Tag: ohms law

Ohm’s Law is pretty straightforward; you multiply ohms by amps to get the voltage. Using variable E to represent voltage, variable I for amps, and variable R for ohms, the equation for Ohm’s Law looks like this: 

E = I × R 

You can figure out the number of amps in a system using basic algebra to turn this multiplication equation into a division one. Divide both sides by R to isolate the amperage (E/R = I). From there, you’d take the voltage reading and divide it by your ohm reading. Your equation should yield the amperage.

However, if you’ve tried doing that and then comparing your answer to the actual amperage measurement, you’ll know that there’s a lot less current than the equation would lead you to believe. Ohm’s Law appears to be inaccurate most of the time, and it’s a bit frustrating because there’s such an emphasis on it in electrical education, but it doesn’t seem to work in the field. Why do we even learn about it in the first place?

The truth is that Ohm’s Law is still valid and works just fine. It’s merely impractical for many of the alternating current (AC) components we work on. That’s because the ohms we measure don’t account for all resistance types that make up total impedance. Inductive reactance is one of those types of impedance, and our multimeters and ohmmeters can’t pick it up. It also happens to be a byproduct of the inductive loads we regularly use.


Maybe I’ve gotten a bit ahead of myself by tacking “inductive” onto words without explaining them. So, what is a load, anyway?

Simply put, a load is a component that does something in an electrical circuit. For example, a lightbulb is a load because it lights up when it receives power. In terms of the work we do, motors and transformers are loads that we regularly use.

There’s quite a difference between lightbulbs and motors or transformers. They each belong to different load categories; lightbulbs are resistive loads, while motors and transformers are inductive loads. Resistive loads have a heating component (toasters, oven coils, and electric heaters are also resistive loads), and inductive loads have an electromagnetic element. 

We see plenty of inductive AC loads in the work we do. (AC refers to alternating current. Inductors don’t have a significant effect on DC circuits.) Inductive loads facilitate magnetism and (usually) mechanical movement.

Transformers are the exception to the movement rule. Transformers only transfer electric energy via electromagnetism and don’t have any moving parts. Still, the point stands that magnetism is the core trait of inductive loads.  


I don’t want to dwell on electromagnetism for too long. Still, I think we should have a solid grasp of its fundamentals before we discuss inductive reactance. 

When current travels through a wire, it will make a small magnetic field. It stands to reason that a coiled wire over a small area would create a larger, stronger magnetic field. After all, the current runs through the wire several times in the same small space. 

The magnetic field expands as the current runs through the coil, and electrical energy accumulates as magnetic energy when the field is at its maximum size. When the current stops flowing, the field shrinks until it disappears entirely, returning all the stored energy to electric energy. It takes a bit of time to store and release the power, so you’ll always see a lag in the current. 


Magnetism vs. heat

As we just explained, inductive loads hold their energy in magnetic fields. This energy storage method is why resistive loads heat up quickly, but inductive loads do not.

The magnetic field’s energy storage impedes the current. The current doesn’t travel from point A to point B in the circuit without experiencing that delay. The delay reduces the total power delivered, so inductive loads don’t heat up to the same degree as resistive loads. 

On the other hand, voltage and current peak simultaneously in resistive loads, which allows all the power in the circuit to be delivered. Resistive loads heat up much more quickly because their circuits don’t have the same delay as inductive load circuits. That’s why solenoids, relay coils, and motors don’t act as heaters that overload constantly. 

Reactance and impedance

Reactance is a component’s opposition to the current flow, just like the lag we talked about in the previous sections. As this description suggests, reactance is a form of resistance. 

Reactance is a type of resistance called impedance. (Remember when I said that the energy storage delay impedes the current?) As such, inductive reactance is impedance from inductive loads. Although lightbulbs and other resistive loads present some form of resistance while operating, the resistance from inductive loads is significantly higher.

The total impedance is a combination of reactance and resistive ohms, so they both make up the total number of ohms.

Like other sources of resistance, we measure reactance in ohms. However, as I said earlier, you can only use your multimeter or ohmmeter to measure resistive ohms. An ohmmeter can’t measure reactance, so there’s no way you can measure inductive reactance beforehand. As a result, Ohm’s Law can’t be used to find current by measuring ohms in most of what we do.

The design of electrical components dictates the resistance and impedance within them. The wires’ winding affects the behavior of inductive reactance, ohmic resistance, and current, as you’ll read shortly.


Inductive reactance and current

You may have noticed that motors draw higher current upon startup. Many people call this the inrush current, which will typically be 4-6 times higher than the standard running current. 

The current is strongest at the start because it takes a little bit of time for the impedance to push back against the current resistance. That usually happens after the motor starts spinning. However, once the inductive reactance has established itself, it strongly resists the current and reduces the amperage as a result. 

If you use Ohm’s Law to find the amperage and yield a number that’s much higher than your ohmmeter’s reading, that’s because you haven’t accounted for the effect that inductive reactance has on current. Again, inductive reactance won’t show up on ohmage readings, but it still impedes the amperage and results in a much lower amperage reading than expected.  


Winding: inductive reactance and transformers

We’ve already established that transformers are the odd ones out because they lack moving parts. Instead, transformers transfer electrical energy from one circuit to another via electromagnetic induction.

