Quote:
Originally Posted by Reinhard
I had checked it specifically before installation, the stock horn itself drew 5A each. So I was sure that the stock wiring harness is absolutely fine. Yes the new horns are also 5A each.
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This time I checked the stock horns in a very crude manner. |
Some
theory, and a few thoughts as to how I would have validated the wiring capacity. Mostly based on some experience I had designing electrical systems for telecom data centres a decade ago - but DC voltage circuits remain the same.
1.
Voltage drop: Any wiring has resistance which is directly proportional to the length of the wire and inversely proportional to the cross sectional area. Now when electricity flows through the conductor from one end to another there is a drop in voltage due to the resistance of the wire, this is again directly proportional to the resistance and the current load through the wire. So in essence, 12V battery / alternator in reality provides slightly less than 12V to the device on the other end of the wiring. Most devices are designed to expect a slight increase/decrease in voltages, but the range is usually not clearly mentioned in most specification sheets.
2.
Joules law of heating All wires generate heat when they carry current. This is directly proportional to the square of the current carried, the resistance of the conductor and the duration of the current carried. Wiring harnesses should be designed to keep the heat generated to a minimum by reducing the resistance, this is can be done by having shorter lengths and keeping the cross section areas larger as-much allowed by design and cost limitations.
3.
Voltage & Current relationship: This is probably the basic, but most important part. When voltages provided to a device are reduced, the device would have a higher current draw. When current draw exceeds the design capacity, bad things happen - typically most of the insulation used fails due to the higher heat generated leading to short circuits. Modern electronics have several fuses and low voltage protective devices to avoid this and increase the safety, but in several automotive applications having some part of the car’s electrical / electronics shutdown during operation is understandably undesirable.
Given these considerations, wiring has to be designed to keep the voltage drop to the minimum required tolerance (I’m not sure of the manufacturers specifications). My personal preference while choosing wiring for the hobbyist electronics projects is to design for a 3-5% drop at peak loads and pick at least 1-2 sizes larger sectional area as much as the design or budget would allow. Invariably I choose several sizes above based on the wiring available on hand.
A. If possible, check the current load expected on the new horn at a 5-10% reduced voltage (10.8 to 11.4V for cars). A spec sheet might have it, but it is not a bad idea to just test it on a workbench like how Reinhard rested his old horns.
B. Measure the distance of the length of wire from the Battery/fuse box to the horn to the grounding point on the chassis. Precision is not necessary, and it’s okay to have error and estimate it on the longer end.
B. Use a DC wiring size calculator/chart (Several available online, one search away) To identify the right cross sectional area for the current load and wire length. Pick a larger size than necessary to be safe.
While all this rational approach might still say that the stock wiring is sufficient for the horn, I would still pick a slightly higher grade/size of wiring irrationally because it’s one of the cheapest electrical safety upgrades, and does give personal peace of mind and satisfaction of having over engineered. It’s no secret that even the best manufacturers design components to meet a target cost, and squeeze design tolerances wherever possible.