Maybe I'm Confused About A 5HP Motor. - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Maybe I'm confused about a 5HP motor.
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08-25-2012, 06:26 PM   #1
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## Maybe I'm confused about a 5HP motor.

When I was in EE school, I was taught that 1HP=746 watts for an ideal motor (no losses). So, 5HP should equal 3730 watts. At 120V, that would equate to 31.1 A and at 240V, that would equate to 15.5 A. With losses, I'd expect the current draw to be higher. At an operating voltage of less than 120V, I'd also expect the current draw to be higher. So, I'm still trying to figure out how a 5HP could run on a 15 A circuit. I don't think the 15 A circuit breaker would ever stay in when the motor starts.

Also, does anybody know what the typical operating voltage is for residential? Is it the same for houses that are closest to the substation as it is for houses that are at the end of the distribution circuit? Why is it or why isn't it?

08-25-2012, 06:47 PM   #2
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What is this motor being used for or on? It is likely advertised as 5 HP but unlikely it is a true 5 HP motor. A true, single phase 5 HP motor operating @ 115 volts will draw 56 amps.

Last edited by brric; 08-25-2012 at 06:51 PM.

08-25-2012, 06:48 PM   #3
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Quote:
 Originally Posted by Bugman1400 When I was in EE school, I was taught that 1HP=746 watts for an ideal motor (no losses). So, 5HP should equal 3730 watts. At 120V, that would equate to 31.1 A and at 240V, that would equate to 15.5 A. With losses, I'd expect the current draw to be higher. At an operating voltage of less than 120V, I'd also expect the current draw to be higher. So, I'm still trying to figure out how a 5HP could run on a 15 A circuit. I don't think the 15 A circuit breaker would ever stay in when the motor starts. Also, does anybody know what the typical operating voltage is for residential? Is it the same for houses that are closest to the substation as it is for houses that are at the end of the distribution circuit? Why is it or why isn't it?
If you are referring to a 5hp motor from a big box store, the motor is probably a 1 HP motor... sales gimmick.

 08-25-2012, 06:55 PM #4 Licensed Electrician     Join Date: Mar 2012 Location: SE Wisconsin Posts: 4,352 Rewards Points: 2,006 Some lawnmower engine manufacturers were just nailed for misrepresenting their HP ratings as well. __________________ __________________________________________________ ______________ Answers based on the National Electric Code. Always check local amendments.
08-25-2012, 10:25 PM   #5
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Quote:
 Originally Posted by brric What is this motor being used for or on? It is likely advertised as 5 HP but unlikely it is a true 5 HP motor. A true, single phase 5 HP motor operating @ 115 volts will draw 56 amps.
I'll need to see the math on that. You must be talking about a 5HP motor with at least a 1.5SF that is actually running at 6.9HP and is 80% efficient. And, perhaps you added a 300' foot run of cable to the motor and you are measuring at the dist panel.

Surely, a master electrician isn't afraid of a little math.

08-25-2012, 10:27 PM   #6
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Quote:
 Originally Posted by Bugman1400 I'll need to see the math on that. You must be talking about a 5HP motor with at least a 1.5SF that is actually running at 6.9HP and is 80% efficient. And, perhaps you added a 300' foot run of cable to the motor and you are measuring at the dist panel.
You need to size motors to the NEC, check out table 430.248....

5 HP Motor @ 120v is 56 amps.

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08-25-2012, 10:45 PM   #7
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Quote:
 Originally Posted by Bugman1400 Surely, a master electrician isn't afraid of a little math.
Simple, you use the NEC, table 430.248.

 The Following User Says Thank You to stickboy1375 For This Useful Post: electures (08-26-2012)
 08-25-2012, 10:59 PM #8 Member   Join Date: Mar 2011 Location: Charlotte, NC Posts: 164 Rewards Points: 105 I agree that is what it says in the NEC Table. Undoubtedly, the math and an amp meter does not support that number for a running motor. But, it makes since that the 56 amp size is needed to survive the startup. Regardless, the NEC's 56 amps is a lot higher than 15 amps stated in a previous Closed thread.
08-25-2012, 11:00 PM   #9
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Quote:
 Originally Posted by Bugman1400 Regardless, the NEC's 56 amps is a lot higher than 15 amps stated in a previous Closed thread.
Yes, because the so called 5HP motor is a marketing scam...

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 08-25-2012, 11:32 PM #10 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 __________________ -KB Life is uncertain -- eat dessert first!!
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08-26-2012, 12:09 AM   #11
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Quote:
 Originally Posted by stickboy1375 Yes, because the so called 5HP motor is a marketing scam...
Forget about perpetual motion, I want to know when they start producing infinite power with 0 input

http://en.wikipedia.org/wiki/Perpetual_motion

There's no way you're running a 5hp motor on a 15A 120v. Maybe it draws 5hp worth of juice when it kicks on for a second and they leave it at that? That would be sleazy snake oil marketing at it's best.

08-26-2012, 06:19 AM   #12

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Quote:
 Originally Posted by Bugman1400 When I was in EE school, I was taught that 1HP=746 watts for an ideal motor (no losses). So, 5HP should equal 3730 watts. At 120V, that would equate to 31.1 A and at 240V, that would equate to 15.5 A.
It all comes down to where are you getting this "5HP" number from. If it is from the rating plate ON THE MOTOR then fine, if it is from the brochure or sticker on the side of the tool then it is a fake marketing number.
NO ONE is trying to argue with you here, nor are they trying to avoid the math, but WHY do any math with useless numbers?
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08-26-2012, 10:47 AM   #13
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Quote:
 Originally Posted by Speedy Petey It all comes down to where are you getting this "5HP" number from. If it is from the rating plate ON THE MOTOR then fine, if it is from the brochure or sticker on the side of the tool then it is a fake marketing number. NO ONE is trying to argue with you here, nor are they trying to avoid the math, but WHY do any math with useless numbers?
I think I lost sight of what stickboy was saying about the air compressors and concentrated on motor theory. I was not aware of the fake HP story. Since I design substations, 5HP motor to me is a 5HP motor unless, it has a higher than 1.0SF. Luckily, kbsparky's link provided a great condensed version. I actually learned something about non-commercial air compressor loads.

08-26-2012, 10:50 AM   #14
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Quote:
 Originally Posted by Bugman1400 I think I lost sight of what stickboy was saying about the air compressors and concentrated on motor theory. I was not aware of the fake HP story. Since I design substations, 5HP motor to me is a 5HP motor unless, it has a higher than 1.0SF. Luckily, kbsparky's link provided a great condensed version. I actually learned something about non-commercial air compressor loads.
Its a joke really, because it causes all kinds of issues...Why this type of sales gimmick is even allowed is beyond me.

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08-26-2012, 10:51 AM   #15
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surely the 56 amp rating you are referring to is for in rush current NOT running amps which begs the intended question..what is the normal running amps for a single phase 120V , 5 HP motor? AND how does lower voltage affect the amperage..I could prolly find the answer elsewhere but the OP is on this forum.. How 'biut it guys..any answers? AND lets not throw in a lot of what ifs, Just think normal?? 5 HP within say. 30 feet of supply voltage..thern work from there..wadda ya think guys AND NO..I'm not an electrician... just tryin' to be reasonable? BUT, as Forrest Gump reminded me , on late nioght TV last night...stupid is as stupid does.hehehe HEY!! I lost my emoticons!! Anyone know where THEY went!! FOUND 'EM

Forgot to ask..also..what are trhe ratings for inrush current on typical breakers OR can you get special ones for heavy in rush current apps?

Last edited by techpappy; 08-26-2012 at 10:55 AM.

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