Comparison of horsepower and amperage draw of motors

This one exposes the fallacy of using horsepower to determine which replacement motor should be used. This video is part of the heating and cooling series of training videos made to accompany my websites: www.graycoolingman.com and www.grayfurnaceman.com to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

[@robertviejo9048]

Thank you so much for your time and expertise. I am an hvac/r tech student, and there is so much to learn. Thanks grayfurnaceman!!!

[@grayfurnaceman]

Welcome GFM

[@tcphvacr9950]

Good video GFM, Ralph at T &N Services preached the same point in the past, like you, because of experience. Comments keep hitting the "better efficiency" defense, and that's what manufacturers want you to think, but it's really not the case. We have a motor shop in St. Louis (Commercial Electric) and when you call for a replacement, he asks (and he knows his stuff) "what's the amperage?" every time. You put a squirrel cage meant for an 11 amp motor on an 8 amp motor, your point will become evident when the motor starts running hot and dies an early life. Good video, thanks for posting.

[@Eddy63]

Great video G-Man ... thanks for the time used to make them . They're a lot of help to the young'ens ...

[@Forge5894]

Excellent video GFM! Thank you posting it here on YT!

[@victorguerra9232]

I didn't know that,,i always go by hp/ rpm,,,,,so what your saying is when you go get a motor,,,you ask for a motor with same amp draw???this would help me alot to know, thank you!!furnaceman,,,,your the man,,much respect!!!!!

[@Straycurrent]

"your mileage may vary ...." An important point. thanks for the instruction

[@garageink74]

Same principle for condenser fan motors? I've also noticed a lot refrigeration guys selecting motors by watts.

[@drummerdem50]

Great advice , will remember this one. Thanks GFM.

[@grayfurnaceman]

Welcome GFM

[@jeremyvoshage2410]

As always, great advice.

[@RockoRocko-rz7kx]

Thanks for the post GFM as I've always went off the rpm and did not even consider this

[@grayfurnaceman]

Welcome GFM

[@sgalvo]

Great video very informative and to the point. I did not know this. Thanks

[@grayfurnaceman]

Welcome GFM

[@kolochodelrio1234]

Another great video. Thank you sr

[@BeaverZer0]

I had this same problem changing a condenser motor the other day because the one I replaced it with pulled way too high amperage even though it was the same horsepower as the one I was replacing. I even went ahead and put in a slightly larger condenser motor and it was still too high the unit had a quarter horsepower and honestly I think it needed a half HP

[@paradoxdea]

Excellent explanation sir. Well done!😊

[@grayfurnaceman]

Thanks for the support. GFM

[@dcrickerson7611]

Great informative video. This would be an easy mistake to make. Thank you GFM.

[@grayfurnaceman]

Welcome GFM

[@joshuak8332]

I always watch your great videos... Thank you (I Always look for HP and RPM...:-(... )..I gonna be checking carefully amps too..

[@arcad1an292]

Great lesson

[@orlandojohnson5742]

I always heard doesn’t hurt to go up a little bit on hp to compensate low amp one! Lennox had a large furnace long time ago…. 3/4 hp! Many techs said it was basically a 1 hp motor!!! U couldn’t put jus any 3/4 in it! JUST LIKE U R SAYING! Thanks & keep teaching! I’ve done HVAC for a while too!

[@grayfurnaceman]

I would agree. HP is not an accurate measure of capacity. GFM

[@jmjomar8]

Great info...thanks

[@randytorboli]

A few years ago I had a condenser fan motor on my ac unit fail and it was the original motor and it was a 1/8 horsepower 1100 rpm motor and the repairman replaced it with a 1/4 horsepower 1075 rpm motor and the amp draws were the same on both motors so is there any chance of having damage done to the unit itself being the rpm and horsepower are different

[@grayfurnaceman]

No, I have done the same thing many times with no adverse effects. GFM

[@randytorboli]

@@grayfurnaceman oh cool that makes me feel better because I actually noticed before the old motor failed that it was not actually spinning at the right room so I'm wondering if the motor had been failing for a few years but not to the point of it actually quitting completely and when it finally died it was squealing really bad

[@user-ff8tp5bm8v]

My name is rod I live in Washington State I need to replace a motor on my oil heater and I'm having problems finding it up here in Washington the model number is 6K149C I found some on Amazon eBay but the last letter on the model number does not match do you recommend where I could pick one of these up or a company that is selling them I've searched the internet

[@Pretzel114]

Thanks GFM!

