Motor load calculations

[ElectricMatt]

I am trying to properly design an electrical system for 2 air compressors 480v - 3p - 50hp. Here is what I have come up with.

Separate branches will be installed for each motor as per specifications.

125 a breaker for feeder protection
90 a time delay fuse for branch circuit

Conductors will be sized as per the overcurrent protection.

Overload protection takes place in the motor control center.

Does this sound right? This is the first time I have done this on my own and am looking for feedback.

Thanks
Matt


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[augie47]

1st and foremost, check the compressor for any name tag or factory requirements. Often there are loads other than the motor and compressor folks often require particular protection.
Generally speaking, forget the motor nametag for anything except overload protection.
All other calculations should be based on the Tables, in your case 430.250 (50HP=65 amp)
Branch Circuit = 81 amps, #4 minimum conductor; SCGF protection 175 amp breaker maximum, 125 amp TD fuse max
for each motor. A 90 amp fuse would be legal but in practicality might be too small considering compressor start up currents.

For feeder protection, assuming you are talking about one feeder for both motors, see 430.62. Your feeder protection will be based in part on your individual protection but 125 way low.
The feeder itself would be a 150 amp conductor.

430.62, 430.52, 430.22 & 430.25 would be your references.

 

[kwired]

I am trying to properly design an electrical system for 2 air compressors 480v - 3p - 50hp. Here is what I have come up with.

Separate branches will be installed for each motor as per specifications.

125 a breaker for feeder protection
90 a time delay fuse for branch circuit

Conductors will be sized as per the overcurrent protection.

Overload protection takes place in the motor control center.

Does this sound right? This is the first time I have done this on my own and am looking for feedback.

Thanks
Matt


Sent from my iPhone using Tapatalk

Minimum feeder conductor size needs to be 125% of largest motor plus full load current of all other motors - taken from 430 part XIV tables. You will need a 146 amp minimum ampacity on the feeder. What overcurrent device should be selected sort of depends on starting method to some extent. If motors are not heavily loaded and are on VFD's or reduced voltage starting methods your 125 amp breaker may work, but is pretty marginal if these run for long time or do see full load or even go into their service factor rating at times. Start both of them simultaneously across the line, would easily happen anytime there is a power failure if they automatically restart, and a 125 amp breaker probably doesn't hold during starting. I'd probably use no less then a 175 amp breaker (but would prefer at least a 200 amp breaker) or 150 amp time delay fuse for the feeder, especially for across the line starting.

 

[ElectricMatt]

1st and foremost, check the compressor for any name tag or factory requirements. Often there are loads other than the motor and compressor folks often require particular protection.
Generally speaking, forget the motor nametag for anything except overload protection.
All other calculations should be based on the Tables, in your case 430.250 (50HP=65 amp)
Branch Circuit = 81 amps, #4 minimum conductor; SCGF protection 175 amp breaker maximum, 125 amp TD fuse max
for each motor. A 90 amp fuse would be legal but in practicality might be too small considering compressor start up currents.

For feeder protection, assuming you are talking about one feeder for both motors, see 430.62. Your feeder protection will be based in part on your individual protection but 125 way low.
The feeder itself would be a 150 amp conductor.

430.62, 430.52, 430.22 & 430.25 would be your references.

So to make sure I am understanding this:

50hp FLC = 65a

Calculations for feeder breaker:
Inverse time breaker = 2.5 multiplier

65 x 2.5 = 162.5a
Next available breaker = 175 a

Calculations for branch circuit fuses:
50hp FLC = 65a

65 x 1.75 = 113.75a
Allowable up to 125a fuse to allow for inrush current.

Is this correct?


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[augie47]

correct other than if its the one motor IMO the correct terminology for the one short-circuit device, feeder or breaker, would be "branch circuit" protection. Your term "feeder" leads one to think there is more than one branch circuit connected. The math, I believe, is correct.

 

[kwired]

So to make sure I am understanding this:

50hp FLC = 65a

Calculations for feeder breaker:
Inverse time breaker = 2.5 multiplier

65 x 2.5 = 162.5a
Next available breaker = 175 a

Calculations for branch circuit fuses:
50hp FLC = 65a

65 x 1.75 = 113.75a
Allowable up to 125a fuse to allow for inrush current.

