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Wire size and length

EFI Wiring Fundamentals

Relevant Module: Electrical Fundamentals > Inductors

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I am having a bit of a hard time understanding the ohms law and how you find these values in a circuit. I’m applying it to the injector circuit of an injector to determine the size of wire I will be using currently it’s using 22 gage and two circuits running 65 vt and has an ohm of 12 please help me understand 😅

Long story short, in my words - Ohm' s law says that when a current passes through a conductor, there will be a voltage drop due to the resistance of the conductor, the energy lost will be in the form of heat, which heats the conductor.

Variations of the law are V=IR, I=V/R, R=V/I - if you know two values you can calculate the third, whatever it is.

The second part is the voltage drop along the length of the wire - the longer the wire and/or the higher the current, the greater the drop. For some things this isn't that important, but for other things it is because a minimum current and/or voltage may be required to operate correctly.

For the last, which is generally things like motors, pumps, fans, etc - P=IV, V=P/I, I=P/V and by substituting from the equations above, we get P= I²/R, P=V²R, etc - you can figure out the rest by simple arithmatic.

In practice, the main things we are concerned about are -

is the wire gauge/CSA (Cross Sectional Area) large enough that the heat released by the current passing through the conductor doesn't damage, or melt, the insulation - this is where we look at the temperature rise values for different currents in the wire. NOTE, this is above ambient, in free air, so in a hot engine compartment or the middle of a wiring loom the rating will be reduced because the wire insulation can' t shed heat as effectively.

is the voltage drop along the wire low enough not to be a problem, for this there should be a chart, or charts, with the voltage drop at different percentages, over a specified length (usually 10 feet/3M) and the maximum current that can be used within that voltage drop.

BOTH are important, as a short wire may be acceptable for the voltage drop but well over the heat rating, whereas a long wire may be fine for the current-heating side, but have an excessive voltage drop.

Oh, P = power, V = voltage drop, I = amperage, R = resistance in Ohms

Thanks for the response I feel like I have a pretty good understanding of that now I geuss what I’m looking for is I have two of the three I have the voltage range0-4.7 and the ohms of the crankshaft position sensor which is 200-300 ohms. From the calculation I did I get .0188 which makes no sense to the standard of the wire size sheet given in the intro course. I’m going off the internet for the ohms not sure if it’s correct but I want to know if I can go from a 20 gauge to a 22 gauge for a sheilded wire

The crank sensor isn't really a "load" in the conventional sense. If it's a powered crank sensor, IE a hall effect sensor, then yes there will be a very small load from the crank sensor, which I will come back to. The "load" in the case of sensors is typically considered to be the ECU or datalogger, but this is an atypical way to look at these circuits.

When it comes to sensor wiring the current draw of sensors is typically regarded to be so low, that this isn't even a concern 99% of time when laying out the harness design. Were talking milliAmps of current here for each sensor, some are even microAmps. The biggest thing to consider here is the physical properties of the wire, more than any current that will be passed through them for sensors. Will the size of wire be strong enough for the environment? Will the jacketing be the right type for the environment that the harness will pass through? Will the wire gauge fit the terminals needed for the sensor properly?

Typically the harness designer/builder will just choose a size of wire that they are comfortable with for the sensor and run with it, and if going by current capacity requirements will actually be many times larger than needed, mostly to fit the physical properties aspect, not the electrical needs aspect. Typical wire sizes for sensors is between 22AWG and 18AWG.

Where you need to consider wire gauge is more for what are known as actuators in the system, like injectors, coils, solenoids, stepper motors etc. The current needed for these parts are also easier to calculate, because you can directly measure the resistance of these devices, and can use the simplified Ohm's Law formulas for them (There are deeper properties of a simple coil that actually change what the final current draw is, but they can be ignored because the difference is so small between measuring simple resistance and taking into account other properties it isn't worth getting too deep into the advanced aspects).

As Chris said, that type of sender is really just a voltage reference sender - basically an on-off switch - and there is minimal current, so the 22 should be fine - but the 18 will be stronger and more resistant to vibration fatigue, and so potentially more reliable.

You guys are way too smart! Much much appreciated. It is a hall sensor but this being my first go around with making a harness I dident quite understand the hall aspect of the sensor now knowing it’s so little amps going through I’m going to go from the stock 20 gauge wire size for the hall sensor to the 3-con 22 shielded wire again thanks for walking me through that I feel a lot better and can move forward

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