ptonsparky
Tom
- Occupation
- EC - retired
Sizing the disconnect fuses for a 3ph. 208V. 1.5HP motor pump
- Thread starter zappy
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Do you mean OL are not built in for 3 phase motors this size?
kwired
Electron manager
- Location
- NE Nebraska
Almost never built in on a general purpose motor. Some OEM's maybe.
Single phase motors is pretty common to see built in overload protection on most general purpose motors. When I run into single phase motors without overload protection they seem to be more of the definite purpose type motors or OEMs and not general purpose motors.
The single phase general purpose ones I have been seeing more of without OL lately are 10 HP and are dual volt 240/480 and am seeing them installed on farm equipment , particularly conveyors, augers and such.
I used to work routinely with fractional HP 3 phase motors. I’ve never seen one with integral OL protection.
kwired
Electron manager
- Location
- NE Nebraska
About only ones I have seen were on some agricultural fans of 1 hp and less. Was Klixon bimetal type like you often see on single phase motors. What they do is put it at the wye point connection of the motor windings, then it will work for either voltage in a dual volt motor, on low volts is only in one of the two "wyes" but still gets the job done.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yeah I have only worked with larger 3 phase motor and they did not have built in OL but I assumed the smaller motors did... I be wrong lol
You can’t do that. The fuse curve doesn’t match the motor curve. That is why you have two devices. The fuse will either nuisance trip on starting or fail to protect against overloads which are fairly long time events. At best you might get away with a time delay fuse at 150% such as a CC then use a motor with a Klixxon thermal switch.
kwired
Electron manager
- Location
- NE Nebraska
The Klixxon switch should be all that is needed for overload protection, the fuse is for short circuit/ground fault for the most part if you have the Klixxon switch, but would not need to be selected at any particular setting other than be no more than NEC maximums.
ptonsparky
Tom
- Occupation
- EC - retired
The advantage of the Klixxon is they eventually quit working after being reset often enough.
kwired
Electron manager
- Location
- NE Nebraska
Kind of a sign of either under sized motor or undersized equipment it is driving as well.
the fuse rating is equal to 1.25 times the full load current. V(V-L) = Line to Line Voltage in volts. For continuous operation, the fuse rating is less than the 125% is not recommended since all the motor is designed to run 120% of the full load rating.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
- Status