Help sizing cables and fuses for batteries and inverter.

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
Need some help determining correct wire and fuse sizes for a vehicle that will have 2 100ah LiFePO4 batteries wired in parallel and a 1500 watt inverter.

I think I have the math correct for the inverter. Please correct if I am wrong.
1500 watt inverter assuming 90% efficiency
1500/.9=138.8 amp.
Inverter fuse size.
138.8 amp x 1.25 = 173.5 amp minimum amp fuse. I could use a 175 amp fuse that is very close to the minimum size, should I round up to 200 amp?
Inverter wire size.
2AWG

Battery cables,
I know I need some help here. Since there are two100ah batteries in parallel I can pull 200 amps I am not sure how the wire and fusing is determined and will I need to have a fuse for each battery?
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
1500 watt inverter assuming 90% efficiency
1500/.9=138.8 amp.
You left out the 12V

1500W / 12V / 90% = 138.8A

The 175A fuse is not the minimum. The minimum is 139A. The maximum is based on whatever wire size you choose (which should be 1AWG or even better, 1/0AWG). That would be a max of 250A (1AWG) or 300A (1/0AWG). You should pick a fuse between the min and max. Too close to the min and you may get nuisance trips. The 125% factor usually gives a good value between the min and max. So if you can get a 175A fuse then it is just fine. Using a 200A fuse would be fine too since it is well below the max.

Battery cables,
I know I need some help here. Since there are two100ah batteries in parallel I can pull 200 amps I am not sure how the wire and fusing is determined and will I need to have a fuse for each battery?
The battery capacity is irrelevant for this. You could have 10 100Ah batteries in parallel and the main battery fuse would be the same as just two batteries.

It's based on the max load the batteries will have to provide in your system. So far we know of at least 139A from the inverter. Do you have a DC fuse box for your system? How many amps will your loads pull? Add that to the 139A of the inverter to get a total. Then base the battery wires and main battery fuse on that total. If you don't have a DC fuse box and just the inverter then you can use the 175A (or 200A) fuse as the main battery fuse and use the same 1/0AWG from the batteries to the inverter.

Once you know the total amp draw from the batteries (either the 139A or some bigger number due to a DC fuse box) then you compare that to the max continuous discharge rating of your batteries. If each BMS can only handle 100A then it would be prudent to put a 125A on each battery in addition to the 175A main battery fuse. If each BMS can handle 200A then there is no reason to fuse each of the two batteries. The one main fuse would be enough since each battery can handle the full max load.
 

Zil

Just another ass on the web
Joined
May 27, 2020
Messages
1,833
Fuses protect wires. 2 awg marine grade cable can carry 210 amperes. I would fuse inverter at 200 amperes. At the battery end of the circuit.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
2 awg marine grade cable can carry 210 amperes.
But you should never use 2AWG for 200A loads unless you want massive voltage drop and the ability to boil water with the heat coming off of the wire. It is safe to fuse 2AWG with a 200A fuse but limit the use of 2AWG wire for 120A loads or less (for reasonably short lengths of wire).
 

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
You left out the 12V

1500W / 12V / 90% = 138.8A

The 175A fuse is not the minimum. The minimum is 139A. The maximum is based on whatever wire size you choose (which should be 1AWG or even better, 1/0AWG). That would be a max of 250A (1AWG) or 300A (1/0AWG). You should pick a fuse between the min and max. Too close to the min and you may get nuisance trips. The 125% factor usually gives a good value between the min and max. So if you can get a 175A fuse then it is just fine. Using a 200A fuse would be fine too since it is well below the max.


The battery capacity is irrelevant for this. You could have 10 100Ah batteries in parallel and the main battery fuse would be the same as just two batteries.

It's based on the max load the batteries will have to provide in your system. So far we know of at least 139A from the inverter. Do you have a DC fuse box for your system? How many amps will your loads pull? Add that to the 139A of the inverter to get a total. Then base the battery wires and main battery fuse on that total. If y ou don't have a DC fuse box and just the inverter then you can use the 175A (or 200A) fuse as the main battery fuse and use the same 1/0AWG from the batteries to the inverter.

