Why did I buy 2AWG battery cables for my inverter?

[chrisski]

It is all about watts in use.

most tv’s use less than 50 watts now... ditto the players... say a max of 300W... that’s around 25A for your short burst loads...
12 could handle it without much trouble.

I just want to caveat that. Those I’m with are not the best at following limits.

For those of us who have teenage kids, they might not be the best at obeying rules when we’re not around. Also, for guests, they don’t understand that the power is limited. That’s another reason I went with my 4/0 wire for my 12 volt 2000 watt system. Even though I tell everyone not to use the microwave, in theory it should handle the load.

Don’t tell them!!

So, if you do go with a much smaller wire than your inverter needs, I would make sure there’s at least a fast blowing fuse in the loop that will limit the current to something less than that wire carries.

 

[Sean Steele]

If you get right down to it, my 2000 Watt inverter can supply more power than the circuit that it is powering is rated for. This inverter will be powering one circuit of outlets that has a 15A breaker in the AC panel. Theoretically, if I go over the 1800 Watts that the circuit breaker normally allows, it could exceed the rating of the AC wiring for that circuit. Given that this is an 18 year old camper, it's probably wired with 14--3. My rAC run between the AC panel and the inverter as well as from the inverter back to the panel housing will be 12-3 because that is what the inverter mfg recommends.

 

[chrisski]

The AC side I’m not worried about. My microwave draws 155 amps on the 12 volt side, which comes out to around 15 amps on the AC side.

 

[Supervstech]

If you get right down to it, my 2000 Watt inverter can supply more power than the circuit that it is powering is rated for. This inverter will be powering one circuit of outlets that has a 15A breaker in the AC panel. Theoretically, if I go over the 1800 Watts that the circuit breaker normally allows, it could exceed the rating of the AC wiring for that circuit. Given that this is an 18 year old camper, it's probably wired with 14--3. My rAC run between the AC panel and the inverter as well as from the inverter back to the panel housing will be 12-3 because that is what the inverter mfg recommends.

Yes, the inverter does have a higher rating than the utility ac 15amp circuit is rated for, but that circuit can output over 200A of surge for a significant time... the inverter cannot.
I have been amazed by what utility power can output under stupid conditions...
tracing breaker trips, I have measures 270A of surge current from cross wired circuits...
and EVERY USA ul listed circuit breaker is rated to interrupt 10,000AMPS of arc... so... guess how many amps are CAPABLE of hitting that circuit!

 

[dmholmes]

There are 2 specs.

For a high frequency inverter the math is like this...
2000 ac watts / .85 conversion factor / 12 volts = ~196 dc amps
~196 dc amps * 1.25 = ~245 fuse amps

AYBC which is for 105C wire https://baymarinesupply.com//bosns_corner_wire_sizes
NEC which is for 90C wire https://www.usawire-cable.com/pdfs/NEC AMPACITIES.pdf

Hopefully those 10 awg wires have 200C insulation on them.
Which inverter do you have?

@smoothJoey, you helped me with this a few months ago, but double-checking now that this thread came up.

For my 1200W inverter:
1200W / 0.85 / 12V = ~118 DC amps
~118 DC amps * 1.25 = ~150 fuse amps

Does that look correct?

I have 2AWG 105C wire (inverter came with 5AWG), 4 feet each so 8 feet roundtrip. Looks within the <3% voltage loss. And 210 ampacity limit in the chart looks good.

I bought a fuse block with 125A fuse. Looks like that needs to be upgraded to 150A.

 

[John Frum]

@smoothJoey, you helped me with this a few months ago, but double-checking now that this thread came up.

For my 1200W inverter:
1200W / 0.85 / 12V = ~118 DC amps
~118 DC amps * 1.25 = ~150 fuse amps

Does that look correct?

I have 2AWG 105C wire (inverter came with 5AWG), 4 feet each so 8 feet roundtrip. Looks within the <3% voltage loss. And 210 ampacity limit in the chart looks good.

I bought a fuse block with 125A fuse. Looks like that needs to be upgraded to 150A.

Assuming its a high frequency inverter its perfect.
If its a low frequency inverter then its
ac_watts * 1.5 low frequency fudge / low voltage cutoff.

 

[chrisski]

The one thing I question is I used the low voltage cutoff instead of 10.5 instead of 12 volts to determine fuse size. My non-adjustable low voltage cutoff is 10.5 by my manual. This will come out to needing a higher amperage fuse at this lower voltage cutoff.

Using a lower amperage fuse is safer, but if the inverter Dc voltage does dip to 10.5, it may cause that lower amperage fuse to blow.

I planned using the low voltage cutoff , but after the system was built, I did not dip below 12, so pushing the inverter near its max limit and it not dipping, I attribute that to a right sized battery bank and thick 4/0 wire which help keeps the volts up.

 

[John Frum]

The one thing I question is I used the low voltage cutoff instead of 10.5 instead of 12 volts to determine fuse size. My non-adjustable low voltage cutoff is 10.5 by my manual. This will come out to needing a higher amperage fuse at this lower voltage cutoff.

You are correct.
A low cutoff of 10.5 volts will increase dc amps which will increase fuse amps.

 

[chrisski]

The more I think about it, may be good to start with 12 volts to get the smaller fuse size, but rate the wires at the amperage you expect to see at 10.5 for low voltage cutoff.

In my case, I planned a 200 amp fuse to have around a fused 1500 watt limit on my inverter, which is 2000 watts. When my voltage only dropped to 12 instead of 10.5, that meant the fuse limits the inverter to 2000 watts.

Of course the primary way I avoid output is not using two high wattage items at once, but others in my RV don’t realize how much watts some items pull, like a crock pot that pulled 1100 watts.

 

[Dzl]

The one thing I question is I used the low voltage cutoff instead of 10.5 instead of 12 volts to determine fuse size. My non-adjustable low voltage cutoff is 10.5 by my manual. This will come out to needing a higher amperage fuse at this lower voltage cutoff.

Using a lower amperage fuse is safer, but if the inverter Dc voltage does dip to 10.5, it may cause that lower amperage fuse to blow.

I planned using the low voltage cutoff , but after the system was built, I did not dip below 12, so pushing the inverter near its max limit and it not dipping, I attribute that to a right sized battery bank and thick 4/0 wire which help keeps the volts up.

Yeah, 12v and 0.85 (in my eyes) are just reasonable example values in the equation. More generally:

[Output Power] ÷ [Conversion Efficiency] ÷ [Low Voltage Cutoff] = [~Max Input Current]

So depending on the inverter specs, conversion efficiency and low voltage efficiency could be something other than 12v and 85%, but those numbers are a decent ballpark for many situations.

 

[JAndle]

4x wire combiner aside, higher surface area allows more heat dissipation and short wires see the PCB as a heat sink at both ends, so ... maybe...? I tend to overdesign, so I am cringing, but maybe it's OK. I assume the ink on the UL/CE certs is dry?