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Thread: Wiring, fuse, amp, relay questions

  1. #1
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    Wiring, fuse, amp, relay questions

    I'm trying to figure out the correct fuses for powering a 400 watt inverter, (800 peak), and a 150 watt PSU. 950 watts divided by 12 volts would be 79 amps, right?

    I was told on the main power lines using 4 gauge wire, I can put 125 amp fuse, right?

    In the pic below fuses 1 and 2 I have a 125 amp fuse, right?

    Fuse 3 is before the 400 watt inverter, which is pulling 66 amps, so an 80 amp fuse, right?

    Fuse 4 is before the 150 watt PSU, is pulling 12.5 amps, so a 15 amp fuse, right?

    Fuse 5 is before the computer, pulling 150 watts I guess, so a 15 amp fuse, right?

    Fuse 6 is before the playstation2, it pulls 76 watts, so an 80 amp fuse right?



    Next question is about wiring the relay. Would this be the correct way?



    Two more questions, the switch I want is for 12 volts, and says 20 amps. Is that ok with a 200 amp relay? What size wire would I run from switch to relay?
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  2. #2
    Maximum Bitrate Change's Avatar
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    Good god man, your math is horrible! Amperage is watts/volts.

    76/12=6.33A for the PS2, use a 7.5A fuse.
    400/12=33.33A for the inverter, probably use a 30 or 35A fuse, since you're probably not going to fully load the inverter.
    You'll probably want an 80-100A fuse on the main power line.

    Also, I'm having trouble reading your diagram and figuring out what is what.

  3. #3
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    Ok, I gave the 400 watt inverter a 66 amp fuse because I was told to go by the peak wattage of the inverter, which is 800. And you're right, the fuse for the PS2 should be 6.33 amps, I forgot to divide that one, thanks for the correction. Here's the updated pic:



    So what about the way the relay is wired, is that right? What size wire? And is a switch rated at 20 amps ok?

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  4. #4
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    1) I would suggest fusing the inverter according to its continuous rating and not its peak. This is because the peak rating is only meant for very short durations. If you run it at its peak output for long periods you risk damaging it. Therefore you need a fuse to blow if this happens. A fuse you've used for its continuous rating wouldn't blow if you only use its peak power for very short periods.

    2) I wouldn't wire up the PS2 like that if I were you. You risk damaging some of the equipment/the car/yourself. The PS2 is a mains powered device so you should just plug it straight into the inverter and just that. Don't attempt to ground it to the vehicle or add extra fuses. If it's mains cable shorts for some reason, it will overload the inverter, hence the reason for 1).

    3) The PSU should just plug straight into the motherboard of the computer, and so again you don't need to ground the computer to the vehicle...the PSU already takes care of that, and for the same reason you don't need the fuse between the PSU and the computer because idealy you will build them both into the same case.

    4) The coils of relays typically only draw less than 1 amp so your switch is easily sufficient. I would use cable rated at 5 amps with a 5 amp fuse. I would also connect the switch to the accessory line and not directly to the battery. If the switch is left on accidently, the the whole purpose of the isolator is lost and both batteries will probably flatten (remember the relay would be drawing current).

    Rob.

  5. #5
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    I really appreciate your help Rob, it's all becoming clear to me slowly but surely. Here's a pic of your advice. I think I almost have it! Is the pic of the relay wiring correct? BTW, my son's name is Rob, I'm building this for him.

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  6. #6
    Variable Bitrate numbers's Avatar
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    You should just check the inverters manual for a suggested fuse. I would not use a 40 amp fuse on a 400 watt inverter. The fuse is not there to protect the inverter. The inverter has it's own fuse(s) for that. (in fact, the fuses on the inverter itself may even be more than 40 amps) The fuse is there to protect the wiring in case of shorts and whatnot. The inverter will protect itself from overloading - not the fuse that you put on it.

    Once you have the relay in hand you'll be able to read the specs for it. It will tell you how much current is needed to operate it, and using that you can decide exactly what your wiring and switch requirements will be.

    Don't take our word for anything. Never underestimate reading the manual.

  7. #7
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    Thanks Numbers, I've been reading for two months, and will read some more when I get the parts in my hand. I just don't feel comfortable ordering anything until I have a solid understanding of what goes where. Never having dealt with car electricity, it's hard to piece all of this info together when it's scattered all over the place. Maybe I should have posted this in the newbie section so some guardian angel could take me by the hand and assure me I won't blow anything up. lol
    A pic is worth a thousand words.

  8. #8
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    Quote Originally Posted by Vee24
    I really appreciate your help Rob, it's all becoming clear to me slowly but surely. Here's a pic of your advice. I think I almost have it! Is the pic of the relay wiring correct? BTW, my son's name is Rob, I'm building this for him.
    Is there a new pic? If you mean the original, then it will work, but it is not ideal, because if you forget to turn the switch off then when you take the key out of the ignition, the batteries will stay connected which will mean they both could go flat. It would be better to connect the switch to the accessory line.

    Rob.

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