Ohms law

[Pinnie]

I’ve heard that ohms law might apply mainly to dc. I’m not sure who told me that or when but I’ve got that in my brain. We use rms ac voltage in the field, which is voltage generated by ac converted to the equivalent voltage in dc (in my understanding).

My question: Is ohms law safe, in all equations, to apply and calculate for ac?

 

[Shaneyj]

The load profile matters.
Ohms law works for purely resistive loads.
Motors as an example are a constant power load so ohms law will not provide an accurate output.


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[Pinnie]

The load profile matters.
Ohms law works for purely resistive loads.
Motors as an example are a constant power load so ohms law will not provide an accurate output.


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So that would be water heaters, ovens, and what else comes to mind?

 

[jim dungar]

For AC circuits we modify Ohm's law to be: V=IZ.
Voltage equals Current times Impedance. Impedance has two components resistance and reactance. Most meters do not measure reactance, directly.

For the most part motors, capacitors, and power supplies are the typical items you might encounter which have enough reactance to affect your calculations.

 

[EngineeredElectric]

Yes to add to that Resistance Reactance thing:
Ohms law in the strictest sense only applies to DC circuits. We simplify many AC systems into a steady state average to make them easier to consider (RMS voltage is one of those steady state averages)

Many of the complex mathematical functions that describe AC systems simplify into Ohms law form when the system involved only has resistive components, and are therefore safely ignored. Most systems in the real world are not purely resistive, there are reactive components as well (Indictors and Capacitors which come in many forms), and there more reactors there are in the systems, the more the ohms law simplification will be inaccurate.

A great example is a transformer. Put your Fluke on the feeders of a transformer that is switched off and cry as the 1ohm reading tells you that the phases are cold shorted and the fuses are going to blow the moment you throw the disconnect. Then turn it on and watch no current flow because the secondary disconnect is still off. Ohms law lied to you. Or, really, this AC system was not one that simplifies to ohms law. You gotta use the real math.


But nobody wants to do that. Or usually even needs to.
What you are getting into is actually very complicated. AC at the physical level is black magic, it gets really absurd. But like most things in engineering it oftentimes just simplifies out in the real world to something that is workable at an instinctual level. The best way to handle it is just find out on the job when your ohms law brain math doesn't work and get used to it.

 

[Strathead]

So that would be water heaters, ovens, and what else comes to mind?

Please give a perspective why you are asking this? Is it to work out some specific information? Technically ohm's law doesn't work for even purely resistive loads as part of an overall system, because there is always a power factor, but for simple things like figuring out the size breaker for a 2200/240V watt water heater at 208 volts it is adequate.

 

[Pinnie]

Please give a perspective why you are asking this? Is it to work out some specific information? Technically ohm's law doesn't work for even purely resistive loads as part of an overall system, because there is always a power factor, but for simple things like figuring out the size breaker for a 2200/240V watt water heater at 208 volts it is adequate.

I was watching a Ryan Jackson video on exactly that, sizing water heater conductors and ocpd. He calculated amperage and resistance using ohms law, as you said, and it got me thinking about it. A lot of the theory I’ve learned (series circuit resistance vs parallel circuit resistance etc.) applies to mainly, possibly exclusively to dc. And then we use rms to get us close enough to do calculations in the field.

So is it close enough to use ohms law for ac? I’m sure most people do it and don’t think twice that are unaware of the nuances.

 

[jim dungar]

So is it close enough to use ohms law for ac? I’m sure most people do it and don’t think twice that are unaware of the nuances.

Yes, it is close enough. You rarely know the resistance of things except heating devices.
Typically if you have kVA, kW, or HP you don't need to know the resistance and reactance as you use the power formula of P=EI.

 

[Pinnie]

Yes, it is close enough. You rarely know the resistance of things except heating devices.
Typically if you have kVA, kW, or HP you don't need to know the resistance and reactance as you use the power formula of P=EI.

What kind of real world scenarios do you run into that you do need it

 

[jim dungar]

What kind of real world scenarios do you run into that you do need it

You need to know resistance and reactance when dealing with power analyses, such as Power Factor problems, Arc Flash and Short Circuit calculations, and detailed voltage drop calcs for motor starting. These are typical for me as a power system engineer, but most trouble shooting I did involved just the functions on my handheld multimeter.

 

[Pinnie]

You need to know resistance and reactance when dealing with power analyses, such as Power Factor problems, Arc Flash and Short Circuit calculations, and detailed voltage drop calcs for motor starting. These are typical for me as a power system engineer, but most trouble shooting I did involved just the functions on my handheld multimeter.

what kind of percentage range of difference do you typically see when you calculate using ohms law versus when you calculate taking reactance into account?

 

[jim dungar]

what kind of percentage range of difference do you typically see when you calculate using ohms law versus when you calculate taking reactance into account?

It depends on the load and what information I am trying to get.
I can't remember the last time I routinely measured resistance with my multimeter, measuring amps and volts has usually been enough.

 

[Pinnie]

It depends on the load and what information I am trying to get.
I can't remember the last time I routinely measured resistance with my multimeter, measuring amps and volts has usually been enough.

Can you explain the nuance in the relationship of resistance to reactance you are implying?

 

[wwhitney]

Can you explain the nuance in the relationship of resistance to reactance you are implying?

If you have an idealized AC voltage source, and connect it to a pure resistance, the current waveform will be in phase with the voltage waveform. They will both peak at the exact same moments in time.

If you connect the idealized AC voltage source to a pure reactance, the current waveform will be out of phase with the voltage waveform. They will peak at different points in time during each cycle.

Of course, any given real world component will have both resistance and reactance, but some items are close enough to the idealized case to just treat them as a resistance only or a reactance only.

Cheers, Wayne

 

[CoolWill]

If E = IZ, then Z = E/I. So you take the volts measured and divide by the amps measured and say that this circuit has x ohms of impedance. It is the vector sum of resistance and reactance. Knowing that, you could take the DC resistance measurement and algebraically work out the reactance.

 

[Pinnie]

is anyone here an electrician or are you all engineers?

 

[jim dungar]

is anyone here an electrician or are you all engineers?

There is no simple explanation for reactance calculations. You need a basic electrical theory course, like the ones offered by Mike Holt.

Most electricians I know have made it through their whole career without dealing with reactance values, once they got their licenses. Several of them kept my number on 'speed dial' for those rare times it came up.

 

[rambojoe]

I gotta say, that is the first question thats made me laugh out loud on this forum... even with 3 broken ribs!!
I think you want the engineers answers..

 

[JoeStillman]

What kind of real world scenarios do you run into that you do need it

It helps to remember that, for purely resistive loads, the ohms are what's constant, not the watts. If you do the algebra, you can prove that the watts for resistive loads are proportional to the square of the voltage. So a 4 kW, 240V water heater will draw 3 kW at 208V.

 

[Pinnie]

You need a basic electrical theory course, like the ones offered by Mike Holt.

Not to sound lazy, but how much effort versus return do you think is there. I am maybe a third through mikes theory book and it feels like a lot of brain power to learn. Plus the theory well is deep, how much theory knowledge do I need? There’s only so many hours in the day. I’m more than capable of mastering theory, but I don’t think I want to be an engineer. I’m leaning towards focusing on studying business. I’m weak there.