I quite often come across a circuit that can be a nuisance.
Example system:
Pump controls, unmanned in remote area, electrical disturbances (lighting, etc.) are substantial, outages are common and therefore generator back up power is present, yet not equipped with ?bump-less? automatic transfer switching. Add to the situation VFDS to further pollute the systems power.
Next derive a 120VAC, 1P control power source from ?sketchy? 480VAC, 3P power. I?ll use a 1kVA encapsulated 480/240-120V 1P transformer (CPT) for this example with a 5A, 2P primary breaker and a 10A, 1P secondary breaker.
The nuisance in the system is the 5A, 2P CPT primary breaker. With so many combinations of power transients and interruptions, the little breaker, when asked to re-magnetize the control transformer, often trips. Even with a C or D trip curve, nuisance tripping is problematic.
Now technically a pair of time delay fuses in lieu of this 5A breaker may be a better solution, however the breaker, serving as both a disconnect and resettable protective device is preferred.
Lastly, I?ll say the wiring from the 480V main to the CPT is only a few feet and is #12.
So to table 450.3(B) I go, to see if the code will allow me to increase the ampacitiy of the CPT primary breaker. It does not, or at least I think it doesn?t.
I generally work on the power quality, surge protection etc. in these systems, but for so many years now, this question always comes up: ?how big a breaker can I put in front of my CPT??
Is there anything in the code or other rules that would allow the 5A breaker to be increased to 8 or 10A? Or is 450.3(B) the answer.
Example system:
Pump controls, unmanned in remote area, electrical disturbances (lighting, etc.) are substantial, outages are common and therefore generator back up power is present, yet not equipped with ?bump-less? automatic transfer switching. Add to the situation VFDS to further pollute the systems power.
Next derive a 120VAC, 1P control power source from ?sketchy? 480VAC, 3P power. I?ll use a 1kVA encapsulated 480/240-120V 1P transformer (CPT) for this example with a 5A, 2P primary breaker and a 10A, 1P secondary breaker.
The nuisance in the system is the 5A, 2P CPT primary breaker. With so many combinations of power transients and interruptions, the little breaker, when asked to re-magnetize the control transformer, often trips. Even with a C or D trip curve, nuisance tripping is problematic.
Now technically a pair of time delay fuses in lieu of this 5A breaker may be a better solution, however the breaker, serving as both a disconnect and resettable protective device is preferred.
Lastly, I?ll say the wiring from the 480V main to the CPT is only a few feet and is #12.
So to table 450.3(B) I go, to see if the code will allow me to increase the ampacitiy of the CPT primary breaker. It does not, or at least I think it doesn?t.
I generally work on the power quality, surge protection etc. in these systems, but for so many years now, this question always comes up: ?how big a breaker can I put in front of my CPT??
Is there anything in the code or other rules that would allow the 5A breaker to be increased to 8 or 10A? Or is 450.3(B) the answer.
