I do a lot of new and retro wiring of pumps somtimes slightly changing size, i see a lot of starter/disconnects. since the starters are normally on a dedicated circuit,from a fused disconnect or circuit breaker, why are they somtimes fused somtimes not? Aren't the proper overloads most important.
fuse or not
- Thread starter jociha
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: fuse or not
Proper sized "overloads" are important to protect the motor/conductor from overloads. The upstream fuse/CB is for short circuit protection. Some motors are listed requiring a fuse upstream, some will permit either a CB or Fuse, and still others will list a CB only. Just part of the listing.
There are other reasons why a disconnect maybe fused upstream, such a short circuit withstand limitations of an unfused disconnect.
Proper sized "overloads" are important to protect the motor/conductor from overloads. The upstream fuse/CB is for short circuit protection. Some motors are listed requiring a fuse upstream, some will permit either a CB or Fuse, and still others will list a CB only. Just part of the listing.
There are other reasons why a disconnect maybe fused upstream, such a short circuit withstand limitations of an unfused disconnect.
bphgravity
Senior Member
- Location
- Florida
Re: fuse or not
Fuses present a problem for three phase motors. If a single fuse blows, you get single phasing which will damage the motor. If fuses are used, phase detectors or monitors should be used in conjuntion.
Fuses present a problem for three phase motors. If a single fuse blows, you get single phasing which will damage the motor. If fuses are used, phase detectors or monitors should be used in conjuntion.
Re: fuse or not
You are correct. The purpose of fuses or circuit breaker in a motor circuit, is to prevent high current in the motor winding during a lock up. The safe high current, and duration, is the determining factors for fuses or breaker trip time requirement.
The running overload heaters are too slow in response.
You are correct. The purpose of fuses or circuit breaker in a motor circuit, is to prevent high current in the motor winding during a lock up. The safe high current, and duration, is the determining factors for fuses or breaker trip time requirement.
The running overload heaters are too slow in response.
Re: fuse or not
Fuses are often required in combination starters for smaller motors to comply with the requirements for short-circuit and ground-fault protection, NEC 430.52.
Take for example a 3 H.P., 460 volt, three phase motor. The feeder is a 20 ampere circuit from a panelboard. The conductors are 12 AWG.
From Table 430.150, the full-load current rating of the motor is 4.8 amperes.
A time delay fuse, sized from Table 430.52 is 4.8 x 1.75 (175%) = 8.4 amperes
8.4 amperes is not a standard size fuse. Exception No.1 seems to allow the next highest standard size fuse, which would be 15 amperes.
The standard sizes of fuses and breakers are listed in 240.6
However, Exception No.2 limits time delay fuses to a maximum of 225%. In this case 4.8 x 2.25 (225%) = 10.8 amperes. A 10 ampere would be the maximum allowed.
I believe that the provisions of 430.52 must be applied even if the motor circuit conductors are protected at their ampacity by the feeder overcurrent device.
I agree with Bennie that fuses used as the short-circuit and ground-fault protection will provide some degree of protection in single-phase conditions, when rated no more than 175%.
In addition, motors starters are available with "smart" overload relay which can detect single-phase conditions.
MKB
Fuses are often required in combination starters for smaller motors to comply with the requirements for short-circuit and ground-fault protection, NEC 430.52.
Take for example a 3 H.P., 460 volt, three phase motor. The feeder is a 20 ampere circuit from a panelboard. The conductors are 12 AWG.
From Table 430.150, the full-load current rating of the motor is 4.8 amperes.
A time delay fuse, sized from Table 430.52 is 4.8 x 1.75 (175%) = 8.4 amperes
8.4 amperes is not a standard size fuse. Exception No.1 seems to allow the next highest standard size fuse, which would be 15 amperes.
The standard sizes of fuses and breakers are listed in 240.6
However, Exception No.2 limits time delay fuses to a maximum of 225%. In this case 4.8 x 2.25 (225%) = 10.8 amperes. A 10 ampere would be the maximum allowed.
I believe that the provisions of 430.52 must be applied even if the motor circuit conductors are protected at their ampacity by the feeder overcurrent device.
I agree with Bennie that fuses used as the short-circuit and ground-fault protection will provide some degree of protection in single-phase conditions, when rated no more than 175%.
In addition, motors starters are available with "smart" overload relay which can detect single-phase conditions.
MKB
BAHTAH
Senior Member
- Location
- United States
Re: fuse or not
Jociha, the reason you see fuses or breakers sometimes and sometimes a non-fused combo/mag is the Engineers, Mechanical and Electrical have lunch together but seldom talk to one another prior to the job bidding. The electrical engineer is covering by specifying a fusible combo/mag just in case the manufacture of the pump or VFD requires fuse protection or he/she needs some flexability with respect to the HP size. I have found that when it comes to motors and related equipment, if it is not the main item in the job the Electrical Engineers don't always have all the information, the result is combo/mags on fart fans. Sometimes the Engineer wants the fuses/breaker or MCP located with the starter so all the protection is in a single location and there is no issue with someone changing a fuse etc at a distribution board that would effect proper protection of the motor or related equipment. Most often the motor circuit is oversized and over protected unless all the proper information is available up front, or there are plans for up sizing the motor in the future.