Is this correct?


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Those values you just calculated are for each branch circuit. The 2.5 multiplier for breaker and 1.75 for time delay fuses is for the most part the maximum setting permitted by code, unless that value won't allow the motor to start. Many device manufacturers may recommend something less. I have a Square D motor slide calculator and they recommend 100 amp thermal mag breaker or 100 amp time delay fuse on a 50 HP 480 volt motor. I have rarely had a device not hold when using that slide calculator, but if you have a really stout supply and very short circuit conductors, you may need to up the trip settings to allow for starting.

correct other than if its the one motor IMO the correct terminology for the one short-circuit device, feeder or breaker, would be "branch circuit" protection. Your term "feeder" leads one to think there is more than one branch circuit connected. The math, I believe, is correct.

I was going to say not so fast there, but now it looks like you added more to your post.

 

[kwired]

One could use 150 amp feeder conductor and have up to 225 amp inverse time feeder breaker for this application, and possibly still be able to increase the overcurrent protection level if it won't allow the motor(s) to start.

 

[ElectricMatt]

correct other than if its the one motor IMO the correct terminology for the one short-circuit device, feeder or breaker, would be "branch circuit" protection. Your term "feeder" leads one to think there is more than one branch circuit connected. The math, I believe, is correct.

One more question just for general knowledge. If I was to run both motors on 1 breaker with a separate disconnect for each. This is how I read it breaks down, is this correct?

125% of FLC largest motor + 100% of FLC each additional motor

Feeder breaker:

81a + 65a = 146a
146 x 2.5 = 365 a
So a 400a breaker

Plus 2 disconnects each with a 125a set of fuses

Is this correct?


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[ElectricMatt]

correct other than if its the one motor IMO the correct terminology for the one short-circuit device, feeder or breaker, would be "branch circuit" protection. Your term "feeder" leads one to think there is more than one branch circuit connected. The math, I believe, is correct.

You are correct it is a branch circuit not a feeder. Poor use of terminology on my part.

It is coming out of a 225 a service adding 2 breakers one for each compressor skid. Each with a disconnect at the skid.

The skid contains nothing other than a motor starter and a small transformer for the motor control circuit.


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[augie47]

One more question just for general knowledge. If I was to run both motors on 1 breaker with a separate disconnect for each. This is how I read it breaks down, is this correct?

125% of FLC largest motor + 100% of FLC each additional motor

Feeder breaker:

81a + 65a = 146a
146 x 2.5 = 365 a
So a 400a breaker

Plus 2 disconnects each with a 125a set of fuses

Is this correct?


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\
close
good on the wire size.. 146 amps
The short-circuit-ground-fault feeder protection, per 430.62 would be based on the largest setting of the branch circuit protective device plus the sum of the FLAs of other motors.
Assume you use a 175 amp breaker for one motor then your feeder breaker would be 175 + 68 =243 for a 225 amp breaker
(there is no "next size up" rule for this)

 

[ElectricMatt]

\
close
good on the wire size.. 146 amps
The short-circuit-ground-fault feeder protection, per 430.62 would be based on the largest setting of the branch circuit protective device plus the sum of the FLAs of other motors.
Assume you use a 150 amp breaker for one motor then your feeder breaker would be 150 + 68 =218 or 200 amp
(there is no "next size up" rule for this)

Okay I understand. Thank you and kwired for your assistance!


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[kwired]

No next size up rule until the 225 amp won't allow the motor(s) to start anyway. Most instances a 225 will work though. If anything you may need to ensure both don't start "across the line" at the same time, which can be done with timing relays in the controls, will probably cost much less then jumping to a 400 amp frame sized breaker.

 

[ElectricMatt]

No next size up rule until the 225 amp won't allow the motor(s) to start anyway. Most instances a 225 will work though. If anything you may need to ensure both don't start "across the line" at the same time, which can be done with timing relays in the controls, will probably cost much less then jumping to a 400 amp frame sized breaker.

Well again thank you for sharing your knowledge!


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