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Current carrying capacity

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  1. #1
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    Default Current carrying capacity

    I'm after a bit of info which I can't find in AS. I am going to test the condition of three sets of Lead Acid batteries (1000Ah each) for capacity and I am using a PL60 (Plasmatronics) regulator with a 30A DC output that will trip the load when the voltage drops below the set point. I am going to use a 1800w inverter (because that is what I have been able to borrow) with a 300w floodlamp as the load. I'll be controlling the discharge and charge (just on and off) remotely via the internet and a web relay.

    Now the question would 4mm 240v cable 1.8M long be capable of carrying 30A DC?

  2. #2
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    Default Current carrying capacity

    I have referred to this site before for automotive wiring.

    http://www.rbeelectronics.com/wtable.htm

  3. #3
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    AS3008 Table 3 (Two single Core Sheathed and Unsheathed V75 etc insulation gives the following ratings for 4mm copper/spaced/touching etc.)

    Spaced 40A, Spaced from surface 39A, Touching 31A, Non metallic enclosure in Air 30A. All the underground arrangements are O.K., the only "fail" is if surrounded by thermal insulation.

    Some derating of around 0.95 from these figures applies if mounted on cable trays with multiple circuits. Probably not a concern for you.

    As 4mm has a resistance of around 5 ohms per kilometre, I guess voltage drop over 1.8 netres also isn't going to be an issue.

    Have fun!

  4. #4
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    Quote Originally Posted by chalkyt View Post
    AS3008 Table 3 (Two single Core Sheathed and Unsheathed V75 etc insulation gives the following ratings for 4mm copper/spaced/touching etc.)

    Spaced 40A, Spaced from surface 39A, Touching 31A, Non metallic enclosure in Air 30A. All the underground arrangements are O.K., the only "fail" is if surrounded by thermal insulation.

    Some derating of around 0.95 from these figures applies if mounted on cable trays with multiple circuits. Probably not a concern for you.

    As 4mm has a resistance of around 5 ohms per kilometre, I guess voltage drop over 1.8 netres also isn't going to be an issue.

    Have fun!
    I know that as I have the standard but if you look at the front is says Alternating voltages so DC would be a different calculation.

  5. #5
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    Quote Originally Posted by Jcsmales View Post
    I have referred to this site before for automotive wiring.

    Wire Gauge Tables
    Thanks looks like it can do the job. Saves me going to buy some 6mm or horror of horrors automotive cable

  6. #6
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    Hi Bros
    Glad you got an answer. I am intrigued why the DC rating would be different to AC rating (AC is RMS which I understand to be the equivalent of DC). Miccomcables (Google) have tables that refer to AC or DC and give the same rating (incidently, they give 32A for 4mm cable). For my enlightenment, any techos out there got views on why AC and DC ratings should be different?

  7. #7
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    DC currents can get alot hotter than AC> Hence why cars arent wired in the same cable as houses.

    For 1.8m I'd just go buy a few meters of battery wire from an automotive store.

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    remember in household solar installations, most people are using 4mm (or larger) DC cabling from the array to the inverter. In a regular 1-2kW system, thats only gunna be about 5.6A........If you ran 2 parallel lots in a 3kw or larger, you would be 11.2 or so. Using 4mm or larger. They arent using a 1mm or 1.5mm wire for these currents.

  9. #9
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    Default Current carrying capacity

    Battery cable would be absolute overkill for a 1.8m run. Automotive 6mm cable (which really is only about 4mm actual wire diameter) is rated at 50A.
    4mm household wiring will be fine as far as I'm aware.

  10. #10
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    would be overkill or better suited?

    Not saying you are wrong or right, but especially giving the 'home handyman' electrician advice that may or may not get him in trouble isnt the best idea.

  11. #11
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    Default Current carrying capacity

    300w light, 12v DC works out to be a 25A draw. If a 4mm wire can carry 50A then I would say yes, battery lead which is for example, 8mm, which is capable of carrying 40A over 7 meters, is overkill.

