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momentary surge and inverter/cable/fuse sizing?

J

JAS

Solar Enthusiast
Joined
Jan 16, 2020
Messages
512
Hey all,

I'm setting up a 12v system to run a few circuits in my home (Internet Router, WiFi, desktop computer). However, I also want to run my refrigerator and separate freezer off of it. I've already hooked a kill-a-watt up to both the fridge and freezer and let it run for several days. The normal draw of each is only around 200 watts (when running). However, the momentary surge hits up around 1000 watts for each.

So I'm thinking... There is the possibility that both the fridge and the freezer can kick on at the same exact time. This draw, plus the existing 200 watts of electronics, can put things momentarily up around 2200 - 2500 watts.

Most Inverters/Chargers or AIO units at the 1000W range spec around a 10-15 Amp AC input breaker.

Question: Is it OK to size the inverter, cabling, and fuses to the normal ~600 watt range? This is assuming that any fuses and/or AC circuit breakers spec's at that range can handle a momentary surge above that rating? IE: A 10A AC Circuit Breaker can handle a momentary surge of 12-15 Amps. I'm assuming the same can be said for the DC side...

Thanks,

Jeff
 
JAS said:
it OK to size the inverter, cabling, and fuses to the normal ~600 watt range?
Click to expand...
Not as described: NO
(What is inverter rating? 600 or 1000?)

I oversize all inverter cables from battery, so for 1000W inverter I’d use cable that supports 2000W and fuse for 1500W load. This accommodates possible future expansion but more importantly any ‘surge’ from inverter load startups.
 
12VoltInstalls said:
(What is inverter rating? 600 or 1000?)
Click to expand...
Ahh, good catch... My "normal load" will be no more than 600W. But, the Inverter size I'm looking at is 1000W. (Specifically, I'm looking at the MPP Solar 1012LV-MS. But, also was debating the Samlex EVO 1212..). So I should have asked if it's OK to size to the 1000W?

But, you are suggesting I size to 2000W? What size wire would that be between the Inverter and Battery? Or should I say... can I get away with 2/0?
 
Look up the ABYC 12V cable chart.
I could tell you but then you won’t know how to figure it next time either

But yes, if it were me I’d cable it for 2000W
You can get away with that sized cable fine
 
JAS said:
Ahh, good catch... My "normal load" will be no more than 600W. But, the Inverter size I'm looking at is 1000W. (Specifically, I'm looking at the MPP Solar 1012LV-MS. But, also was debating the Samlex EVO 1212..). So I should have asked if it's OK to size to the 1000W?

But, you are suggesting I size to 2000W? What size wire would that be between the Inverter and Battery? Or should I say... can I get away with 2/0?
Click to expand...

The Samlex evo and the MPP 1012LV are about as different as 2 devices of similar rating could be.

The Samlex is a low frequency inverter with significant surge capacity.
The MPP is cheap and cheerful to rhyme with scrap.

But here is the math that will cover both.
1200 ac watts / .85 conversion factor / 12 volts low cutoff = 117.647058824 service amps.
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps.
The wire and fuse need to be sized for fault amps.
The bms needs to be sized to at least service amps.

Feel free to ask follow up questions if anything isn't clear.
 
OK, I got ya...

Bummer though, I just read elsewhere that the MPP will only allow a 2 gauge wire to fit through the hole in the case :rolleyes:

Can I get away with running about 6" of 2 gauge to get it out of the case and then connect it to 2/0 to go the other 3' ?
 
JAS said:
OK, I got ya...

Bummer though, I just read elsewhere that the MPP will only allow a 2 gauge wire to fit through the hole in the case :rolleyes:

Can I get away with running about 6" of 2 gauge to get it out of the case and then connect it to 2/0 to go the other 3' ?
Click to expand...
How long is the run between the inverter/charger and the battery?
150 amps can run on as little as 4 awg wire which is smaller the 2 awg.
2 awg would be my suggestion but we do need to know the round trip length of the circuit in case voltage drop is the determining factor.
BTW 2/0 awg is ~twice the circumference of 2 awg.

I encourage you to experiment with this calculator.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.
baymarinesupply.com baymarinesupply.com
 
smoothJoey said:
The Samlex evo and the MPP 1012LV are about as different as 2 devices of similar rating could be.
Click to expand...
Yeah, I get that.

smoothJoey said:
The MPP is cheap and cheerful to rhyme with scrap.
Click to expand...
Aren't you being a bit harsh here ;) Seems to be many people happily using these for years...

The EVO prices have been creeping up over the last few months (plus I need the $120 monitor screen to go with it and an additional $120+ SCC) and the MPP Solar unit is now back in stock and half the price :)

smoothJoey said:
But here is the math that will cover both.
1200 ac watts / .85 conversion factor / 12 volts low cutoff = 117.647058824 service amps.
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps.
The wire and fuse need to be sized for fault amps.
The bms needs to be sized to at least service amps.
Click to expand...
So based on your math, (If I'm understanding correctly) you're saying to size based on the rated load (and not necessarily on the momentary surge load), correct?
 
JAS said:
Aren't you being a bit harsh here ;) Seems to be many people happily using these for years...
Click to expand...
I call it like I see it.
JAS said:
So based on your math, (If I'm understanding correctly) you're saying to size based on the rated load (and not necessarily on the momentary surge load), correct?
Click to expand...
Correct.
Typically the fuses and breakers we use they on 100% current at ~5 minutes.
So unless the inverter has significant surge out to 5 minutes then it doesn't effect the math.
High frequency inverters usually have 2x surge for ~20 milli seconds.
That is 1 cycle of ac power and is only relevant to the marketing department.
 
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JAS said:
:) You did. And that is where I was leaning when I started the thread... but, 12voltinstalls got me 2nd guessing myself
Click to expand...
Sorry. That is just my thing- overgaging low voltage.
In a way, the math he uses leans toward arriving at a decent headroom. It just doesn’t have the maybe-not-likely future bigger inverter room.
smoothJoey said:
150 amps can run on as little as 4 awg wire which is smaller the 2 awg.
2 awg would be my suggestion but we do need to know the round trip length of the circuit in case voltage drop is the determining factor.
BTW 2/0 awg is ~twice the circumference of 2 awg
Click to expand...
But here is the math that will cover both.
smoothJoey said:
1200 ac watts / .85 conversion factor / 12 volts low cutoff = 117.647058824 service amps.
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps.
Click to expand...
This is the ABYC chart:
It’s too bad it won’t provision for 2/0 but on your shorter length the benefit is limited. 2ga on the chart works and it also fits.

I’m sorry to have confused you.
 
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