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Branch Circui Protection Device UL508A

Branch Circui Protection Device UL508A

Branch Circui Protection Device UL508A

Hi all,
I have the following issues which I try to solve and for which I can not find clarification in the Standard UL508A

1. Sizing Branch Circuit Protection Device for motor load.
I have a motor and I know its hp (horse power) but I don't have FLA (Full Load Amper) on nominal plate. What to do in this case when hp value doesn't correspond to values from table 50.1. For example my motor has 6hp. Should I take next bigger value from the table? Or Is it allowed just to calculate FLA assuming that I know all parameters required to calculate FLA (Power, Voltage, Power Factor, Efficency)? How does the UL 508A stand to this approach?

I found the following equation (I don't know if it is correct) FLA=746 x hp / V x Eff x PF
Eff - efficiency factor
PF - power factor

2. Proper selection of BCPD.
Is it possible to use as a branch circuit protection the Miniature Circuit Breaker with appropriate characteristic like FAZ-D... for my load or Motor Circuit Breaker like PKZM0... in branch consisted of Motor->Overload Realy->Contactor->BCPD. In my case becuase of requirements of my client I can not use neiter Self-Manual Motor Controller (Type E) nor Type F Combination Controller. I have some doubts becuase I can not conform my suspections that these devices can be used as BCPD in this application.

RE: Branch Circui Protection Device UL508A

You may want to look in the NEC instead. Without nameplate data, but knowing the motor is 6 HP, single phase; assume 1000 VA /HP = 6000 VA in your case FLA = 6000/240 = 25 A
NEC Table 430.52 says max inverse CB = 2.5 x 25 = 62A or 70A next higher size. If it is a standard motor in a standard application, you can probably make the OCP device 50A or 60A. If I understand your 2nd question correctly, the answer is yes if the OCP device exhibits the inverse time/current performance. Alternately, you can use the other devices shown in Table 430.52

Good luck


RE: Branch Circui Protection Device UL508A

I have always taken the tactic that when in doubt, size devices and conductors to the next highest standard HP rating, you can't go wrong that way. So in this case, use 7-1/2HP for your 6HP motor. The reason you have a "6HP" motor is simply because it is a 50Hz kW sized motor that, when run at 60Hz and US voltages, the HP is technically increased, because HP is a function of torque AND SPEED. So since the speed increases by 5/6ths, so does the HP. The torque is the same however, so the current is the same. Sizing devices / conductors for the next size up means no "interpretations" of what that all might entail. you cannot however provide proper selection / setting of the OL protection until you know the EXACT nameplate FLA rating of the motor.

As to the Branch OCPD, the device you chose MUST be listed under UL489, UL508, or UL 248 for a fuse (plus UL 512 for the fuse holder and UL 98 or 508 for the disconnect). From my experience (I used to work for K-M), the FAZ MCBs from Moeller were only UL 1077 listed as "supplementary protectors", expressly forbidden in UL 508A, paragraph 31.1.6 (a). There are several manufacturers who now offer those same style MCBs that ARE listed as "circuit breakers" under UL 489, and those are perfectly acceptable. Moeller / Eaton may very well offer them in the FAZ line, I just don't know, I haven't used their products in a long time now .

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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