[@grayfurnaceman]

Welcome GFM

[@ShuhDonk]

I don't quite understand the comparison between the two 3/4 hp motors. I understand one claims to draw less power, but that does mean its not as powerful at the other, it is just more efficient, correct? If they both really put out 3/4 hp but one runs more efficient that is the one I would want to use. Please install both these motors and use them as intended and monitor amp draw and see if the 8 amp struggles to do its job and is pulling more power than its rated for maybe maybe. Also how are the manufactures measuring hp? Are they measuring in the same way? Could be the 8 amp is not really 3/4 based on the way GE measures hp.

[@grayfurnaceman]

The idea that 2 electric motors of the same design made by competing manufacturers would have such a wide variance in efficiency is just not true. Differences in efficiency are in the single digits at most. If you look at the 2 motors in the video, the difference in size is obvious. I will have a video comparing these motors in operation in the near future. GFM

[@mikumiku925]

You are awesome!!!! Thank you sir!!!!!! 😄😄😄😄😄

[@part1801]

Is is ok to replace a motor 6.5 amp motor with a 13 amp motor, everything else is the same? I was shipped a very similar but slightly bigger bodied blower motor with the same exact wiring, but it's a different part and model number. I'm thinkin maybe they're interchangeable?

[@grayfurnaceman]

I would be concerned about that much of an increase. I have had registers blown out of the floor. GFM

[@part1801]

@@grayfurnaceman thanks for the reply sir

[@rusticsurviving2606]

Are you saying it has to be exact amps... I need to replace a condenser fan motor..1/4hp 1075 RPM 2amp..I found a 1.8amp.. everything else is the same..should I buy it or not..I want it to last..thank you

[@grayfurnaceman]

This is a hard one. Most motors do not draw the rated load amps. They are a little under. To be sure, I would want the same or a bit over on the amp draw. GFM

[@rusticsurviving2606]

That is what I thought..I just wanted to double check my understanding of your video...Thank you for the information

[@Elisummit845]

What about putting the 11amp in place of the 8amp? Should be no issue, correct?

[@grayfurnaceman]

Should be no problem. GFM

[@Elisummit845]

@@grayfurnaceman thank you ☺️.

[@20antman]

Great video. Hp rating can be very misleading. This is something that many techs aren't taught. The 2 center motors are pretty much rated 8amps in your video. The right one is labeled 3/4hp. Shows it's more of a 1/2 hp. Or just falls into the 3/4 hp rating.

[@grayfurnaceman]

I found out that the hard way with an overdrawing replacement motor. GFM

[@joeshearer1247]

Anthony Reardon hvacr I think it's a way for manufacturer to game the system

[@20antman]

Yea it bit me on an AAON unit. Then i learned my lesson.

[@lee-johnson]

So many guys put a motor in and don't check the amps. Whether it's a condenser fan or blower we have to check the amps.

[@grayfurnaceman]

Agreed GFM

[@danielthechskid]

Another way to look at it, 1 horsepower is 746 watts. Because not every motor ever made was designed to be exactly 746x the horsepower rating you end up with this result. Say you have a motor rated to output 425 watts of shaft power as that's what the original equipment design needed, now you have to decide whether to round that down to a 1/2 HP 373w or round up to a 3/4 559.5w. If you round down then someone may end up using a replacement that is too small, if you round up and someone tries to re-purpose this motor it may end up overloaded. That is why you sometimes ignore nominal horsepower ratings.

[@funnythingsincorporated7948]

This is confusing. I thought if I'm using a lower amp motor, I'm using less watts thus saving electricity and lower electric bills. It is still spinning 1075rpm so what's the different? As long as the capacitor matches what the motor needs and the motor matches the volt the system needed, I should be fine, right?

[@grayfurnaceman]

The amount of energy used is dependent on the load placed on the motor. Not the size of the motor. An electric motor tries to adapt to the load. If I use a 10 amp motor on a 10 amp load, all is well. If I use a 5 amp motor on a 10 amp load, the 5 amp motor will attempt to adapt to the larger load but will overheat. There is no free lunch. GFM

[@Z-Ack]

What is the load for a blower motor? The size of the blower or fan blades? The mirv filter rating? The ductwork? Closed vents? Mine just whistles when it has too much of a “load”….

[@Lickemstick]

I love your videos. That being said... You're not taking into account motor efficiency my friend. A 3/4 hp motor can draw whatever current it's engineer's designed it to have. Different materials in the windings and such can have a drastic effect on efficiency.

[@grayfurnaceman]

As I have responded before to others, efficiency differences in the same type (in this case PSC) of motor are very small. Its the rating that is variable. I will be doing more on these motors and maybe I can win you over on this. GFM

[@nellAyaR2034]

How is using the 8.0 amp motor to replace the 11.2 amp one going to burn up the motor? They both run off 120v right? Might not get the same efficiency out of it, but how is that going to burn it up?

