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Old 08-26-2012, 10:55 AM   #16
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Maybe I'm confused about a 5HP motor.


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Originally Posted by techpappy View Post
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!!
A 5 HP motor at 120v runs at 56 amps... well, slightly under, but we still have to figure our branch circuit, ground fault & short-circuit protection at 56 amps.


Last edited by stickboy1375; 08-26-2012 at 10:59 AM.
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Old 08-26-2012, 11:01 AM   #17
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Maybe I'm confused about a 5HP motor.


WOW!! I guess I'm just more used to 3 PH equipment...Yipes that's a lot...so, then what's the inrush current going to be and are normal breakers OK to handle that? May be go with 220 Volt motor? Would that be easier? Definitely 3 PH would but, doubt if it's available.
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Old 08-26-2012, 11:06 AM   #18
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Maybe I'm confused about a 5HP motor.


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Originally Posted by techpappy View Post
WOW!! I guess I'm just more used to 3 PH equipment...Yipes that's a lot...so, then what's the inrush current going to be and are normal breakers OK to handle that? May be go with 220 Volt motor? Would that be easier? Definitely 3 PH would but, doubt if it's available.
To expand on what I previously stated, you take the HP of a motor, you look up table 430.248, that will list all the voltages and amperages of each HP rating...

So we take a 5 hp rated motor @ 120v, it comes in at 56 amps... now, to size the branch circuit, we must add 125% to that number... (that percentage covers the inrush)


56*1.25 = 70 amps, that is our MINIMUM wire size...

for ground fault and short circuit protection we add 250%,

56*2.50 140 amps, that is our MAXIMUM breaker size....

Here is a pretty graphic to give you another example...

Last edited by stickboy1375; 08-26-2012 at 11:17 AM.
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Old 08-26-2012, 11:21 AM   #19
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Maybe I'm confused about a 5HP motor.


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Originally Posted by techpappy View Post
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?
The answer to the first part of your post is the math I was referring to. The formula can be found in any UGLY's book. For single phase, the formula is the following:

I=(HP x 746)/(E x Efficiency x PowerFactor)

For three phase, you add a 1.73 on the bottom:

I=(HP x 746)/(1.73 x E x Efficiency x PowerFactor)
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Old 08-26-2012, 11:26 AM   #20
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Maybe I'm confused about a 5HP motor.


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The answer to the first part of your post is the math I was referring to. The formula can be found in any UGLY's book. For single phase, the formula is the following:

I=(HP x 746)/(E x Efficiency x PowerFactor)

For three phase, you add a 1.73 on the bottom:

I=(HP x 746)/(1.73 x E x Efficiency x PowerFactor)
You have to use the NEC... bottom line. I dont know if these would end up being the same results, but reguardless, you must use the NEC tables.

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Old 08-26-2012, 12:16 PM   #21
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Maybe I'm confused about a 5HP motor.


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You have to use the NEC... bottom line. I dont know if these would end up being the same results, but reguardless, you must use the NEC tables.
If the motor is used in a residential application (w/ a molded case breaker), you're right. A commercial, industrial, or utility where the motor may be on a contactor then, the NEC table doesn't apply.
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Old 08-26-2012, 12:17 PM   #22
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Maybe I'm confused about a 5HP motor.


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If the motor is used in a residential application (w/ a molded case breaker), you're right. A commercial, industrial, or utility where the motor may be on a contactor then, the NEC table doesn't apply.
How do you figure that one? There is NO residential vs commercial applications here. You may want to actually read the NEC... You have to follow article 430.

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Old 08-26-2012, 12:43 PM   #23
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Maybe I'm confused about a 5HP motor.


OK here's the straight up formula Hp X 746 divided by Voltage =
5 X 746 = 3720 WATTS divided by 120 = 31.24 Running Amps You say that the NEC pretty much doubles that..I believe you but, talk about over kill ..what with Breakers/ GFCI's/AFCI's ..you'd think there'd be enough protection...must have been a committee of lawyers and engineers NOT tradesmen
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Old 08-26-2012, 01:20 PM   #24
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Maybe I'm confused about a 5HP motor.