  12. #12
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    Quote Originally Posted by Brian7886 View Post
    would be overkill or better suited?

    Not saying you are wrong or right, but especially giving the 'home handyman' electrician advice that may or may not get him in trouble isnt the best idea.
    I'm not a home handyman

  13. #13
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    Quote Originally Posted by chalkyt View Post
    Hi Bros
    Glad you got an answer. I am intrigued why the DC rating would be different to AC rating (AC is RMS which I understand to be the equivalent of DC). Miccomcables (Google) have tables that refer to AC or DC and give the same rating (incidently, they give 32A for 4mm cable). For my enlightenment, any techos out there got views on why AC and DC ratings should be different?
    I just thought I would ask as DC isn't a sine wave so it should run hotter for the same current. I can get plenty of short lengths of 4mm ( I know i can parallel them) but I was just wanting to use one cable for the DC side of the inverter the AC side is just an extension lead.

    I didn't think I would generate this sort of interest as it is usually me answering posts not the other way around.

  14. #14
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    Quote Originally Posted by Jcsmales View Post
    300w light, 12v DC works out to be a 25A draw. If a 4mm wire can carry 50A then I would say yes, battery lead which is for example, 8mm, which is capable of carrying 40A over 7 meters, is overkill.
    Remember I am using an inverter and the 300w light is on the 240v side, I'm just doing it on the cheap by using borrowed gear for a one off job.

  15. #15
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    But his post is out there for all to see.

    Cant just throw out info with a 'she'll be right' attitude for all to see, especially if some drongo takes the advice to heart and goes and lights himself up.

    Back up any answers with rulebook/fact rather than comparisons.

  16. #16
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    HI Bros

    300W load plus say 10% max effeiciency loss in the inverter =330W /12V =27.5A


    The 4mm2 has a current rating around 35A & has a resistance of 4.13Ohms per Kmetre ,
    You will have about 3.7W of heatghenerated in each mettrtre of cable4
    IT will be ok !!
    Watts lost =IxI xR
    or IxE[ voltage drop]no matter what the current being ac or Dc

    do not worry about being DC or be confused about the rediculous Auto cable method of rating!!!


    PeterQ

  17. #17
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    Quote Originally Posted by Brian7886 View Post
    But his post is out there for all to see.

    Cant just throw out info with a 'she'll be right' attitude for all to see, especially if some drongo takes the advice to heart and goes and lights himself up.

    Back up any answers with rulebook/fact rather than comparisons.

    Hi Brian
    First, Bros is talking about 12V not 240V wiring
    But ,If you are so concerned about diyers why do YOU give ANY detailed info ??
    Have a look at some of YOUR posts
    PeterQ

  18. #18
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    Quote Originally Posted by Brian7886 View Post
    remember in household solar installations, most people are using 4mm (or larger) DC cabling from the array to the inverter. In a regular 1-2kW system, thats only gunna be about 5.6A........If you ran 2 parallel lots in a 3kw or larger, you would be 11.2 or so. Using 4mm or larger. They arent using a 1mm or 1.5mm wire for these currents.
    HI
    And the reason IS????
    PeterQ

  19. #19
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    AC or DC shouldn't make any difference to the current carrying capacity of copper wire in this case. 10 amps AC as measured by a meter has the same heating affect in a resistance ( in this case the wire) as 10 amps DC. This is the basis of the term RMS when measuring sinewave voltage and current, even a cheap non RMS meters which usually use average voltage or current will be very close to the RMS value when measuring sinewave AC.

  20. #20
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    RMS is the equivalent of DC, not 'close' but by definition. The clever poster who observed that the current is a function of the LOAD, not the CAPACITY gets full points. The clever poster who added efficiency also.
    For mine, 4 sq mm can handle 38A and will drop 0.67V over 2m. There are many posts in this string (by both old and new members) that, in hindsight, shouldn't be here. The best we can do in future is NOT POST anything inflammatory or critical - eyes on the prize, guys - answer the OP's question.

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