Reverse-engineering motor HP

[LarryFine]

Can one extrapolate a motor's highest likely HP rating by using the breaker rating? In other words, if I have a 208v 3ph blower motor successfully starting on a 15a 3p breaker, approximately how large could the motor be?

 

[bkludecke]

What type of C/B? This would be a really vague and inaccurate way to get very unreliable data. You're my kinda electrishun:grin: . My guess would be a max of 5 HP.

 

[Rick Christopherson]

While not perfect, a good approximation for estimating a motor's maximum horsepower is to use 0.8 for powerfactor and 0.8 for efficiency, and base the calculation on the motor's nameplate current value. If you really want to push a motor to its maximum, use 0.85 for either, or both.

HP = I x V x pf x eff / 746

A motor's hp rating is not an absolute, but depends on how far the manufacturer is willing to push the motor into its thermo-electric curve while still maintaining a warranty. In other words, you can get more power out of the motor, but it may damage the windings in the process.

Oops, sorry I forgot the 3-phase conversion. For a 3-phase motor:

HP = I x V x 1.73 x pf x eff / 746

Alright, a second edit: based on your numbers, a 15 amp motor on 208 V 3-phase, this motor is between 4.6 and 5 hp.

 

[iwire]

HP = I x V x 1.73 x pf x eff / 746

In this case that will be of little use.

All Larry has is the breaker size, we have no idea if the original installer used the correct breaker size.

If Larry takes some current readings he might approximate what HP is being used but not what rating the motor is. Unless it happens to running at it's rated output.

 

[Rick Christopherson]

All Larry has is the breaker size, ....

I agree. If all he has is a breaker size, then the motor could conceivably be 1/4 hp. Moreover, if the motor is never driven to its maximum HP, then it could be even larger than 5 hp without tripping a breaker.

Based on breaker size, it could be anything. That's why I mentioned looking at the nameplate current.

Without knowing the manufacturer's nameplate current, it is conceivable to load this motor down to nearly LRA, but doing so is going to let the magic smoke out. It is important to know what the rating is of the windings.

 

[coulter]

Can one extrapolate a motor's highest likely HP rating by using the breaker rating? ...

As noted, not very well.

Can you get a look at the overloads? As noted, they could be the wrong ones, but couple that with an amp reading maybe would give you a better idea.

carl

 

[LarryFine]

Okay, my bad. The job is about 50 miles away, so I won't be able to eyeball it until I get there. The info I have was related by phone. I'm sure there is no controller and/or overloads. Here's a bit more info:

This is a church kitchen system. The intake and exhaust blowers are each powered by plain ol' Siemens 3-pole plug-in breakers. I have to add contactors for the fire-suppression system, as well as for manual control.

Right now, the blowers are wired directly to the breakers, but through J-boxes for the insertion of the contactors (I was told). I want to size the contactors properly, since they'll be used for everyday use.

My question is whether one could determine the highest motor power likely to be used where 15a breakers would be reliably large enough. I'll take relay-contact current-rating recommendations as well.

For example, one 3-p contactor is rated at 20a and 5HP. Is there any advantage to using the next one up, at 25a and 7.5HP? The cost difference is $3.25. After that, 30a and 10HP. Where do you draw the line?

Danke,

 

[frenchelectrican]

Larry.,

Most motor on 208 v 3? on 15 amp breaker typically are 2 or 3 hp range but some case they go much as 5 hp [ if started unloaded that fine ]

For example, one 3-p contactor is rated at 20a and 5HP. Is there any advantage to using the next one up, at 25a and 7.5HP? The cost difference is $3.25. After that, 30a and 10HP. Where do you draw the line?

I useally draw my line depending on how it lay out manly on the conductor size and motor rating.

if both motors are 2 hp size you can used the 20 amp contactor device but when you are expecting to upsize sometime future yeah go with 25 amp contractor device

But really IMO for commercal kitchen exhaust fan and intake normally i useally wired sperated contractors due the Ansul system requirement and also local codes as need to.


Merci,Marc

 

[LarryFine]

But really IMO for commercal kitchen exhaust fan and intake normally i useally wired sperated contractors due the Ansul system requirement and also local codes as need to.

Yes, these will be separate contactors, and for that exact reason. I will use a single switch for both, but they must be separately operable, and I'll use only one microswitch for everything, and save the second one in case they have a building alarm system.

I love seeing how complicated the system can be and still use only one of the micro's.

 

[frenchelectrican]

I love seeing how complicated the system can be and still use only one of the micro's.

Yeah i can see what ya mean with a simple mircoswitch can do with it,

One of my freind he describing the Ansul system to me right down to last connector [ he do it little diffrent than i useally noramally do ] { a funny twist with it he did asked the fire inspector to qoute which code and somehow he stumbled all he say " it been done like that " but i am sure the fire inspector will give him the correct info }

Merci.

 

[MJJBEE]

Okay, my bad. The job is about 50 miles away, so I won't be able to eyeball it until I get there. The info I have was related by phone. I'm sure there is no controller and/or overloads. Here's a bit more info:

This is a church kitchen system. The intake and exhaust blowers are each powered by plain ol' Siemens 3-pole plug-in breakers. I have to add contactors for the fire-suppression system, as well as for manual control.

Right now, the blowers are wired directly to the breakers, but through J-boxes for the insertion of the contactors (I was told). I want to size the contactors properly, since they'll be used for everyday use.

My question is whether one could determine the highest motor power likely to be used where 15a breakers would be reliably large enough. I'll take relay-contact current-rating recommendations as well.

For example, one 3-p contactor is rated at 20a and 5HP. Is there any advantage to using the next one up, at 25a and 7.5HP? The cost difference is $3.25. After that, 30a and 10HP. Where do you draw the line?

Danke,

My Handy Dandy table says the FLA of a 5 HP motor is 16 amps at 208 V therefore if you install the 5 HP contactors you should be OK. I hate doing jobs site unseen although as an engineer we get to do them just off drawings. If I had to venture a guess I would say it's a 3 Hp motor. However I would rather be safe then sorry and bring the 5 HP contactor. I don't know that a larger contactor would buy you anything for this application occasionally it is advantageous to use larger contactors I mostly think about it when I know I'm going to change the motor in a few months.