Motor protection

[Brandon Loyd]

I have a compressor motor without thermal overload protection built in. I need to install a breaker and wire. Do I size my breaker at 125%. It has a 400 amp breaker now with a 300 amp trip. Wire is 3/0 TW and Motor FLA is 173amps. I am thinking a 225 amp breaker with 3/0 THHN, but I beleive if I go with a 250 amp breaker, I can keep the same Frame size and mounting equipment. Would a 250 amp breaker with 4/0 THHN be ok?

 

[augie47]

IMO, If it's a package unit you would follow the nameplate info per Art 409
(MCA / MOCP ).
If it's a motor, use 430.25o to obtain FLA.
As a "guide", a 150 HP 480v motor would be 180 amps requiring a 4/0
minimum conductor and a maximum breaker @ 450 amps.

sounds like your 3/0 TW wire may be a bit small....(and old )

 

[Brandon Loyd]

You lost me. It is a 480V, 150HP, 173amp continuous duty motor. Where does it say I can go up to a 450 amp breaker? Isn't the wire always going to be sized based on the the overload protection, not the FLA?

 

[templdl]

Remember that breakers protect wire.
Use a combination motor starter that includes a magnetic only motor circuit protector and a contactor with an overload relay with heaters sized to protect the motor.

 

[don_resqcapt19]

First you need some type of motor overload protection and second the conductors for motors are sized at 125% of the full load current as found in the tables at the end of Article 430. The branch circuit short circuit and ground fault protection is sized based on T430.52.
The table full load current for a 3 phase 150 hp 480 volt motor is 180 amps. 430.22 requires the conductor to have an ampacity of 125% of the FLA or 225 amps. 4/0 copper with a 450 amp thermal magnetic breaker.

 

[Brandon Loyd]

This motor is only fed by a failing breaker. I need to replace the breaker and wire. If I install a 450 amp breaker with 4/0(250 amps THHN), then my wire is not protected. I now the motor is only rated at 180 amps, but isn't it true that if the motor begins to deteriorate, the amp draw will rise, which could eventually exceed the rating of the wire which will not be protected by a 450 amp breaker. Or are we assuming if that happens, the heat from the wire will trip the thermal overload in the breaker. It just doesn't seem like 250 amp rated wire can be protected by a 450 amp breaker. If so, why not install a 50 amp breaker with #14 wire?

 

[Brandon Loyd]

I understand the consept of a motor starter with heaters. The breaker protects against short circuit, and the heaters protect the motor and wire against overload. I am just struggling with the idea of protecting small wire with a large breaker.

 

[augie47]

This motor is only fed by a failing breaker. I need to replace the breaker and wire. If I install a 450 amp breaker with 4/0(250 amps THHN), then my wire is not protected. I now the motor is only rated at 180 amps, but isn't it true that if the motor begins to deteriorate, the amp draw will rise, which could eventually exceed the rating of the wire which will not be protected by a 450 amp breaker. Or are we assuming if that happens, the heat from the wire will trip the thermal overload in the breaker. It just doesn't seem like 250 amp rated wire can be protected by a 450 amp breaker. If so, why not install a 50 amp breaker with #14 wire?

The motor is required to have Overload Protection per 430.32.
Normally on compressors this is provided by a magnetic motor starter.
In those sitautions the Code allows that overload protection to protect the wire also....a "downsteam protection" as it were (barring some highly unusal event, this overload device will prevent the wire from being overloaded).
. The GFSC (breaker or fuse) in that situation is only providing Short-Circuit/Ground fault protection and with that device sized to handle the motor inrush it is still capable of protecting the wire. (a short on a 4/0 wire will open a 450 amp breaker)
If you have No overload protection (almost unheard of on compressors in my experience) and elect to use fuses/breaker for overload protection then
you you size that per 430.32-> 430.38 (125%-140%)....

Is there a motor starter/contactor ? Are you sure it does not have overloads ?

 

[don_resqcapt19]

... It just doesn't seem like 250 amp rated wire can be protected by a 450 amp breaker. If so, why not install a 50 amp breaker with #14 wire?

There are some circuits where you are permitted to do that. Just like your circuit the breaker is only providing short circuit and ground fault protection and another device is providing the overload protection.

A device that provides overload protection can be installed at any point on the circuit. The device that provided short circuit and ground fault protection can only be installed a the supply end of the circuit.

 

[Brandon Loyd]

I must be dense today. This breaker is directly feeding the motor without ANY other overload protection, starters, or heaters. Yes or No, 4/O (250amps) wire is legally protected by a 450 amp breaker? If yes, then the current 300 amp breaker is an ok size since it is not nuisence tripping at startup?

