Cable Selection & Installation Tips


  • VOLTAGE DROP & THE CROSS SECTIONAL AREA
  • Good Battery Cabling

    The DC cabling between the battery and the inverter needs to be of sufficient cross sectional area (the total cross section of all the individual strands) in order to carry the current drawn by the inverter efficiently. As a general rule the thicker the better works but such issues as the number of copper strands within the cable and the purity of the copper itself can also affect the perfomance. A cable with a cross section suitable for the current demands of the application (as shown in the table) will minimise the input resistance, high quality copper further enhances the running efficiency. With the growing popularity of powerful in-car audio systems, high quality battery supply cables are readily available.

    A fairly common mistake is that of under estimating the current drawn by the inverter input stage. A 12 Volt system typically draws 1 Amp for every 10 Watts of output power supplied giving a maximum load current of some 20 Amps for a small 200 Watt unit and nearly 40 Amps at it's surge limit. Now although a lot of cable stock may specify current ratings much higher than this it does not necessarily mean it is suitable for this application. Many such cables are for domestic wiring such as ring mains and cooker spurs operating at mains voltages were a drop of a few volts under a load of say 20 Amps would not be a problem. This same voltage drop at 20 Amps on for example a 12 Volt battery supply to the inverter would present the inverter with what appears to be a discharged battery.

    Poor & Dangerous Battery Cabling


    Input

    V
    O
    L
    T
    A
    G
    E

    WATTAGE

    200W

    700W

    1500W

    12V

    6mm2

    16 mm2

    32 mm2

    24V

    3mm2

    10 mm2

    24 mm2

    48V

    1.5mm2

    6 mm2

    16 mm2

    96V

    1 mm2

    3 mm2

    6 mm2

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  • BATTERY CABLE LENGTH ISSUES
  • Often it is not convenient or possible to arrange for the battery supply to be near the load and in these cases the wiring will have to be extended. At first it would seem logical to site the inverter close to the load and keep the battery out of the way, even outside for ventilation, however this is not the best solution electrically. The effect of having the relatively high current battery cables extended too far will cause excessive voltage drops and symptoms like that of using cable of insufficient cross section. As in the previous section the voltage drop at the input stage of the inverter gives the appearance of a discharged battery. If the input cabling must be extended then the cross sectional area chosen should be increased to compensate.

    Poor Extension Solution

    A better solution to the problem is to site the inverter right next to the battery with short battery cables and extend the output cable to the load. The reason behind this is that although the lengthening of any cable causes a drop in voltage to appear across it under load, the output voltage presented to the load is relatively high so the effects of any drop are less noticable. With this arrangement of short cabling to the battery, only a minimum length of the relatively expensive battery cable is required for the installation.

    Preferred Extension Solution

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  • LOW CURRENT SWITCH CONTROL
  • Where it is desirable to control the inverter with a switch close to the load then units incorporating a remote switch wire is recommended. This feature allows the inverter to be switched on and off from a distance without extending the main current carrying cables that would cause a voltage drop at the input. The switch need only be a small low current type and the wiring to need only be of minimal cross section. This arrangement would require a second fuse to be placed in this switch line to protect the thin cable, a value of 1 Amp would be sufficient as only a minimal current is ever drawn through this circuit.

    Remote switching of the inverter

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  • THE BATTERY CONDITION ALARM
  • The battery condition alarms where fitted are to give an audible warning of a battery requiring charging and to switch off the inverter when the battery falls to a certain discharge condition. This circuit acts upon the voltage present at the input of the inverter so any voltage drop introduced by insufficient battery cabling will also have an influence on the alarm operation and in certain cases cause the inverter to cut out when a load is applied. Typically for a 12 Volt system the unit will operate normally until the battery has discharged to a point where the voltage presented to the inverter falls to approximately 10.5 Volts, at this point the alarm will begin to sound quietly prompting the user to charge the battery. If no action is taken or the charging arrangements don't supply current at the rate the inverter requires the battery voltage will fall further and the alarm volume will increase until at approximately 9.5 Volts the inverter output will switch off. The inverter will remain in this state until both the battery is recharged and the inverter is manually switched off and on to reset the circuit. This action prevents the battery from becoming deeply discharged which can shorten it's service life or prevent for example it's ability to crank over an engine.

    Alarm And Shutdown

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  • FUSE PROTECTION OF BATTERY CABLING
  • All our units have internal fuses rated to blow in the event of a failure inside the inverter or excessive overload conditions. These fuses cannot protect the battery cabling against fire in the event of a fault caused by for example a cable chaffing on a bulkhead or being crushed. For this reason you should fit an in-line fuse of a rating suitable for protecting the battery cable but not such as to introduce excessive voltage drop in the cabling. For example a 200 Watt inverter for a 12 Volt battery system is internally fused at 30 Amps so with suitable heavy cabling the whole system could be protected by a 60 Amp in-line fuse at the battery post. In systems using paralleled batteries we would suggest an in-line fuse for each battery. In systems using series batteries a single fuse anywhere in the circuit will make it safe in the event of a fault. A short circuit will thus result at most in a pop from the in-line fuse, however the risk of an unfused system developing a short circuit which would unleash a huge bolt of current, rapidly heating all but the very heaviest cables is not one worth taking.

    Fuse Protecting Cables Burnt Unprotected Cables

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  • SERIES WIRING OF BATTERIES
  • With reference to the section regarding extension of cables, one of the advantages of higher input voltages is that battery cable voltage drops are much less of a problem. For example a 24 Volt inverter would draw typically half of the current that a 12 Volt inverter would do for the same output allowing slimmer, longer and more easily routed cables to be used in the battery enclosures. Of course you would require two 12 Volt batteries instead of a single one but their capacities would only need to be half as large (or you would by the same reasoning get twice the capacity of the 12 Volt system using the same type of battery) as they share the work. The fusing arrangements are simple as one fuse will protect the entire battery cable and can be situated anywhere in the circuit such as between the batteries as in the illustration. Also the charging arrangements would have to be taken into account, the preferred method would be to keep the batteries in series when charging also, using a 24 Volt charger to keep all the cells balanced and in the same condition. For a permanent installation the advantages of higher voltage battery systems are well worth consideration as they have many advantages over a 12 Volt system when higher powers are required.

    Series Battery Connection

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  • PARALLEL WIRING OF BATTERIES
  • When greater battery capacity is required batteries may be connected in parallel, this also gives greater peak cranking current capabilities. Wherever possible each battery should be connected with a seperate fuse and the individual branches of the circuits brought together as electrically close to the inverter input as is practically possible.

    Good Parallel Cabling
    Compromised Parallel Cabling

    The looping in of other batteries to form the parallel branches through a common fuse is a compromise and will give reduced performance. As can be seen the total current drawn from all the individual batteries must flow through the single fuse and cables linking the closest battery too the inverter so voltage drops will be higher.

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