Electricary Calcs

Discussion in 'Motorhome Chat' started by Brakers, May 1, 2016.

  1. Brakers

    Brakers Funster Life Member

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    Hi Fellow Frat Clubbers

    As some of you might know I have a Autotrail Cheyenne 840D and in previous posts have been discussing various problems I though I had with my battery system. Anyway going on from that Techno commented on my wiring to my leisure batteries was to use his words Sh*t, and to be honnest I agree.

    Apparently Autotrail in their infinite wisdom wired up the two leisure batteries with separate feeds of 4mm wire to each of the batteries.
    I have connected the batteries together using 25mm multi-core cable.
    I am going to have an 1800wat inverter fitted, also some other work done, I was hoping to upgrade the leads from my batteries to the EC325 to a more robust wire but have been told by Southdown motorhomes who will be carrying out the work this cannot be done because of the connections to the EC325. What a bummer.
    Anyway my question is to all you techy people out there does anyone know what current can safely be put through these 4mm wires. I’ve tried to check on the internet but most sites refer to house electrics.

    Many Thanks Tony
     
  2. John Laidler

    John Laidler Funster

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    You certainly won't run your 1800W inverter through such tiny wires. This page has a calculator which shows the losses through the wires. It suggests losses of around 2 or 3 percent are acceptable but the length of the cable is also a factor. For example a 4mm cable just 2m long is good for around 25 amps but if 4m long it is down to just around 12 amps. An 1800W inverter will need a minimum of 150 amps.

    However, your inverter would be wired with its own leads to the battery, ideally through an isolating switch and be sited as close to the batteries as possible.

    http://www.12voltplanet.co.uk/cable-sizing-selection.html
     
    Last edited: May 1, 2016
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  3. Brakers

    Brakers Funster Life Member

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    I don't expect to run the inverter to it's full capacity, 25amps will more than likely be over the top of what I want the inverter to run, am I correct in saying to run the inverter at it's full load then I would need wire to withstand 150amps, is that correct or am I way off?
    Many Thanks Tony
     
  4. John Laidler

    John Laidler Funster

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    I edited my post while you were posting. The inverter should have its own wiring capable of coping with the maximum load and be protected by fuses. Anything less would be a fire risk.

    I suggest finding someone who can install it correctly otherwise it could be dangerous given the currents involved and heat generated if something is wrong.
     
  5. Brakers

    Brakers Funster Life Member

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    Is there a way round this, I'm no good with electrics so have to rely on other people. Can the inverter be wired straight from the batteries bypassing the EC325 with largher wires?
    Many Thanks Tony
     
  6. Brakers

    Brakers Funster Life Member

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    Thats the trouble with typeing someone posts while you are typing
     
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  7. Lenny HB

    Lenny HB Funster

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    This site has a lot of usefuf info and a volt drop calculator.
    A 4mm cable for the main feed I would class as not fit for purpose, needs to be a at least 10mm sq.,
     
  8. Brakers

    Brakers Funster Life Member

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    Yes your right, this is me getting things wrong again, Southdown Motorhomes said they will fit the inverter, I just asked them if the wireing to the batteries could be upgraded and they said as my previous post, they did not say they would wire the inverter through those wires, that was me assuming the power would have to go through those wires. I sure they will put a separate feed to the inverter. But I will ask the question.
    Many Thanks Tony
     
  9. John Laidler

    John Laidler Funster

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    Yes. :) That's how it should be done - with fat boy wiring.
     
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  10. Brakers

    Brakers Funster Life Member

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    I don't think I understood what was going to be fitted, as DBK said the inverter will need to have it's own wireing and I'm sure Southdown will do that but as I said I will ask the question.
    Many Thanks Tony
     
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  11. Lenny HB

    Lenny HB Funster

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    That is the standard way of wiring an inverter, my 1000w inverter is wired direct to the batteries with 25mm sq., cable via a 100 amp fuse and a battery cut out switch.

    Yep always happens.:)
     
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  12. Lenny HB

    Lenny HB Funster

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    One more comment "Southdown", you must be well healed will cost you an arm and a leg.:D
     
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  13. Brakers

    Brakers Funster Life Member

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    When I think of it the vehicle has full LED lighting except for the 240v mains lighting so I expect for what the 4mm wireing has to handle it would be adequate.
    Many Thanks Tony
     
  14. Brakers

    Brakers Funster Life Member

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    Well I've got the arm but i'm not sure about the leg. LOL.
    If I were fit enough, and confident I would do it myself.
    Many Thanks Tony
     
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  15. Brakers

    Brakers Funster Life Member

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    Hi DBK am I correct in saying your the one who has an 1800T inverter fitted with a separate consumer unit fitted?
    Many Thanks Tony
     
  16. John Laidler

    John Laidler Funster

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    It will be fine for use as intended I think. The only time it will get a heavy load is if the batteries run down a bit and you start the engine, at which point the charging currents might be over twenty amps or thereabouts but probably not for long and the wires should be protected by suitable fuses in the worst case.
     
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  17. John Laidler

    John Laidler Funster

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    You've thrown me there, I guess it depends on the inverter model and how it is wired - probably best to consult an expert at this point as there are different ways of wiring in an inverter. The simplest just have sockets on you plug things into directly. Others make all the sockets in the MH live but doing that means you have to handle what happens when on EHU.
     
  18. Brakers

    Brakers Funster Life Member

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    Hi DBK Yes you are correct in saying about what happens when you are hooked up to the EHU that is why I'm haveing one that detects if the 240v is conntected and switches accordingly.
    Many Thanks Tony
     
  19. Lenny HB

    Lenny HB Funster

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    Sorry don't agree, with lights on, Truma heating fan running full blast, turn on a tap water & pump draws a few more amps you will get quite a volt drop, depending how long the cable runs are.
     
  20. Brakers

    Brakers Funster Life Member

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    I do understand what you mean, but I must assume the manufactures of this vehicle have done their calculations as there must be hundreds of these vehicles running round with the same wireing as me. I would like the wireing upgraded but Southdown said it can't be done. So I've got to rely on autotrail's calculations.
    Many Thanks Tony
     
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