Battery to Inverter Breaker Sizing

Quanta · January 15

I've got a Progressive Dynamics 1800 watt pure sine wave inverter, that provides a 3600 watt surge. The specs state the inverter will draw 180 amps when providing the 1800 watts output.

Manual indicates 200 amps. What size breaker should I use to avoid tripping at the rated 2X rated power surge output?

Link to manual: https://www.progressivedyn.com/wp-content/uploads/Support/manuals/PD1600-Operation-Guide-701580.pdf

Quanta.

BB. · January 15

Fuses and breakers have time vs current curves... And there are standard/slow/fast acting versions out there... Also, these over current protective devices are not "that accurate" (fuzzy trip times vs current--In the breaker trip time chart in link below, a 200% overload for DC breaker can trip between 0.06 and 110 seconds and be "within specifications")...

Anyway, Type T fuses are really nice for protection, and here is LittleFuse's Time vs current graph (page 8 of PDF if the screen shot is not readable for 200 Amp version):

https://www.littelfuse.com/assetdocs/jlln-datasheet?assetguid=2925cc6d-2278-44e0-8fdd-106f087f707e

Image: https://us.v-cdn.net/6024911/uploads/editor/bt/6k8ceg88jhtz.png

A 200 Amp T fuse will pass 400 Amps for 0.06 seconds (minimum/max/nominal?) and 300 Amps for around 3 seconds...

Most inverters will have a surge current rating of approximately 2x their full power rating, and the max time is usually a couple seconds...

And here is Midnite Solar's (Carling is Midnite's supplier for (some/many/all?) their breakers) generic Time vs Current trip charts for various types of Breakers:

https://www.midnitesolar.com/pdfs/Carling_A-DTime-Delays.pdf

And their 250 Amp DC breaker (make sure you get the properly rated breaker... AC breakers are not (usually) rated for DC circuits:

https://www.solar-electric.com/mnedc250.html

Most of the time, AC inverters will "trip" before the protective fuse/breaker... Their electronic sense circuits are much faster and more accurate than a generic OCP device.

A good place to start is with the Inverter's manual... You generally will not get into problems (safety/liability) if you follow their requirements.

And when buying breakers, you may have to do some research/make phone calls to figure out which breaker type is being sold (slow, medium, fast, etc. trip)...

Here is a long thread about fuses and breaker selection:

https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

Too much information?

-Bill

Quanta · January 15

-Bill
"Too much information?"

Not at all. I spend something like, easily six hours/day, studying everything from the state of the world, to biochemistry. Previous career was in electronics and some process control on the design, build, install side, so the terms don't mystify me. I guess I'll finally break down and contact the manufacture' for the information once I take another look at the manual.

A 200 amp breaker with something like a 2 second delay is likely what I'm looking for, but will check with the manufacture'.

Thanks all,

Quanta.

Quanta · January 15

Just got off the phone and manufacture' recommends a 200 amp fuse over a breaker. I'll check the littlefuse products.

Regards.

Photowhit · January 19

Things just don't happen that fast. I'd just use a breaker. With a 12 volt battery bank you likely won't want to be running near it's 180 amp capacity much...

BTW That's likely 2/0 wire for short distance and 4/0 for much more than 4 feet.

Quanta · January 22

Photowit: With a 12 volt battery bank you likely won't want to be running near it's 180 amp capacity much...

1800 is the running watts. Surge is 3600. I won't be running constant 1800, however, why not 1800? Is it due to the spec that it is 1800 running- 3600 surge?

Thanks.

Photowhit · January 23

It's because that's the wrong system voltage for a constant 180 amp draw!

A huge 12 volt battery would be in the neighborhood of 800 amp hours in lead acid, Drawing 180 amps would be drawing more than 20% of it's capacity per hour. To do this you would have to take into account the rating of the battery which is based on 1/20th of the capacity, not 1/5th. This reduces the effective capacity greatly. Likely have issues with voltage sag after an hour with a lead antimony battery.

Increasing the system voltage allows the use of higher single cell strings. So a large 24 volt battery your 1800 watt load is reduced 90 amps per hour, and a 48 volts 45 amp hour. At 45 amp steady draw you would want a 900 amp hour 48 volt battery, still a very large battery, think 3000 pounds.

Huge loads like this do exist in some situations, think forklifts. Forklifts are usually rated at a 5 hour discharge rate. My 24 volt, 660 amp hour at 20 hour discharge is a 504 amp hour at a 6 hour discharge. Using 80% of the battery capacity some forklifts get recharged after an 8 hour shift.

Hope this helps explain why you wouldn't want to run near capacity with a 12 volt 1800 watt inverter.

Quanta · January 23

Yes, this is very helpful. I haven't connected anything yet, but have in the past run 12 volt systems.

I did a rough calculation on the running watts I'll require, and this is only for power outages; perhaps a few days, or worst case scenario- Some long term black out. Over all, I'll likely be running around 800 watts if the fridge and freezer are running at the same time. Beyond those draws, the only others are, the computer, a big screen LED TV, and LED lighting and, miscellaneous low wattage charging devices.

The battery bank consists of: 12, 12 volt, DC-27 sealed lead acid batteries that would be wired into two banks.

Thanks for the assistance,

Quanta.

Photowhit · January 23

As the fridge and freeze will be short term loads running on thermostat, they won't be running a long time. With heavy wiring, as a backup system 1800 might be okay. Also the max loads are likely when in defrost mode. Might be worthwhile checking to see what the typical cycling load is, might be only 200 watt running the compressor. A Kill-A-Watt meter can be helpful for this.

Check out your battery bank for the capacity for the amount of time you can expect them to run. Use the Kill-a-watt meter or check the energy star rating for the annual expected watt usage and work backward for daily load. When estimating the capacity run time, I would use 50% for true deepcycle lead acid batteries, 80% for quality LiFePo4 batteries.

Battery to Inverter Breaker Sizing

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