| Author | Message | ||
| doug yes (Dougg)
Registered Member Username: Dougg Post Number: 127 Registered: 1-2007 Posted From: 174.131.41.29 Rating: N/A |
We would like to run 1 roof air conditioner off the alternators.(Dual Bosch 150 amp each) There is no big bus air. The roof unit pulls 14.5 amps @ 115 volts. 1.Would a 12volt, 3000 watt pure sine inverter cover the start load? 2.What size breaker should we use for the inverter? 3. Would a 24 volt inverter be worth the extra $300? Thanks all. | ||
| Terry (T3rry)
Registered Member Username: T3rry Post Number: 29 Registered: 2-2010 Posted From: 68.147.23.190  Rating: N/A |
14.5a*115v=1667w/12v=138 amps on 12v | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1062 Registered: 1-2003 Posted From: 72.171.0.149  Rating: |
Doug, Remind us what bus you have. If the alternators are 12-volt, you should use a 12-volt inverter, and if they are 24-volt, use a 24-volt inverter. It will cost more than $300 to convert between voltages on the DC side, and you'll lose 10% of your power. Roof airs generally draw upwards of 20 amps when starting. That's 2,400 watts -- most 3kW inverters will start and run a single roof air. The size breaker or fuse you would use for the inverter depends on the size wires you run to it as much as how much current it can draw. The current depends om 12 vs. 24 volts; the latter will be half the current for the same wattage. A typical 3kW 12-volt inverter has a surge rating of upwards of 5kW and is usually fused at 400 or 450 amps. I doubt you will want a DC breaker that size. If you are talking about the breaker on the AC side, that is dictated by the roof air manufacturer. IIRC, most call for 20-amp HACR breakers. -Sean http://OurOdyssey.BlogSpot.com | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 974 Registered: 8-2006 Posted From: 99.37.31.150 Rating: N/A |
Terry, I feel there is an error in your reasoning, and an omission of efficiency loss as well. Modern US appliances are rated at 120 Volts, which would make the roof air draw 1745 Amps instead of 1667. This makes it easy to compute 12 to 120 Volts, as you just multiply by 10. So, a roof air that draws 14.5 Amps on 120 VAC, will draw 145 Amps on 12 VDC, IF the inverter were perfect. Inverters aren't perfect, so we will have to figure the actual amp draw on a warm day will be about 160, instead of 138. G | ||
| Tom Christman (Tchristman)
Registered Member Username: Tchristman Post Number: 206 Registered: 1-2006 Posted From: 66.218.33.156 Rating: N/A |
Please remember that 12vdc is only when the straight batteries are being used. If this is going to be used going down the road with the engine alternator, then amperage is figured with 14vdc. So the 14.5 amps @ 120vac is in fact 1740 watts, that translates into 124.3 amps at 14vdc when running. I know-splitting hairs. Good luck, TomC | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 981 Registered: 8-2006 Posted From: 99.37.31.150 Rating: N/A |
There was a reason I put the post in the way I did, and that reason was that I was providing information for a successful installation! A lot of us can do Ohm's law, but let's think about this. Within the last few posts is a request for help from someone who got a low voltage warning from his inverter immediately after turning it on. Ends up his wire from the batteries is way too small for the length of run. So, I ask: What will happen if you size your wire and battery fuse for the Amp draw at 11 volts, (when the inverter will still run just before it shuts down for true low battery while boondocking,) and you run your inverter and A/C while driving? The answer? It will work just fine. Now, you size your cable and fuse for 14.2 Volts, ignore inverter loss, and try to run your A/C while parked? When the batteries get down a ways, either the inverter shuts down for low voltage before the batteries are actually discharged, because of the loss in the small wire, or the 200 Amp fuse blows. Thats why I post conservative calculations. G |