Yes I will have a DC fuse box, a Blue Sea 5025.
Loads

Lights. 10 watts x 5 hours a day = 50 watts = 4.2 amps day.

Phones/Tablet. 15 watts 3hr day = 45 watts = 3.75 amps day.

Fridge. Specification says a max of 55 watts running MAX cooling mode, as low as 33 watts running eco mode but the compressor doesn't run 24 hours a day. Any ideas how much time a day the compressor runs, 20%-30%-40% of the time? Not sure how to find this number, I could guess and say the compressor will run 40%, any suggestions? Assuming the compressor runs in eco mode 40% of the day, 7.2 hours and uses 40 watts = 288 watts day = 24 amps. The fridge

139 from the inverter plus the above DC loads brings us to 170.95, if my math is right.

I am not sure how to address the Fridge unknown?







Once you know the total amp draw from the batteries (either the 139A or some bigger number due to a DC fuse box) then you compare that to the max continuous discharge rating of your batteries. If each BMS can only handle 100A then it would be prudent to put a 125A on each battery in addition to the 175A main battery fuse. If each BMS can handle 200A then there is no reason to fuse each of the two batteries. The one main fuse would be enough since each battery can handle the full max load.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
Your calculations for DC loads isn’t correct. You sort of combined two sets of calculations into one. The two calculations you need are 1) the max current load at any given time and 2) the amount of power used in a day.

For the first, simply look at the max total wattage. If the lights are on while using your computer and the fridge is cycling then you will be using 10W + 15W + 55W = 80W. At 12V that’s up to about 7A. So the inverter is 139A and the DC loads are 7A for a total of 146A. So size the wire and fuses based on that.

For the second you need to work out how much time each load will be used. Your calculations were headed in that direction. But for the lights, 10 watts for 5 hours is 50 watt hours, not 50 watts. At 12V that would use about 4 amp hours each day from your battery. These results don’t affect the choice of wire or fuse size. These results determine how much battery and solar you need. All of that should ideally be worked out before buying anything so what you get can meet your expected needs.

For a fridge you might want to assume a 33% duty cycle but it can really vary. So 55W x 24 hours/day x 33% = 440Wh/day.
 

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
Your calculations for DC loads isn’t correct. You sort of combined two sets of calculations into one. The two calculations you need are 1) the max current load at any given time and 2) the amount of power used in a day.

For the first, simply look at the max total wattage. If the lights are on while using your computer and the fridge is cycling then you will be using 10W + 15W + 55W = 80W. At 12V that’s up to about 7A. So the inverter is 139A and the DC loads are 7A for a total of 146A. So size the wire and fuses based on that.
Is this correct?
Battery. 175 amp fuse used at the battery, 2/0 AWG wire.
Inverter. 175 amp fuse used on Inverter 2/0 AWG wire away from inverter.

For the second you need to work out how much time each load will be used. Your calculations were headed in that direction. But for the lights, 10 watts for 5 hours is 50 watt hours, not 50 watts. At 12V that would use about 4 amp hours each day from your battery. These results don’t affect the choice of wire or fuse size. These results determine how much battery and solar you need. All of that should ideally be worked out before buying anything so what you get can meet your expected needs.

For a fridge you might want to assume a 33% duty cycle but it can really vary. So 55W x 24 hours/day x 33% = 440Wh/day.
This would indicate 44.55ah day

My original intent was to purchase 1, 208ah battery but it was suggested on the forum that I would be better off buying 2. 100ah batteries. Both the 208ah and 100ah batteries have a max current draw of 100amps but wiring the two 100ah batteries in parallel will allow for a max current draw of 200ah. I went ahead and purchased the 2 100ah batteries.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
Is this correct?
Battery. 175 amp fuse used at the battery, 2/0 AWG wire.
Inverter. 175 amp fuse used on Inverter 2/0 AWG wire away from inverter.
1/0 AWG would be enough but 2/0 AWG is certainly better. It will remain cooler even when the inverter is maxed out.

You really only need the 175A (or 200A) fuse at the battery. Adding another for the inverter is actually a bit redundant but it doesn't hurt.

