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diy solar

Undersized battery cable?

wiseacre

Solar Enthusiast
Joined
Aug 8, 2021
Messages
492
Location
North of the Adirondacks
Obviously I made a mistake. I thought increasing the cable size that came with the inverter from 6awg to 2/0 was a good idea and adequate. But after looking at Will's link to cable size https://www.altestore.com/diy-solar...inverter-cables-sizing-and-breakers-or-fuses/ it looks like I should be using 4/0 cable.

I have a 3500 watt inverter (surge 7000w)
24v battery (2s, 12v) Gel
I have 2/0 cable connecting and a 300 a fuse

So if I understand correctly I should at least replace the 300a fuse with a 200a.
That seems to limit the inverter to 2500w?
Ideally, ordering some 4/0 wire would correct my mistake.

I'm learning, so any comments are appreciated
 
Obviously I made a mistake. I thought increasing the cable size that came with the inverter from 6awg to 2/0 was a good idea and adequate. But after looking at Will's link to cable size https://www.altestore.com/diy-solar...inverter-cables-sizing-and-breakers-or-fuses/ it looks like I should be using 4/0 cable.

I have a 3500 watt inverter (surge 7000w)
24v battery (2s, 12v) Gel
I have 2/0 cable connecting and a 300 a fuse

So if I understand correctly I should at least replace the 300a fuse with a 200a.
That seems to limit the inverter to 2500w?
Ideally, ordering some 4/0 wire would correct my mistake.

I'm learning, so any comments are appreciated
Your calculations are a bit off.

Assuming a high frequency inverter...
The surge rating for high frequency inverters is on the order of milliseconds and is therefore not significant.
3500 ac watts / .85 conversion efficiency / 20 volts low cutoff = 171.568627451 service amps.
171.568627451 service amps / .8 fuse headroom = 214.460784314 fault amps
That means a 215 amp fuse and wire that can handle 215 amps minimum.

Based on those calculations the minimum that you could use is 1 awg pure copper wire with 105c rated insulation.
Of course nobody wants their wires to get that hot so better to go up a size.
This calculator will let you explore voltage drop.
 
Last edited:
The above answer left me more confused, assume I take the short bus to school

additional inverter info:

3500W 12V/24V/48V to 110V Off Grid Pure Sine Wave Inverter
Continuous Power: 3500W
Output Volt: 110V AC
Waveform Pure Sine Wave
Surge Power: 7000W
Frequency: 110V-60Hz
Input Range: 10-15V DC/21-30V DC
Working Input: 12V/24V/48V DC
Non-load Consumption: <25W
Efficiency: >90%
Fuse: 20A/35A
Over Heating Shut Down:>75℃
Over Load Shut Down: 3600W Over 10 Second
Over Thermal: Shut Down The Output
Working Temp.:-10℃~+50℃

I am currently using 2 Gel batteries (12v 100Ah) in series to get 24v 100Ah
Battery cables are 2/0
fuse is 300a
cable length is appx 2 feet from battery to inverter

from my very very limited understanding I should be using a 200a fuse for that size cable (2/0)
if I use a 200a fuse how likely will it blow using this inverter?
Not that it's likely but what if I used (2) 1500watt space heaters at the same time?
Ideally what should my cable size be?
 
The above answer left me more confused, assume I take the short bus to school

additional inverter info:

3500W 12V/24V/48V to 110V Off Grid Pure Sine Wave Inverter
Continuous Power: 3500W
Output Volt: 110V AC
Waveform Pure Sine Wave
Surge Power: 7000W
Frequency: 110V-60Hz
Input Range: 10-15V DC/21-30V DC
Working Input: 12V/24V/48V DC
Non-load Consumption: <25W
Efficiency: >90%
Fuse: 20A/35A
Over Heating Shut Down:>75℃
Over Load Shut Down: 3600W Over 10 Second
Over Thermal: Shut Down The Output
Working Temp.:-10℃~+50℃

I am currently using 2 Gel batteries (12v 100Ah) in series to get 24v 100Ah
Battery cables are 2/0
fuse is 300a
cable length is appx 2 feet from battery to inverter

from my very very limited understanding I should be using a 200a fuse for that size cable (2/0)
if I use a 200a fuse how likely will it blow using this inverter?
Not that it's likely but what if I used (2) 1500watt space heaters at the same time?
Ideally what should my cable size be?
Assuming that the 2/0 wire is pure copper with 105c rated insulation it is good for 330 amps.

The fuse's primary job is protect the wire and a 300 amp fuse can do that.
The fuse does not protect the inverter.
2/0 awg pure copper wire is more than adequate to supply power to your inverter.
The batteries are not likely up to the task.
1 awg is technically big enough.
1/0 awg is even better.
2/0 awg is better still.
 
Assuming that the 2/0 wire is pure copper with 105c rated insulation it is good for 330 amps.