[@grayfurnaceman]

The rated load of the motor does not mean that is the most it will draw. If it is applied to a higher load than it is designed for, it will draw higher amperage. Hope this helps. GFM

[@richardaguilar7652]

@@grayfurnaceman Okay, so if my new AC motor is rated at 12.4 amps, then my AC blower won't necessarily need all the 12.4 amps to run? Correct?

[@grayfurnaceman]

@@richardaguilar7652 Amperage draw is dependent on load. Less load, less amperage draw. GFM

[@Z-Ack]

Why would it over draw the motor? The motor is only going to draw what it draws.. the motor is the load.. theres is no removing an 11a motor and replacing with an 8a. The new motor will draw only 8 a…. It might be over worked or insufficient for the air speed which might make the motor draw more current if there was some kind of restriction it had to power through.. but the only way a motor is going to draw more than its rated amperage is if there is a resistance to the induction or spin…. Or if its in a hot environment not rated for its housing..

[@Icytits]

The blower design is the motor load. Like you said, it's going to spin at 1075 give or take some slippage. However, the blower is designed to move a certain amount of air. Air has weight. Weight takes energy to move. That motor is going to overdraw because the blower design demands it. Take an amp draw once in a while.

[@johnc4352]

These are alternating current motors. Horsepower is horsepower is watts which is known as real power. Volts times amps is known as volt- amps and this is apparent power. The difference between real power and apparent power is what is known as the power factor. The 3/4 horsepower motor that draws the less amps is just a motor with a better power factor. The motor that draws 8 amps to make the 3/4 horsepower is a better made motor, it will not draw 11 amps.

[@danielthechskid]

Excellent point but the truth is somewhere between this and what is said in the video. Techs don't have the time to install a lower amperage motor as a trial to see whether or not it is overloaded or just has better power factor and efficiency so just matching it up on amps and physical size as close as possible is the best course of action. Imagine if after installing the trial motor and reassembling the equipment it did end up overloaded, they would have to disassemble and pull the blower again to make it right. Why do the job twice when you can do it right the first time?

[@grayfurnaceman]

These motors are all PSC and there is not that much difference in their efficiency. Power factor has virtually no effect on the efficiency of modern single phase motors. In about one month, I will be publishing a video comparing the amp draw when these different motors are installed in the same blower. Maybe it will clear this up. GFM

[@danielthechskid]

grayfurnaceman When you do be sure to have them plugged in to something like a Kill-A-Watt meter which will show all details (Amps, watts, VA, power factor) and put this to rest.

[@luissalguero686]

you're saying that Power factor has no effect at all?

[@grayfurnaceman]

Luis Salguero I am saying power factor is a bit overused as measure of efficiency. When comparing motors, the power factor is about the same for all PSC motors. GFM

[@vainwretch]

What you are saying is not accurate. Yes each one draws different current and I would say the one with higher current rating has more torque and is why It is a larger motor. So that being said it will spin a higher displacement and larger fan. Using the smaller motor causes it to not get to speed as quickly but has the ability spin at the same RPM if rated the same and could and would burn out because of the lack of generating a heavier fan. Current draw is only a factor of the maximum current that motor will draw . I will use audio amplifier analogy I have 2 audio amps ,one is 1000 watts the other 2000 watts. They both use the same 120 volts. If I run the smaller amp to push 700 watts of sound it will get hot and be more punishing on its longevity than if I use the larger one at 700 watts. It has a higher rating. Yes it is important to use the larger motor if that is what was in the original unit because the larger fan requires more torque and will shorten the life of the smaller amperage motor trying to spin a higher flow fan. It is more about CFM's because a 20 amp circuit means that there is 20 amps always available, Does not mean that 20 amps will go into that motor unless it shorts out . But even with this in consideration a modern more efficient motor may be able to create the same torque as an older less efficient motor using less current. Science : )

[@grayfurnaceman]

I have no idea of what your point is. Yes, the larger amp draw motor will have higher torque. Yes, the smaller amp draw motor will overdraw amperage. Increased amperage with constant voltage equals more power usage. The reason for this video is to point out the fallacy of using HP ratings to determine which motor to use as a replacement. Amperage draw is king in the service industry. Motor efficiency, if the type of motor is the same, is a very small factor in efficiency. I have another video on this coming out soon that may help. GFM

[@meerscan9101]

I halfway disagree with you, because when you walk into the supply house, the first thing they will ask is, "What horsepower motor do you need?"

[@grayfurnaceman]

Who is the expert here? The parts man or the service tech? The service tech has to install the part and make it work. The parts man gives you what you ask for. GFM

[@meerscan9101]

true, thats why I said I halfway disagree... 90% of time go OEM, 10% go macgyver