75a at 90c. The chart that says 105a was a single conductor vs 75a up to 3 conductors. 6 ga is probably fine, especially if he never pushes the converter. I am not the guru you are so I will bow to your wisdom.

 

[John Frum]

I am sure you are better able to read that chart properly, but when I look at table 310.15(B)(16), it says 75a at 90c. The chart that says 105a was a single conductor vs 75a up to 3 conductors. 6 ga is probably fine, especially if he never pushes the converter. I am not the guru you are so I will bow to your wisdom.

Look at Table 310.15(B)(17) (formerly Table 310.17)

 

[John Frum]

This one says 120 amps@105C but nobody wants their wires hotter than boiling water....
Unless they are trying to make coffee.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

 

[Stepandwolf]

This one says 120 amps@105C but nobody wants their wires hotter than boiling water....
Unless they are trying to make coffee.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

I have been using the chart from Blue Sea which I gave the op. Instead of using Blue Sea's own diagram, I used their calculator. I input 12v, 70a, and 3' and it came back with 8ga...and it said "Capacity per ABYC Standards: 80 amps." American Boat & Yacht Council being the authority they use as they cater to the marine community and I consider the marine community to live at a higher standard than residential and commercial applications.

 

[HRTKD]

The Bay Marine Supply calculator is based on the same standards. On their page it says, "Chart is based on USCG, ABYC, and UL standards for marine-rated (105°C dry / 75°C wet) wire."

 

[John Frum]

I have been using the chart from Blue Sea which I gave the op. Instead of using Blue Sea's own diagram, I used their calculator. I input 12v, 70a, and 3' and it came back with 8ga...and it said "Capacity per ABYC Standards: 80 amps." American Boat & Yacht Council being the authority they use as they cater to the marine community and I consider the marine community to live at a higher standard than residential and commercial applications.

its actually 70 service amps / .8 fuse headroom = 87.5 fault amps.

 

[2manyquestions]

Everyone, many thanks for all the input...I have learned a lot. Now I am awaiting a few items before I begin wiring it up. I will share a pic when it is done