 

[augie47]

I would have to say NO.
The 450 amp breaker is far above the size allowed by 430.32 for an overload device.
IMO, if you elect to use the breaker as your OL device, you would select it per 430.32 and then install a wire to match that ampacity.
(a 225 breaker would be your "starting point" and the 4/0 would work but I doubt a 225 will work with a compressor inrush.

To me, the correct thing would be to add a motor starter with OL or a drive with OL

 

[don_resqcapt19]

There is no rounding up to the next size for overload protection. The maximum standard breaker than you can use for this application would be 225 amps.

 

[augie47]

There is no rounding up to the next size for overload protection. The maximum standard breaker than you can use for this application would be 225 amps.

Don, does 430.32(C) not give you a little "wiggle room" if the 225 does not allow starting ?
(I've never seen this application on a motor of any size so I have not studied as I should)

 

[texie]

Wow, I can't imagine running a motor/compressor this big with out a proper controller and overload protection. It would seem that one would want at least an across the line mag starter for some basic control as opposed to getting the breaker and conductor sizing just right to be in compliance with art 430 and then be forced to control it by manually throwing the breaker. Off the top of my head it would seem using the breaker as a switch for control purposes is a violation anyway unless it is SWD rated. Anyone seen a 250A 480V breaker rated SWD? One would be hard pressed to convince me this is good practice regardless if you could make technically compliant. I can see it now- "hey Joe we're low on air pressure over here! Go throw that breaker over there on. Wait! No, not that one! Boom!"

 

[don_resqcapt19]

Don, does 430.32(C) not give you a little "wiggle room" if the 225 does not allow starting ?
(I've never seen this application on a motor of any size so I have not studied as I should)

Yes it does, but only up to 140% of nameplate FLA. In this case that is 242.2 amps. There is no provision that lets you round up to the next standard size so the biggest breaker you can use for overload protection is 225 amp.

 

[augie47]

Yes it does, but only up to 140% of nameplate FLA. In this case that is 242.2 amps. There is no provision that lets you round up to the next standard size so the biggest breaker you can use for overload protection is 225 amp.

Thank you. Kudos

 

[stew]

If that motor starts on a 225 amp breaker i would be very surprized. It would have to start completly unloaded or the inrush is going to trip this breaker. I am also of the opinion that a compressor this size must have some sort of mag starter associated with it. I would be very surprized if it did not and if it in fact does not then one needs to be added. Otherwise how does the system start? manually every time you need air? Heck just put a mag starter with the proper overloads and a 300 or 350 amp breaker on the 4/0 cu conductors and you then have a properly set up system IMHO>

 

[kwired]

Wow, I can't imagine running a motor/compressor this big with out a proper controller and overload protection. It would seem that one would want at least an across the line mag starter for some basic control as opposed to getting the breaker and conductor sizing just right to be in compliance with art 430 and then be forced to control it by manually throwing the breaker. Off the top of my head it would seem using the breaker as a switch for control purposes is a violation anyway unless it is SWD rated. Anyone seen a 250A 480V breaker rated SWD? One would be hard pressed to convince me this is good practice regardless if you could make technically compliant. I can see it now- "hey Joe we're low on air pressure over here! Go throw that breaker over there on. Wait! No, not that one! Boom!"

SWD rating is intended for frequently using the breaker to switch electric discharge lighting not for motor switching.

If that motor starts on a 225 amp breaker i would be very surprized. It would have to start completly unloaded or the inrush is going to trip this breaker. I am also of the opinion that a compressor this size must have some sort of mag starter associated with it. I would be very surprized if it did not and if it in fact does not then one needs to be added. Otherwise how does the system start? manually every time you need air? Heck just put a mag starter with the proper overloads and a 300 or 350 amp breaker on the 4/0 cu conductors and you then have a properly set up system IMHO>

I find it hard to believe it does not have high temperature or oil pressure limits that ordinarily open the control circuit to a contactor or provide input to shut down a VFD.

 

[Cow]

It sounds like the OP just needs to buy a size 5 pump panel and be done with it.

 

[ezbz]

Breaker Trip Curves

Breaker Trip Curves

I would agree that a circuit breaker is not an acceptable device for controlling a motor load, but most manufacturers produce circuit breakers with different trip classes (some even adjustable) for inductive loads or motor loads. If he were to get one with a Class 10 rating a 225 would probably do the job.