whatsupdoc Posted March 30, 2013 Share Posted March 30, 2013 Hi - Now I've got the lesisure batteries sorted (and fused!) I want to run a 1500 watt (pure sine wave) inverter intended for an 800 watt microwave (and I know it's a colossal drain, but it will be for only occasional use of a few minutes - and supplemented by 250 watt solar panels). What size inline fuse should I use? My calculations suggest a current of about 75 amps (I may well be wrong!) - ignoring the initial surge. The leads that came with the inverter (from Sunshine Solar) are stamped 100 amp. Are they likely to be beefy enough? Link to comment Share on other sites More sharing options...
Brian Kirby Posted March 30, 2013 Share Posted March 30, 2013 Not my field, but "O" level physics says 1,500W/12V = 125A. So, on the face of that piece of simple arithmetic, 100A cable seems insufficient. There is also a question of cable length, which greatly influences voltage drop at these high amperages. If the distance to the inverter is more than a very few feet, you'll need heavier cables again. Also, I think you said you had fused your batteries at 50A each end of the + cables? Don't forget that with batteries in parallel half the inverter current (in theory) should be drawn from each battery, and half 125A would be 62.5A, so those 50A fuses seem a bit suspect. In practise, if one battery develops a fault, I think the inverter would probably try to pull the lot from the one good battery, meaning the same cable and fuse rating should probably be applied throughout. But, as I say, this is not my field. Link to comment Share on other sites More sharing options...
whatsupdoc Posted March 30, 2013 Author Share Posted March 30, 2013 Thanks, Brian. I based my calculations on the 800 watts of the microwave at 90? efficiency. The inverter will be adjacent to the batteries, so cable length shouldn't be an issue. It all seems a bit of a minefield ... hopefully somebody may help to clarify! Link to comment Share on other sites More sharing options...
sambukashot Posted March 30, 2013 Share Posted March 30, 2013 I would connect the sine wave invertor to the engine battery and run the microwave only with the engine running, check your alternator output first!You do not want to be running down the leisure batteries deep cycle or they will only last months not years... Link to comment Share on other sites More sharing options...
whatsupdoc Posted March 30, 2013 Author Share Posted March 30, 2013 Yes, I did consider that, but I read somewhere that if the vehicle electrics management detects what it thinks is an abnormal load it assumes there's been a malfunction and starts to shut things down ... Link to comment Share on other sites More sharing options...
sshortcircuit Posted March 30, 2013 Share Posted March 30, 2013 I have understood that to calculate the load by a microwave is to add 50% which for 800w would equal 1200w so about 100 amps going full tilt. It all depends on what type of fuse you have used as they are not very accurate and depending on what type can carry a current of 50 amps + for a considerable time. As Brian has said the inverter at full load will take about 62.5 amps which the 50amp fuse might be able to take for a short period. The fuse it there to protect the cable only and should be rated under the current capacity of the cable so under 100amps will be ok. The inverter has its own internal circuits to prevent overload I would not bring the solar panel into the equations as you will need absolutely brilliant sunshine to get 250w Link to comment Share on other sites More sharing options...
lennyhb Posted March 30, 2013 Share Posted March 30, 2013 Start up current on microwaves is between 50-100% more than their rated current except for invertor type which are about plus 30 % plus. You need to look at the fusing current graph for the type of fuse you are fitting for example quick blow fuse will often require twice the rated current for 10 seconds to blow but would still blow if overloaded by 20% for severel minutes. Each type of fuse has different current/rupture graph. A fuse rated about 30% above the continuous power should be OK providing it can handle the start up current without rupturing. The fuse must always be below the rating of the cable to prevent a fire risk. Link to comment Share on other sites More sharing options...
lennyhb Posted March 30, 2013 Share Posted March 30, 2013 Stupid ipad touch screen (lol) Link to comment Share on other sites More sharing options...
BGD Posted March 31, 2013 Share Posted March 31, 2013 Or just buy a 12 volt DC microwave rather than a mains operated one? Link to comment Share on other sites More sharing options...
lennyhb Posted March 31, 2013 Share Posted March 31, 2013 12volt microwaves are only mains ones with an invertor built in that then means you have to run very heavy cables to the microwave from the battery by-passing the control panel/charger, doesn't really solve any of the problems. Link to comment Share on other sites More sharing options...
Mel B Posted March 31, 2013 Share Posted March 31, 2013 Just out of interest, what sort of food are you thinking of using the microwave for? :-S Link to comment Share on other sites More sharing options...
Clive Posted March 31, 2013 Share Posted March 31, 2013 Alowing for the full output of the inverter and an efficiency of say 85% comes to 143 amperes. BUT modern posh inverters allow for the battery voltage falling off under heavy load and just take more amps. So I would want to use an air fuse of 150 A minimum. It very much depends on the CSA of the cables between the inverter and the batteries as this is what the fuse is protecting, not the inverter. C. Link to comment Share on other sites More sharing options...
lennyhb Posted March 31, 2013 Share Posted March 31, 2013 Going by Clive's recommendation cables needs to be at least 25mm sq (170 amp) that will be fun running them and terminating that size cable has an OD of 9.8mm. Link to comment Share on other sites More sharing options...
whatsupdoc Posted April 1, 2013 Author Share Posted April 1, 2013 Many thanks to everyone for the constructive advice. It seems clear that the 100 amp leads are not up to the job, so I'm getting some beefy ones - for both the inverter and battery pairing, and once I've checked further I'll get some decent size fuses. Link to comment Share on other sites More sharing options...
sshortcircuit Posted April 1, 2013 Share Posted April 1, 2013 You should be asking yourself why would the inverter manufacturer be supplying cables which "you" do not think "are up to the job." What battery source do you have to supply the inverter? An inverter at full tilt will rapidly deplenish the batteries and whilst you may initially get a large current this will fall considerably unless you have a large bank of batteries Link to comment Share on other sites More sharing options...
whatsupdoc Posted April 2, 2013 Author Share Posted April 2, 2013 Well ... I took the advice of people on the forum - and they know a lot more about this than me. Link to comment Share on other sites More sharing options...
w1ntersun Posted April 2, 2013 Share Posted April 2, 2013 Hi, I read on one of the battery sites ( can,t remember which) but to run a 1500 watt invertor they reccomend a min of 4 100 amp batteries to avoid battery damage. Richard Link to comment Share on other sites More sharing options...
Keithl Posted April 2, 2013 Share Posted April 2, 2013 w1ntersun - 2013-04-02 9:25 AM Hi, I read on one of the battery sites ( can,t remember which) but to run a 1500 watt invertor they reccomend a min of 4 100 amp batteries to avoid battery damage. Richard Elecsol!!! Link Keith. Link to comment Share on other sites More sharing options...
sshortcircuit Posted April 2, 2013 Share Posted April 2, 2013 w1ntersun - 2013-04-02 9:25 AM Hi, I read on one of the battery sites ( can,t remember which) but to run a 1500 watt invertor they reccomend a min of 4 100 amp batteries to avoid battery damage. Richard From a battery manufacturer ???? It all depends on how long you will run the inverter and the type of microwave. My micro works in pulses when it draws a good bit current and then idles for a time which means it is not a continual draw from the battery/s. In addition it is normal for the inverter to shut down when voltage drops to far so should not flatten battery. The distance the inverter is from the battery will also decide the size of cable required. Greater distance=thicker cable Link to comment Share on other sites More sharing options...
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