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Originally Posted by techpappy View Post
OK here's the straight up formula Hp X 746 divided by Voltage =
5 X 746 = 3720 WATTS divided by 120 = 31.24 Running Amps You say that the NEC pretty much doubles that..I believe you but, talk about over kill ..what with Breakers/ GFCI's/AFCI's ..you'd think there'd be enough protection...must have been a committee of lawyers and engineers NOT tradesmen
I dont think the NEC doubles it at all, all the motors I've ever wired were pretty close to the nec tables. just slightly lower.

And I dont think you understand what the breaker is actually providing for a motor circuit.

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Old 08-26-2012, 02:17 PM   #25
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Maybe I'm confused about a 5HP motor.


Quote:
Originally Posted by techpappy
OK here's the straight up formula Hp X 746 divided by Voltage =
5 X 746 = 3720 WATTS divided by 120 = 31.24 Running Amps You say that the NEC pretty much doubles that..I believe you but, talk about over kill ..what with Breakers/ GFCI's/AFCI's ..you'd think there'd be enough protection...must have been a committee of lawyers and engineers NOT tradesmen
Motors are inductive loads so your formula isn't valid. Straight up or not. Current will be higher than a resistive load.

Your flippant suggestion about gfci and Afci seemed equally ill advised. I doubt folks who's lives were saved by these devices would agree with you.
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Old 08-26-2012, 02:25 PM   #26
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Maybe I'm confused about a 5HP motor.


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How do you figure that one? There is NO residential vs commercial applications here. You may want to actually read the NEC... You have to follow article 430.
Have you ever wired an MCC with a 1, 2, or 3 motor contactor? It depends on the thermal elements used. It is possbile to use a 30A thermal element that has a long trip time to allow for motor inrush. No 56 amps here.
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Old 08-26-2012, 02:57 PM   #27
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Maybe I'm confused about a 5HP motor.


Stick boy...I do know the difference between resistive load protection and short protection..just that i'm not used to 5HP at 120 Volts..not many people would use that application...also, not directed at you, but I didn't feel I was being flippant about GFCI's ands AFCI's what I was meaning to say is that they are an improvement on or provide additional safet therefore it seemd to me that excessive wire size or breaker sizes weren't necessary because they are additonal safeties that CYA in spades ..AND so why all the additional load factors? Wouldn't these devices be brought into play if overload or direct short occurs? Also I know that a load can vary on a motor depending on the application...maybe that's why the added protection..e.g., fan wheel vs. driving a large saw or lathe..so, I guess the extra amps ARE needed..I think I get it now...Still, wouldn't it be better to go to 220 V motor, in a domestic application to be able to save on wire size..depending on distance from panel, of course.
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Old 08-26-2012, 03:02 PM   #28
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Maybe I'm confused about a 5HP motor.


Hey Bugman.thanks...that's where I was coming from..thought the breakers would act like the thermal OL's...don't they? i.e., like the old dual element fuses we used to use for all motor circuits...
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Old 08-26-2012, 03:11 PM   #29
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Maybe I'm confused about a 5HP motor.


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Hey Bugman.thanks...that's where I was coming from..thought the breakers would act like the thermal OL's...don't they? i.e., like the old dual element fuses we used to use for all motor circuits...
Not at all... in some situations, the motor contains the overload protection, other times, we have to provide it... either way, you still use the NEC to size everything.


Lots of people dont understand wiring motors... its very common misconceptions on what to provide and when.
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Old 08-26-2012, 03:14 PM   #30
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Maybe I'm confused about a 5HP motor.


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Have you ever wired an MCC with a 1, 2, or 3 motor contactor? It depends on the thermal elements used. It is possbile to use a 30A thermal element that has a long trip time to allow for motor inrush. No 56 amps here.
It's all the same...






http://ecmweb.com/code-basics/motors...trollers-oh-my


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