This would indicate 44.55ah day
If the total was about 535Wh/day then yes, at 12V it is about 44Ah/day. But that's just the DC loads. Have you worked out how many watt hours your AC loads will be? If you haven't done a full energy audit then please see the following:


My original intent was to purchase 1, 208ah battery but it was suggested on the forum that I would be better off buying 2. 100ah batteries. Both the 208ah and 100ah batteries have a max current draw of 100amps but wiring the two 100ah batteries in parallel will allow for a max current draw of 200ah. I went ahead and purchased the 2 100ah batteries.
Since your battery loads can be up to 146A, a single battery with a max continuous discharge current of 100A is insufficient. Two such batteries in parallel will give you 200A (not Ah) of max continuous discharge current. So if your goal was a 12V 200Ah battery bank then you do need two 12V 100Ah batteries in parallel to meet the needs of your max loads.
 

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
If the total was about 535Wh/day then yes, at 12V it is about 44Ah/day. But that's just the DC loads. Have you worked out how many watt hours your AC loads will be? If you haven't done a full energy audit then please see the following:

At your suggestion I completed the energy audit being sure to include both AC and DC loads.
System size calculations
Full Load W-hrs.1222.5W-Hrs.
Full Load Watts1140Watts
AC Watts1050Watts
AC Surge wattage0Watts
AC Watt-Hrs612.5W-Hrs.
DC Watts90Watts
DC Watt-Hrs610W-Hrs.
Inverter Sizing
Inverter efficiency85.00%%
Min continuous Inverter sizeWatts
Min Surge Inverter size (Input Watts)1235Watts
Battery Sizing
Daily Storage Requirement1331W-Hrs/Day
Max discharge from full CHARGE85.0%%
# days of storage capacity1.5Days
System Voltage12.0Volts
min Battery capacity W-Hrs2348.1W-Hrs.
min Battery capacity A-Hrs195.7Amp Hours
Max continuous current to DC Loads7.5
Max continuous DC current to Inverter0.0
Max continuous current from bat.7.5Amps
Max Continuous discharge rate0.04C

I want to point out that mt AC loads are from a coffee maker 700W and rice cooker 350W. Coffee maker could be used daily. rice cooker maybe a couple of times per week.

This system also has no solar, perhaps at a later date but only room on the van roof for a single 100W panel so will relay on charging when driving for now.


Reading all posts from this thread I noticed in post #2 you stated
"If each BMS can only handle 100A then it would be prudent to put a 125A on each battery in addition to the 175A main battery fuse. If each BMS can handle 200A then there is no reason to fuse each of the two batteries. The one main fuse would be enough since each battery can handle the full max load".

Confirmed SOK battery spec "maximum continuous discharge current 100A".
following your suggestion to include a 125A fuse at each of the two batteries, would a battery post mounted BRMF fuse kit mounted to each battery be acceptable or better to you a MEGA or ANL fuse? Can you please explain the rational for adding these fuses?

With the above information from the energy audit would I still be able to use 1/0 wiring (2/0 even better if I wanted) and a 175A main battery fuse?

Anything else that needs to be considered?
 
Last edited:

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
With the above information from the energy audit would I still be able to use 1/0 wiring (2/0 even better if I wanted) and a 175A main battery fuse?
Assuming you still plan on a 1500W inverter and let's say 100W of DC loads then you are looking at 1500W / 12V / 85% = 150A from the inverter and 100W / 12V = 8A for the DC loads for a total of about 158A being pulled from the battery. 1/0AWG or 2/0AWG is still a good choice. A 200A main battery fuse should be the right size.

Confirmed SOK battery spec "maximum continuous discharge current 100A".
following your suggestion to include a 125A fuse at each of the two batteries, would a battery post mounted BRMF fuse kit mounted to each battery be acceptable or better to you a MEGA or ANL fuse? Can you please explain the rational for adding these fuses?
Since each battery can only provide 100A of the total load and you might have up to a 158A load, the idea of fusing each battery is if you have a load over 100A and for some reason one of the batteries fails. Suddenly the remaining battery will be asked to provide too much current. The smaller battery fuse will protect the battery from the excess load. The battery's BMS will also protect the battery from excess discharge. The fuse is meant to be the first line of defense. The BMS should always be the last line of defense. My understanding is that an MBRF fuse mounted directly on the battery's positive terminal would be a good choice. You still want a 200A Class T fuse as the main battery fuse.
 