Transformers have two sets of windings: primary and secondary. The primary winding connects directly to the AC supply, and the secondary winding connects to the load (output terminal). A magnetic core binds the primary and secondary winding.

When a transformer has no load on the secondary winding, it draws almost no current on the primary winding. That’s because the impedance on the secondary winding is extremely high, and it becomes a near-perfect inductor when there is no load. 


Did we really go through all this information just to prove that Ohm’s Law is not a sham? Absolutely. Our ohmmeters and multimeters only give us part of the picture, and it’s unfair (and inaccurate) to judge the validity of Ohm’s Law with our limited measurements. 

As you can see, the electrical world is complicated, and there’s a lot more to resistance than the ohms we can measure with our devices. Just remember that the amps don’t magically disappear; they get impeded by inductive reactance.

In HVAC and electrical school, one of the first things you learn about electricity is Ohm’s law:

Volts = Amps x Ohms 

Pretty simple Right? and Watt’s law is just as easy:

Watts = Volts x Amps

With this new found knowledge the student walks confidently into the real world with two equations and some elementary Algebra skills expecting to be able to predict Volts, Amps, Watts and Ohms using this secret knowledge.

Then they Ohm out a residential compressor or other single phase motor windings for the first time…

They might read something like:

Start to Common = 4.2 Ohms

Run to Common = 2.6 Ohms

Start to Run = 6.9 Ohms

So they measure their voltage, grab a calculator and calculate.

Run Winding 243V ÷ 2.6Ω = 93.4A

Start Winding 243V ÷ 4.2Ω = 57.85A

Well, THAT makes no sense…. so they stop using Ohms law and settle with believing that electricity is magic and ohms law is broken.

In actuality, Ohms law does work it’s just that the loads we measure vary and don’t all behave in the same way.

Inductive Loads

In the case above, a compressor/fan motor/contactor coil etc… is an inductive load. That means that its job is to convert electrical energy to electromagnetic force. While an inductive load does have SOME resistance when de-energized that can be measured with an Ohmmeter, the majority of the electrical resistance only shows up once current is applied.

In an inductive (magnetic) load, this resistance that shows up once it’s powered on is called inductive reactance and is still measured in Ohms.

In the case of the example above, the run winding is connected directly between L1 & L2 and when the circuit is completed by the contactor in the example above the run winding WILL actually draw really high amps for a split second (first electrical cycle) until the inductive reactance kicks in.

In the start winding the current is limited by a combination of resistance, inductive reactance and the capacitance of the capacitor. Add in the fact that the applied Voltage across the run winding is actually higher than the L1 – L2 voltage due to back EMF (CEMF) and its enough to confuse anyone.

Then you throw in Power Factor to the mix in an inductive load… this means that even when we multiply Volts x Amps in an inductive AC circuit what we see in VA isn’t necessarily what you get in Watts.

It takes a lot of measurements and math to figure this all out and unless you are an engineer it’s much easier to measure what you have on a functioning component rather than attempting to calculate amperage and wattage based on voltage and ohms.


We are taught that resistive loads (loads that create light and heat) are much more simple. There is none of this inductive reactance or power factor nonsense in a resistive load like a light bulb or a heat strip.

But wait… there’s more

So this light bulb is measuring 12.2 Ohms confirmed with two different meters and it is rated at 14.4 volts. So we do the simple math:

Amps = 14.4v ÷ 12.2Ω 

Therefore Amps = 1.18
So we put it to the test by feeding this bulb exactly 14.4V from a DC power supply

Annnnddd… Not even close

We expected an amperage of 1.18 and got an amperage of 0.1


Incandescent light bulbs are a resistive load but they are also made with a filament of the element tungsten which has PTCR (positive temperature coefficient resistor) properties. This means that the resistance of a tungsten incandescent light bulb goes up by 10 to 15 times from its cold temperature to its hot temperature. In the case of the bulb above we now its cold resistance is 12.2 ohms, but by working ohms law backward we can also tell that its HOT resistance is 142.57 ohms which means that the hot resistance is 11.6 times higher than cold in this particular instance.

Not all resistive loads behave in this way though. Let’s look at a heat strip.

Amps = 240v ÷ 15.5Ω

Amps = 15.48 

Watts = 240v x 15.48

Watts = 3715 based on Ohms & Watts law

Don’t worry… This is a 7.2kw (7200 Watt) heat strip divided into two 3600 watt strips and we are only reading one half (3.6kw)

To bench test it further I applied one-tenth of the designed voltage (24 Volts) to see how it would respond.

1.532a @ 24v = 15.32a @ 240v 

The reason the math still doesn’t line up perfectly is that even in a heat strip the resistance increases as the temperature increases but in a much smaller fashion than in a light bulb.

When cold the math predicts it is a 3.7kw heater, when warm at 24v applied it predicts 3.67kw and at 240v the resistance increases to its full rating at 3.6kw.

All of this to say that Ohms & Watts law are useful and accurate but they are impacted by real “under load” forces in such a way that it much more realistic to make measurements on a functioning device and work backward than to use ohms to work upwards from an ohm bench test and a voltage.

— Bryan




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