Jociha, the reason you see fuses or breakers sometimes and sometimes a non-fused combo/mag is the Engineers, Mechanical and Electrical have lunch together but seldom talk to one another prior to the job bidding. The electrical engineer is covering by specifying a fusible combo/mag just in case the manufacture of the pump or VFD requires fuse protection or he/she needs some flexability with respect to the HP size. I have found that when it comes to motors and related equipment, if it is not the main item in the job the Electrical Engineers don't always have all the information, the result is combo/mags on fart fans. Sometimes the Engineer wants the fuses/breaker or MCP located with the starter so all the protection is in a single location and there is no issue with someone changing a fuse etc at a distribution board that would effect proper protection of the motor or related equipment. Most often the motor circuit is oversized and over protected unless all the proper information is available up front, or there are plans for up sizing the motor in the future.
masterinbama
Senior Member
- Location
- Huntsville Alabama
Re: fuse or not
one reason you see fused and non fused disconnects is with a non fused disconnect you can have a higher horsepower motor.take a 20 hp 208 volt motor for example you can use a 100 amp non fused disconnect but with fuses you need a 200 amp
one reason you see fused and non fused disconnects is with a non fused disconnect you can have a higher horsepower motor.take a 20 hp 208 volt motor for example you can use a 100 amp non fused disconnect but with fuses you need a 200 amp
Re: fuse or not
Motor windings, with rotor removed, are randomly tested with a variable power supply applying current until destruction. The size of fuse is then determined by its time/trip characterists.
The purpose is to prevent damage to windings, from overheating, during a locked rotor event.
Thermal trip breakers do not always meet the requirement.
This is why it is important to observe the nameplate on air conditioner units. The compressors are susceptable to locking.
Motor windings, with rotor removed, are randomly tested with a variable power supply applying current until destruction. The size of fuse is then determined by its time/trip characterists.
The purpose is to prevent damage to windings, from overheating, during a locked rotor event.
Thermal trip breakers do not always meet the requirement.
This is why it is important to observe the nameplate on air conditioner units. The compressors are susceptable to locking.
scott thompson
Senior Member
- Location
- Anaheim, California
Re: fuse or not
As Bennie stated, A/C Compressors are highly susceptable to extended Locked Rotor time periods, and flat-out failing to run out from Locked Rotor (Stalled / Unmovable Rotor).
OCPD at the equipment should be able to allow the high Overcurrent and Long Time frame of starting, but also prevent excessively high LRA and be dialed into the unit's rated FLA, which will offer running protection.
If fused properly (within close tolerances of the running load amperes), the problem of Single Phasing a Three Phase Motor can be covered.
Once again as Bennie stated, the Locked Rotor starting situation is similar to a Short Circuit, which will tend to blow out at least Two fuses (connection is L-L). If the fuses are sized too large, they may not react in time to an attempted 1? start (or fried rotor), and results in a loss of Smoke from the Motor's Windings.
Lose too much smoke and the flames may be released too!
If a Polyphase (3?) Induction Motor experiences a Single Phase situation while it's running, the lines which are still "Live" will draw an excessive current, while the "Dead" line will have normal or lower current flowing in the Motor's winding, and outward to whatever it can flow through.
Voltage will be near system voltage.
Proper fuse size selection will blow the fuse(s) in the "Live" lines.
Just my $0.02
Scott35
As Bennie stated, A/C Compressors are highly susceptable to extended Locked Rotor time periods, and flat-out failing to run out from Locked Rotor (Stalled / Unmovable Rotor).
OCPD at the equipment should be able to allow the high Overcurrent and Long Time frame of starting, but also prevent excessively high LRA and be dialed into the unit's rated FLA, which will offer running protection.
If fused properly (within close tolerances of the running load amperes), the problem of Single Phasing a Three Phase Motor can be covered.
Once again as Bennie stated, the Locked Rotor starting situation is similar to a Short Circuit, which will tend to blow out at least Two fuses (connection is L-L). If the fuses are sized too large, they may not react in time to an attempted 1? start (or fried rotor), and results in a loss of Smoke from the Motor's Windings.
Lose too much smoke and the flames may be released too!
If a Polyphase (3?) Induction Motor experiences a Single Phase situation while it's running, the lines which are still "Live" will draw an excessive current, while the "Dead" line will have normal or lower current flowing in the Motor's winding, and outward to whatever it can flow through.
Voltage will be near system voltage.
Proper fuse size selection will blow the fuse(s) in the "Live" lines.
Just my $0.02
Scott35
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