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Quality Parts & Tools for building and repair of automotive wiring harnesses for Car Van Truck Bus & Boat
Breakdown callout service to Telford,Shrewsbury,and Shropshire areas.
Tel 01952 247007 for mobile auto electricians in Telford & Shropshire.
Trailer connection wiring info is on the "Towing Electrics " page
Building a kit car or restoring a vintage classic automobile ? Auto electrical spare parts , Crimp Terminals, Battery Cable,Wire Connectors, Car Fuses, Fuseholders, Switches ,Relays ,Insulation Tapes ,Heatshrink Tube , Ties , Fasteners , Lights,Tools,
Specialist Vehicle Wiring Components for Car,Van,Truck,Bus,Tractor,Plant , Marine Boat, Motorcycle Electrics available online at SMAK
www.autoelectrics.smak.co.uk Fast Delivery Mail Order Distribution of quality workshop consumables & car electrical spares.
Points to consider when installing automotive electrical equipment
Layout of wiring,switchgear,power sources,accessories, to avoid excessive cable runs
Accessibility for service and maintenance
Current & voltage rating of components and cable
Exposure of connections and electronics to fluids which may cause corrosion or electrolytic action
Temperature and adequate ventilation
Radio frequency interference (RFI) which may be generated by switching of electrical circuits.
Hazards in areas from gases which may be explosive or flammable
Mechanical hazards which may cause cables to become chaffed or damaged
Security
Electrical Circuit Theory
Ohms Law States:
The current flowing in a circuit is directly proportional to the voltage applied and inversely proportional to the resistance
at a constant temperature.
Ohms law may be used to calculate the electrical loads applied by components during the design of a circuit:
For example ;
Vehicle lighting system installation
A vehicle is fitted with a work lamp, which is fitted with a 12volt 55watt bulb. The current drawn by the lamp can be
calculated using Ohms law by :
Current (I) = Power in watts (W) divided by Voltage(V).
Current (I) = 55 / 12 = 4.58amps
From this the cable size , fuse, and switch capacity required can be calculated.
Changing the rating of the bulb to 100 watts would have the following effect:
Current (I) = 100/12 = 8.33 amps current draw .
The resistance of the lamp can be calculated by
Resistance(R)=Voltage (V) divided by Current (I)
For the 100watt lamp this would be 1.44 Ohms.
If the original cable installed was only capable of handling a 5 amp current flow this would now need to be replaced to
take into account the extra electrical consumer load.
Voltage drop in a circuit can be calculated using the following formula:
CURRENT FLOWING X TOTAL CABLE RESISTANCE = VOLT DROP
Increasing the length of a cable run will increase the cable resistance and may require an upgrade to a heavier guage of cable.
Calculation of the total length of a cable run must take account of distance from power supply to consumer and return distance from consumer back to power supply.
To fit additional worklamps on a vehicle without replacing original cabling , Worklamps utilising modern LED technology could
be used with power consumption as low as 7 watts.
Automotive
electronics books at amazon.co.uk 
Various designations exist for vehicle wiring and termination:
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BS AU7a
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British Standard AU7a is for colour coding of wiring used on older british vehicles.
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DIN 72 552
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DIN terminal numbers are used for defining identifying connection to various devices eg, Switches,relays,control units.
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DIN 47 002
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DIN colour codes for electrical wiring
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12N
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Trailer wiring plug and socket for 12 volt rear lighting circuits to ISO 1724
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12S
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Trailer wiring plug and socket for 12 volt supplementary system wiring eg Reverse lamps, power supply, fridge etc. to ISO
3732
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13S
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13 pin plug and socket wiring for including lighting and auxilliary circuits ISO 11446
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24N
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24 volt commercial vehicle trailer wiring plug and socket for lighting and trailer braking control ISO 1185
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24S
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24 volt Supplementary S type plug and socket for reverse lamps, rear fog lamps , and supplementary circuits. ISO 3731
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ISO 7638
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24 volt Antilock braking system plug and socket.
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ISO 12098
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Commercial vehicle plug and socket wiring for trailer lighting , Auxilliary circuits, axle lift, traction control, CANbus
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Typical cable rating for single core pvc auto cable to BS6862
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Cable size
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Typical current rating
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9/0.3mm CSA=0.65mm(sq)
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5.75 Amps Single Core Cable
(resistance/metre @ 20degC of 0.0294 Ohms)
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14/0.3mm 1.0mm
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8.75 Amps Single Core Wire
(resistance/metre @ 20degC of 0.0189Ohms)
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28/0.3mm 2mm
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17.5 Amps Single Core Cable
(resistance/metre @ 20degC of 0.0094 Ohms)
28/0.30MM MINI CABLE REEL RED 17AMP
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35/0.3mm 2.5mm
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21.75 Amps
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44/0.3mm 3.00mm
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25.5 Amps Single Core Automotive Cable
(resistance/metre @ 20degC of 0.006Ohms)
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65/0.3mm 4.50mm
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35 Amps Single Core Auto Cable
(resistance/metre @ 20degC of 0.0041 Ohms)
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84/0.3mm 6.0mm
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42 Amps Single Core Cable
(resistance/metre @ 20degC of 0.0031Ohms)
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97/0.3mm 7.0mm
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50 Amps Single Core Automotive Cable
(resistance/metre @ 20degC of 0.0027Ohms)
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84/0.4mm 10.00mm
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70 Amps
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266/0.3mm 20.0mm
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135 Amp Starter cable
(resistance/metre @ 20degC of 0.0010 Ohms)
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37/0.90mm 25.0mm
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170 Amps Starter cable
(resistance/metre @ 20degC of 0.0008 Ohms)
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61/0.9mm 40.0mm
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300 Amps Starter cable
(resistance/metre @ 20degC of 0.0005Ohms)
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61/1.13mm 60.0mm
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415 Amps Starter cable
(resistance/metre @ 20degC of 0.0003 Ohms)
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To determine the correct size of replacement cable use cable manufacturers specific data for their product and the current
loading.The above table is an approximate guide only.Cable placement, length, and duty cycle of the circuit should also be
taken into account when specifying cable. Cable used for motor vehicle wiring has multiple strands to provide flexibility, for example a cable specified as 28/0.3 will consist of 28 strands of 0.3mm diameter wire providing a nominal conductor cross sectional area of 2mm(sq)
Voltage drop on most circuits should be no greater than 10% of the system voltage ie 1.2 volts on a 12 volt circuit.
Starting and charging circuits (ciruits with a motor or generator) must be uprated so voltage drop is no greater than 0.5volts.
Mobile auto electrical services include fault finding and diagnostics , wiring circuit repairs to systems where wiring insulation
has chafed or melted causing burnt wires.
Auto Electrical Accessory supply and installation.
Locating
bad connections on wiring systems .
Mechanical repairs.
Breakdown callout available to Telford ,
Shropshire & surrounding areas.
Telephone 01952 247007
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