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jibjabhater 
Member - Posts: 12
Member spacespace
Joined: July 10, 2008
Location: Alaska, United States
Posted: September 21, 2011 at 6:11 PM / IP Logged  

I was wondering if someone could steer me in the right direction? I am installing a 2400 watt peak inverter on my diesel truck. I am going to use it to power a small 1000 to 1200 watt 110 volt heater as an auxillary heater on these cold alaska mornings. The directions that came with the inverter said I need to have a 500a fuse/breaker. From the little bit of research I have seen on here, I believe it should be a 10% rating of the inverter. Meaning I should only need 240a fuse/breaker. My questions are:

What size fuse will I need?

If I am mountaing the inverter in the cab of my truck approximately 6 ft from the battery what gauge will I need? It has two terminals on the inverter for power and ground?

How can I hook this up using a relay to my truck accessory so I can start it with my auto start and have it kick on?

Thanks in advance for the help.

oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 21, 2011 at 10:59 PM / IP Logged  
The 10% rule is really that input current is approximately 10% of output power - ie, 2400W/10V = 240A. (From P=VI => I=P/V where 10V is used as a simple approximation to 12V, 13.8V, 14.4V etc - especially when the voltage can dip to 10V (during cranking) else the device has an efficiency of ~80% (as may be typical for amplifiers, or inverters at "optimum" load).
But then add 10% - 30% for fuse headroom - ie, a fuse should be be sized as to NOT run at more than ~70% of the typical load.
Plus add fuse capability to withstand inrush currents.    
And in the case of inverters, allow for peak.
But your 2400W peak is what we call a 1200W inverter (peak typically being double the RMS rating, and "peak" being a vague or ambiguous description which is unacceptable as a product description here - only RMS counts.)
Hence I would argue a 120A input, hence a 150A fuse or larger.
The fuses probably have a similar "peak" profile to the inverter so they should cancel. (IE - a 150A fuse might withstand 300A for a few seconds.)
But finally, you current depends on your load which is (say) 1200W, but that matches the inverter rating. If it were 600W, you could halve the input fuse size. (And the cabling, but I'd rather cable for the full rating and use a smaller fuse which can be up-sized later if needed.
Also you need less copper if the inverter can be mounted closer to the battery/alternator, though engine-bay inverters are not a good idea! Plus since it's for heating, put the inverter's 20%-30% inefficiency in the cabin.
I'll let others suggest wire gauges etc.
This is a very inefficient (and expensive) form of heating, but that's another issue.
jibjabhater 
Member - Posts: 12
Member spacespace
Joined: July 10, 2008
Location: Alaska, United States
Posted: October 13, 2011 at 11:30 PM / IP Logged  

So taking the advise, I think I am going to go with a 200-250 amp circuit breaker as opposed to a fuse unless it gets to pricy.

My second question is, this will be a switched power source ( when truck is running) what size relay should I use. I have checked some of the stats and I can get a reasonably priced 300 amp relay for around 25 bucks. My problem is they state it is only rated 150 amps continous and 300 surge. I saw the next step up was a 500 amp. Should I follow the go big or go home philosphy? Thanks for any help you can send my way.

oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 13, 2011 at 11:41 PM / IP Logged  
As per your other thread:
    1100W => 110A so 150A is fine.
    With a 150A fuse/breaker in case it overloads.
If you ran the inverter at its full 2200W, you'd need a 250A relay etc.
But remember, the fuse/breaker should match or be smaller than the relay capacity (and wire capacity etc). The fuse/breaker should be the weakest (smallest) link.
js305 
Member - Posts: 26
Member spacespace
Joined: April 29, 2007
Location: United States
Posted: October 29, 2011 at 9:47 PM / IP Logged  
I sell inverters (won't mention the brand, but they are all similar). The only reason for the 500 amp fuse in the main line going from the battery to the inverter is in case of a catastrophic failure in the inverter or a dead short in the battery wiring to the inverter. You can use a smaller fuse and be fine. As I said, this big fuse will keep your truck from catching fire if there is a short somewhere before it gets to the inverter. Most inverters have fuses on them somewhere, as do the ones I sell. They are on the output side and usually around 15 amps or so. You do realize you will have to keep your engine running or risk running your battery down in a very short time frame. Even with multiple batteries and isolators you get really short times with these big inverters.
As far as what gauge wire to use, just use the biggest you can connect to the terminals on the inverter. There should be a chart of some sort with the instructions. Always error in the favor of bigger wire when dealing with DC voltage.
Joe in Texas
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 29, 2011 at 10:18 PM / IP Logged  
I agree with JS.
Rule of Thumb is that input current (Amps) = output power divided by 10.
IE - 1200W out = 1200/10 = 120A.
As a comparison, highbeam headlights are (say) 2x65W (130W = 13A) maybe with up another 2x55W low beams (110W = 11A) and dash etc (50W = 5A) - hence maybe 25-30A total. The 1200W inverter is over 5x that draw.
Conversely 120A is about half a common starter motor (Jap 4-cylinder).
That should give an idea of how long a battery will last.
And few alternators at idle will supply 120A.
Hence why isn't vehicle heating isn't done electrically when engine heat is available.
FYI - It seems that in the USA, inverters are sold as "peak" output.
Most other countries would not get away with that, but inverter peak is simply 2x RMS, hence your "2400W peak" inverters are "1200W inverters" - ie, 1200W RMS.
FYI-2 - That double RMS should hold true for inverters until they start pulling the wank that audio manufacturers do - ie, where peak output is no longer music output (2x RMS), but some random unqualified figure like "for 2% of the cycle with a nordic wind under a full moon" etc.

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