Thermal-Magnetic Breakers vs Time Delay Fuses

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fifty60

fifty60

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For short circuit protection on motor applications, the NEC allows you to use a larger value (250% of FLA) when using a Thermal Magnetic Breaker than when using a Time Delay Fuse (175% FLA). Does this mean that you SHOULD use a larger value breaker than you would a fuse?

For example, if I have a motor that is operating well (no nuisance trips) with a class CC fuse sized at 175% FLA, and I want to switch to a thermal magnetic breaker, can I expect it operate just a well at 175% FLA?

Or do no nuisance trips at 175% FLA for a fuse not translate to no Nuisance trips at 175% FLA with a breaker? Does the 250% rating allowance for a breaker only mean that I CAN size it that large and still be safe per the NEC, or should I expect to have to size my breaker larger than a fuse.

Thanks in advance.
 
Looking at only the amp rating of devices is not sufficient for evaluating their performance.
You need to compare their individual Time Current Curves (TCCs). A TCC is a plot of how long a device can carry an amount of current. Most fuse catalogs have TCC's for the popular fuses, while most circuit breaker curves are available from their on-line 'tech support' library.

For short circuit protection of motors I would want to compare how the circuit breaker and the fuse perform at at least 3 different points: such as 10x the FLA of the motor, 20 seconds, and 0.1 sec.
 
It is of my opinion that for the best motor protection a combination motor started is used consist ion of a an overload relay with the correct heaters or an electronic OLR and a motor circuit protector (MCP) magnet only circuit breaker set per NEC 439-52. I usually comment that the MCP be set just above the motor inrush peak current just enough to prevent nuisance tripping.
set as such should the MCP trip it is with almost all certainty that it tripped because a motor winding started to fail resulting from a line to ground fault. Commonly such a failure escalates as the arcing starts to increase. As I recall it was Westinghouse developed the mag only breaker (MCP) in order to provide more comprehensive motor protection to better prevent fores resulting in motor failure. It is also of my opinion that a properly set MCP is able to provide motor short circuit protection which is far superior than that which can be provided with fuses. When an MCP pick up a fault an initiates a trip it clears the fault in a few cycles.
As such a combination motor starter consisting of a properly adjusted MCP, contactor, and an OLR would provide motor overload and motor circuit protection.
 
Motor inrush current is also impacted by transformer impedance, conductor sizes types and lengths and other loads and how they effect overall impedance of the supply. Low impedance in the supply means more available inrush current and lower setting overcurrent devices have greater risk of tripping than same device type with a higher setting.

This kind of activity is easy to see sometimes when on construction sites. Sometimes a power tool will throw breaker when attempting to start. Plug same breaker into an extension cord and breaker holds. The increased resistance in the cord is enough to keep the inrush low enough to remain below the breakers trip curve through the whole starting cycle. Of course if you add too much resistance there will be a point where time becomes a factor in tripping the breaker if the motor doesn't accelerate fast enough.
 
To directly address the "Can vs Should" question, I direct you to Jim Dungar's signature line.

In a combo starter, the fuses or CB are TYPICALLY only there to provide the instantaneous short circuit protection, because the Overload Relay is taking care of the long-time thermal over current protection of the motor windings and if the proper conductors were used, they are protected as well. So to that point, the next important issue is coordination with the next device up stream, so that any fault is cleared as close to the fault as possible. With a CB that has adjustable instantaneous trips, such as an "MCP", that is more easily accomplished than with fuses.

BUT, you cannot "role your own" MCP combo motor starters, because MCPs can ONLY be used in a tested and listed factory assembled combination motor starters. So if you are making one yourself in the field, that's where the 250% limit on a CB comes to play, because you are forced to use a Thermal Mag breaker, and not all thermal mag breakers have adjustable magnetic trips. In a TM breaker without adjustable mag trips, in order to GET the mag trip setting high enough, you usually MUST use a larger breaker, ergo the higher allowance in the NEC. So if you have a TM breaker with an adjustable mag trip setting, then as long as that setting range is sufficient to avoid nuisance tripping while fitting into the other rules, use whatever is smallest. If it is non-adjustable, you will likely be hard pressed to use anything smaller than the 250% from my experience anyway.

Fuses come with the primary weakness of causing single phasing, so they require added phase loss protection in 3 phase motor controls, which is often ignored because of cost and leads to motor failures. But as a general rule, fuses can be useful in current limiting of very high available fault currents. The average CB is good to maybe 65kAIC, the average motor fuse will be good to 200kAIC. The instances where that is critical however are not as prevalent as the fuse mfrs want you to believe.
 
for small motors it does not matter much. use the largest breaker you feel comfortable with that is allowed by the NEC. you cannot coordinate these size breakers reliably anyway.

for larger motors it is desirable to coordinate with upstream devices. having said that, I rarely do as most of the time I have no idea what is upstream.
 
Jraef said:
To directly address the "Can vs Should" question, I direct you to Jim Dungar's signature line.

In a combo starter, the fuses or CB are TYPICALLY only there to provide the instantaneous short circuit protection, because the Overload Relay is taking care of the long-time thermal over current protection of the motor windings and if the proper conductors were used, they are protected as well. So to that point, the next important issue is coordination with the next device up stream, so that any fault is cleared as close to the fault as possible. With a CB that has adjustable instantaneous trips, such as an "MCP", that is more easily accomplished than with fuses.
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Click to expand...
It is to be noted that any of the individual components of a combination motor started do not have an interrupting rating included/as a part of their nameplate rating. However, an enclosed listed combination motor started will have an interrupting rating included normally placed on an inside surface of the enclosure. UL tests are done with these items mounted in an enclosure.
 
Thanks everyone. Article 440 words the OCPD differently for Hermetic Compressors. The 250% option for thermal magnetic breakers is not there. The way I read it, regardless (fuse or circuit breaker) I have to start out at 175% of the RLA, and then I can increase it to 225% RLA if needed. Can anyone explain why there is a difference between the 2 articles when it comes to max OCPD sizing? Does 440 just not go into as much depth as 430? Also, I have a possible scenario that I would like advice on. If my RLA is 16A, that multiplied 1.75 is 28A. Do I have to round down to the 25A breaker if I think that may be too small, or is it within the spirit of the NEC to go up to the 30A breaker as a preventative measure. The 30A breaker is well within 225% RLA. If I think rounding down may cause nuisance tripping in the field, does that mean I can then increase within the 225% limit. Or does it actually have to fail in the field with the 25A breaker?
 
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