The fuse's primary job is protect the wire and a 300 amp fuse can do that.
The fuse does not protect the inverter.
2/0 awg pure copper wire is more than adequate to supply power to your inverter.
The batteries are not likely up to the task.
1 awg is technically big enough.
1/0 awg is even better.
2/0 awg is better still.
Perfect, thank you so much
confirmed what I originally thought

Yea, my batteries are woefully inadequate but all I could afford at the time.
I am looking at prebuilt LiFePo4 batteries. I wouldn't dare a DYI battery with my lack of working knowledge and equipment.
24V 200AH LiFePower4 Battery by EG4 is on the top of my list
 
The above answer left me more confused, assume I take the short bus to school

additional inverter info:

3500W 12V/24V/48V to 110V Off Grid Pure Sine Wave Inverter
Continuous Power: 3500W
Output Volt: 110V AC
Waveform Pure Sine Wave
Surge Power: 7000W
Frequency: 110V-60Hz
Input Range: 10-15V DC/21-30V DC
Working Input: 12V/24V/48V DC
Non-load Consumption: <25W
Efficiency: >90%
Fuse: 20A/35A
Over Heating Shut Down:>75℃
Over Load Shut Down: 3600W Over 10 Second
Over Thermal: Shut Down The Output
Working Temp.:-10℃~+50℃

I am currently using 2 Gel batteries (12v 100Ah) in series to get 24v 100Ah
Battery cables are 2/0
fuse is 300a
cable length is appx 2 feet from battery to inverter

from my very very limited understanding I should be using a 200a fuse for that size cable (2/0)
if I use a 200a fuse how likely will it blow using this inverter?
Not that it's likely but what if I used (2) 1500watt space heaters at the same time?
Ideally what should my cable size be?
It won’t matter what size wire you use, those batteries won’t let you exceed 100AMPS…
So sustained use they limit your inverter to around 2500W… and the batteries will only support that load for one hour.
 
Perfect, thank you so much
confirmed what I originally thought

Yea, my batteries are woefully inadequate but all I could afford at the time.
I am looking at prebuilt LiFePo4 batteries. I wouldn't dare a DYI battery with my lack of working knowledge and equipment.
24V 200AH LiFePower4 Battery by EG4 is on the top of my list
Average joe just released a video reviewing some 200Ah LFP kits… large amounts BMS rated at 500A I think too.
 
I think, because I don't know the specs of your batteries, the amperage draw at high wattage is a lot for a couple of batteries equaling 100 ah at 24 volts. Based off me seeing 155 amps at 12 volts with 1800 appliance watts, for 3500 watts, you'd see perhaps 200 - 250 amps. Although its 1800 appliance watts, I have about 2100 watts being drawn from the inverter to account for inverter losses of about 15%.

That amperage would skyrocket if the voltage sagged to the inverter cutoff. Same wattage higher amps with the lower voltage. That is what the formula was for. I got numbers a little different:

3500 ac watts / .85 conversion efficiency / 20 volts low cutoff = 205.88 service amps.
205.88 service amps / .8 fuse headroom = 257. 35 fault amps
That means a 215 amp fuse and wire that can handle 260 amps minimum.

So my guess is if you use the inverter it does not go above 2000 watts very much at all.

This is the chart I use to determine wire size based off ampacity:

1629055455497.png
With using a fuse to limit power, this is the blow time for a class T fuse

1629056516573.png
So your 200 amp fuse could run at 300 amps for over 8 minutes before it got hot enough to melt. Because of that, I think a 200 amp fuse is step to limiting the inverter, but because it could run at 300 amps for minutes, discipline is more import in not turning those appliances on.
 
Divide the max theoretcial inverter load (watts) by the battery voltage and then divide the answer by 0.85 for efficiency loss
 
It won’t matter what size wire you use, those batteries won’t let you exceed 100AMPS…
So sustained use they limit your inverter to around 2500W… and the batteries will only support that load for one hour.
Good point, something I did not know enough to consider.

Upgrading my batteries is my prime objective.
These are at the top of my list
https://shop.signaturesolar.us/products/24v-200ah-lifepower4-battery-by-eg4

My original Gel batteries would need to be donated to my son-in-law for trolling motors
 
So your 200 amp fuse could run at 300 amps for over 8 minutes before it got hot enough to melt. Because of that, I think a 200 amp fuse is step to limiting the inverter, but because it could run at 300 amps for minutes, discipline is more import in not turning those appliances on
The additional info helps towards understanding, thanks.

discipline is more import in not turning those appliances on
IRL my usage is limited by my battery. Normally I'm using a little less than 1500w a day for my aquariums. In case of a power outage I'd limit large appliances like a refrigerator and freezer to one at a time until my battery ran out of juice.

So now I feel reassured and can untie the knot in my panties ?
 
Average joe just released a video reviewing some 200Ah LFP kits… large amounts BMS rated at 500A I think too.
I appreciate the heads up

Someday, maybe. A DYI battery right now scares me silly.
I suppose I could follow instructions but would still need to buy equipment and from what I've read on the forum about wait time and damaged cells I think I'm better off going prebuilt for my modest needs.
 
