Inverter -- fuse size

[sailorman]

Hi all
in the process of fitting a 1200watt sine wave inverter .
would appreciate advise on fuse size.
i have bought a 175 amp fuse but now think its a little on the high side /forums/images/graemlins/confused.gif
cheers
roger

 

[boatmike]

I would suggest a calculation for an inverter to take account of 80% efficiency would be 1200w/12 = 100amp/0.8= 125 amps. I would fit a 150 amp fuse to allow for 20% overload. Most inverters are protected by overload cut out devices anyway so this is not going to cause damage to the inverter.

 

[MedMan]

It sounds high to me. 1200 watts at 12 volts will draw 100 amps plus, say 10% for inefficiency gives a maximum of 110 amps.

Two thoughts. Is it rated for a continuous load of 1200 watts or is that the maximum draw? Secondly, if it's not a silly question, what do the installation instructions say?

 

[sailorman]

thanx chaps that was quick as usuall.
1/ no instructions as s/hand.
2/ max load microwave @ 800watt output
3/ the m/wave works fine but need to fuse supply cable.
4/ have fitted isolator switches to both pos & neg cables

 

[TrueBlue]

I got quite frightened at the cost - but more importantly at the size and possible heat generation of a fuse and assembly (unless fuse is bolted in), so I fitted a 100A. circuit breaker from Maplins - as supplied to boy racers with their 500+ watt amplifiers. Perfic, drops out instanta.

ps: my systems are 24V.

 

[sailorman]

fuse in a pukka fuse holder all bolted-up ( blue sea -manufactured in usa )

 

[pvb]

Blue Sea...

No problems using the Blue Sea fuse holder (which probably has a Littelfuse mega fuse in it). These are excellent products. Your 175A fuse will be OK for this purpose - you're fusing the cables, after all. Guess you'll turn off the isolator switches when inverter isn't needed. The 800W you quote for the microwave is probably its output power; it'll take more like 1200W when running.

 

[sailorman]

Re: Blue Sea...

thanks for the reply
your right on all counts.
the 175 amp fuse is rated @ 130% for 10 minutes which seems a long time if the battery is boiling due to a malfunction /forums/images/graemlins/shocked.gif

 

[pvb]

If there\'s a short...

If there's a short, the fuse will blow instantly. I have Mega Fuses on my boat, and the only time I've had one blow was when I accidentally caused a short when working on the alternator - the 200A fuse blew instantly! Don't think a malfunction in the inverter is likely to be a problem.

 

[sailorman]

Re: If there\'s a short...

really concerned that if not onboard @ inverter isotated what would happen if a "short" occoured( 10 mins is a veeeeeery long time with 330 Amp hr batteries)

 

[pvb]

Don\'t worry...

If there's a short, the fuse will blow! For the cost of a fuse, you can easily test it and set your mind at ease.

 

[ashanta]

Mike,

I really appreciate your explaination in easy language. Please tell me what 0.8 means in yor calculation. i understood everything up to then. (i'm obviously not an electricion/electronics guy)

Many thanks.

Peter.

 

[ashanta]

Just realised!

I'm a dozy sod! You were taking 80% in your calculation, I do apolologise to be appearing dim witted,

Thanks.

Peter.

 

[sailorman]

thanks all
Im a whizz with PCs ( can balls them up quicker than most )
its the high amperage short circuit that was the concern.

 

[Anonymous]

It's actually quite an involved subject and the correct choice depends on the design of the inverter. If it was mine, I would ask the inverter manufacturer to tell me what fuse or breaker to use.

 

[boatmike]

0.8 is an arbitary figure to allow for 80% efficiency of the conversion from DC to AC. This is the figure I normally use for square wave inverters. Full sine wave or simulated sine wave inverters can be up to 95% efficient but I would still use 80% to calculate fuse size. As someone else said you are really protecting the cables, which I am sure you realise need to be akin to main battery cables in size. So the fuse should be as near to the battery terminals as possible. There is nothing wrong with using your 175amp fuse therefore. It will still blow if the cables short or the inverter blows internally, but I would favour dropping it to 150amp as these are commonly available in mega fuse ratings. For comparison my inverter is 1500watt and the stated efficiency is 90% and I have a 175amp fuse fitted. Doing the same calculation on mine the result is 1500/12= 125amps
125/0.8= 156 x 1.2= 185amps. Rather than fit a 200 I have dropped it to 175 which is again a commonly available fuse which actually gives an overload of only 12%. But in my case the tested efficiency of the inverter is known to be 90% rather than the calculated 80% so this is fine
Hope this is clear....

 

[sailorman]

thanks will change to 150 amp