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
Class T fuse. I will follow your advice, there are countless posts on the forum recommending a class T fuse be used on the battery main line. My confusion with the use of a class T comes from reviewing many of the electrical systems that are built by YouTubers that have strong relationships with a major battery supplier and are using ANL fuse for main battery fuse.

Since I will be sleeping about a foot above the batteries, I want to be safe so class T fuse it is.

Thank you for all of your help!
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
An ANL fuse is fine for lead-based batteries. LiFePO₄ can put out so much more current quickly during a short. A Class T fuse is able to handle a much higher current. That is why you see so many recommendations for Class T fuses when working with LiFePO₄ batteries.

Compare the AIC rating of the various fuses to see the differences.
 

Cuca

New Member
Joined
Sep 26, 2021
Messages
69
Assuming you still plan on a 1500W inverter and let's say 100W of DC loads then you are looking at 1500W / 12V / 85% = 150A from the inverter and 100W / 12V = 8A for the DC loads for a total of about 158A being pulled from the battery. 1/0AWG or 2/0AWG is still a good choice. A 200A main battery fuse should be the right size.

@rmaddy
Would it be acceptable to us a 225A class T fuse in place of a 200A? (I am using 1/0 wire). With the supply chain in its current condition I find everyone is out of stock on class T fuse holders that accept up to a 200a fuse, even vendors that say it is in stock are out when called asking to confirm.

Evidently 225A fuse is larger than a 200A and I can find 225A fuse holders in stock.

Since each battery can only provide 100A of the total load and you might have up to a 158A load, the idea of fusing each battery is if you have a load over 100A and for some reason one of the batteries fails. Suddenly the remaining battery will be asked to provide too much current. The smaller battery fuse will protect the battery from the excess load. The battery's BMS will also protect the battery from excess discharge. The fuse is meant to be the first line of defense. The BMS should always be the last line of defense. My understanding is that an MBRF fuse mounted directly on the battery's positive terminal would be a good choice. You still want a 200A Class T fuse as the main battery fuse.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
Would it be acceptable to us a 225A class T fuse in place of a 200A? (I am using 1/0 wire).
If you are using marine grade pure copper wire with 105ºC rated insulation then it can be fused up to 250A.
 

Zil

Just another ass on the web
Joined
May 27, 2020
Messages
1,833
But you should never use 2AWG for 200A loads unless you want massive voltage drop and the ability to boil water with the heat coming off of the wire. It is safe to fuse 2AWG with a 200A fuse but limit the use of 2AWG wire for 120A loads or less (for reasonably short lengths of wire).
I am not recommending wire gauges. I am recommending fuse size for the wire gauge already listed in OP post.
I do not believe the wire recommended ampcity will heat the wire to boiling water. Up until this post, you have not discussed the key question. The length of the wire to determine voltage drop. Marine grade 2 awg would be fine if the inverter was less than 10 feet round trip from the battery. Fused at 200A for a 140 amp load.
Yes, voltage drop produces heat, but not boiling water heat in the proper ampacity wire.
 
Last edited:

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
3,393
Location
USA
I am not recommending wire gauges. I am recommending fuse size for the wire gauge already listed in OP post.
I do not believe the wire recommended ampcity will heat the wire to boiling water. Up until this post, you have not discussed the key question. The length of the wire to determine voltage drop. Marine grade 2 awg would be fine if the inverter was less than 10 feet round trip from the battery. Fused at 200A for a 140 amp load.
Yes, voltage drop produces heat, but not boiling water heat in the proper ampacity wire.
Sorry I misread your original post. Yes, 2AWG can be fused up to 200A. My comment about the wire getting really hot was in regard to trying to run a 200A load through 2AWG.
 
Top