Your calculations are a bit off.

Assuming a high frequency inverter...
The surge rating for high frequency inverters is on the order of milliseconds and is therefore not significant.
3500 ac watts / .85 conversion efficiency / 20 volts low cutoff = 171.568627451 service amps.
171.568627451 service amps / .8 fuse headroom = 214.460784314 fault amps
That means a 215 amp fuse and wire that can handle 215 amps minimum.

Based on those calculations the minimum that you could use is 1 awg pure copper wire with 105c rated insulation.
Of course nobody wants their wires to get that hot so better to go up a size.
This calculator will let you explore voltage drop.
Hello Smooth,
First paragraph was obviously meant to impress.
It is too much information for an "idiot" like me.
However, I really appreciate the calculator.
I am learning. Maybe I'll understand before I die.
Pray for me. :)
 
Hello Smooth,
First paragraph was obviously meant to impress.
It is too much information for an "idiot" like me.
However, I really appreciate the calculator.
I am learning. Maybe I'll understand before I die.
Pray for me. :)
You could ask questions about the specific bits that don't grok.
 
I am so new... ordered some 1/0 4'ft pure copper cables to
connect my battery to my inverter. I am in the midst of a deep
dive into understanding how less or more current works, and
its relationship with accuracy, cable size and length, and voltage drop.

In any case, I need to work on this some more so I can ask
understandable questions. I was also told by someone outside
of Wm Prowse's study that I should use 2/0 battery cables, but I
am not going to be using above 400w until I understand how this all works.
Perhaps when I increase to a 24v system I will use a cable that large.

On Wm Prowse's 400w solar power system assembly
tutorial, he uses 2 gauge cables to hook up his system
to a 12v battery. So I have taken note of this. Perhaps
it is because his cables were longer than 4 feet, on his
tutorial, he needed a larger cable than a 4 gauge.
-----
2000w Pure Sine Inverter (4K surge)
40A EPEVER Charge Controller
12v Sea Blue Fuse Block w/ negative bus
175a circuit breaker between 12v 100a Battle Born Battery
50a Bussman fuse between Charge controller and inverter.
2 - 100A 12v solar panels, claiming to generate 5.4A
2 pair - 100A 55A - 40ft 8-gauge Panel Extension Cable Wire
----
I am starting with one 12V 100A Battery, and two
100A solar panels. I just want to make sure I have
enough protection for the components while I start
this and see what I need next. I look forward to working
my way through this.

I am guessing I could use battery cables (4-6' ft) that are
4 gauge, 2 gauge, 1/0, or 2/0.
Since the cables are so short, perhaps the current in each
one is different per gauge.

According to charts I am looking at,
this particular chart does not give cable length
but max current. How can I calculate the length
of the cable based on this?
1) a 2/0 cable has max. current of 190A
2) a 1/0 cable has max. current of 150A
3) a 2 gauge cable has max current of 94A
4) a 4 gauge cable has max current of 60A

---
I realize I need to purchase another battery as well
as 2 more 100A solar panels, but I am just trying to
see if I can start here and get it up and running without
causing any damage to my instruments. I have cables
here of 4 gauge, 1/0, and another set of 2/0 coming
in a couple of days.

QUESTION:
Am I correctly seeing that the longer the cable, the
smaller the gauge can be (not below 4 gauge) to run
my battery on this system? And that the shorter
cables allow for more efficiency because of less
voltage drop, as well as heat?

I realize I just gave you a lot of things, and some
of it is mixed up, but I appreciate your willingness
to help.

Thank you for your response.
Any light you can shed will be most helpful.

Best,
 
QUESTION:
Am I correctly seeing that the longer the cable, the
smaller the gauge can be (not below 4 gauge) to run
my battery on this system?
The length of a wire has no appreciable effect on its ampacity rating.
Smaller gauge is tricky.
The smaller(numerically) the gauge the thicker the wire.
All else being equal the thicker the wire the more current it can carry.
There are 3 main factors that determine the ampacity rating of a wire.

1. The conductor material.
To keep things simple lets only consider pure copper wire.

2. The insulation.
To keep things simple lets only consider insulation rated for 105 Celsius.
That means the insulation won't melt until its hotter than boiling water.

3. The thickness of the conductor.
Thicker is better for ampacity and for voltage drop.

And that the shorter
cables allow for more efficiency because of less
voltage drop, as well as heat?
Heat is considered a constant.
The more wire the more resistance(voltage drop) but also the more surface area to dissipate heat.
 
Thank you Mr. SmoothJoey,
You have given me a few very valuable pieces to the puzzle.
I am going to hook up my system to the battery, starting with
4 gauge, 4ft, and check it out.

Feeling like I am in the middle of a marathon.
Learning is like coming to life.

Thanks again!
 
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