In this building I have a 3phase 480v exhaust fan. The FLA is 3amps. It has overload protection. The breaker that feeds it is on the first floor and it is a 20 amp breaker. The disconnect, starter and fan is in the attic. The disconnect is fused 20amps and the conductors are 12 THHN. I would say this is ok, but the starter on the inside where is says what size overloads to use also says maximum fuse size 10 amps. Does this mean that the fuses need to be 10 amp fuses?
Motor circuit protection
- Thread starter JdoubleU
- Start date
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eric9822
Senior Member
- Location
- Camarillo, CA
- Occupation
- Electrical and Instrumentation Tech
20 Amp fuses for a motor with 3A FLC are a bit excessive. Check out Table 430.52. 300% for nontime delay, 175% for time delay.
- Location
- Illinois
- Occupation
- retired electrician
Yes, you need to follow the instructions on the starter even if the NEC would permit you to use a larger fuse. The maximum code permitted fuse for this application would be 15 amps, assuming that this is a 3 phase 1.5hp motor.
Now if I took out the fuses and just used the breaker for protection would this be ok. Not that I would do that just creating senerios to understand the code.
- Location
- Logan, Utah
No, 110.3(B) requires that we follow the installation instructions of listed equipment. So if the instructions say "Max fuse 10 amps" Then you are limited to that.
Chris
- Location
- Illinois
- Occupation
- retired electrician
In addition to what Chris said, the 20 amp OCPD it too big for this circuit no matter what.
- Location
- Massachusetts
It's funny I was just repairing a control panel that had Allen Bradley IEC style contactors and overloads. The motors FLAs where under 2 amps and the overload settings where correct.
But the manufacturer of the control panel ignored the Allen Bradley instructions which specified 10 or 12 amp fuses ahead of the contactors. The panel is supplied with a 30 amp circuit.
Well both motors had shorts and the overload units pretty nush exploded while the 30 amp breaker did not trip.
The customer is having me add the fusing. I will try to post some pictures.
But the manufacturer of the control panel ignored the Allen Bradley instructions which specified 10 or 12 amp fuses ahead of the contactors. The panel is supplied with a 30 amp circuit.
Well both motors had shorts and the overload units pretty nush exploded while the 30 amp breaker did not trip.
The customer is having me add the fusing. I will try to post some pictures.
- Location
- Massachusetts
Here are the pictures I mentioned.
Here is the control panel, those familiar with IEC OLs will spot the 'penny in the fuse box'.
One of the contactors removed
The side of the contactor, the overload section melted.
The rear side
I pulled the overload off the contatctor, you can see the heat from the OLs damaged the contactor.
I removed the front of the overloads
I think if proper fuses where installed ahead of this there would have been no damage.
Here is the control panel, those familiar with IEC OLs will spot the 'penny in the fuse box'.
One of the contactors removed
The side of the contactor, the overload section melted.
The rear side
I pulled the overload off the contatctor, you can see the heat from the OLs damaged the contactor.
I removed the front of the overloads
I think if proper fuses where installed ahead of this there would have been no damage.
Bob, I agree with you. Without proper fusing, motor shorts often are catastrophic to starters, especially IEC starters. Installing the fuse size (and often class) recommended by the manufacturer is of paramount importance.
- Location
- Massachusetts
I hate IEC stuff, can't stand it. Plastic flimsy junk.
Of course how big would this box have to have been with NEMA parts.
mcclary's electrical
Senior Member
- Location
- VA
I see the penny,,,,instead of using the NC contacts on the overload they mated under a terminal
- Location
- Massachusetts
We have a winner.
Yeah this is a fan control panel and the HVAC guys did some work in it before I was called in.
What about the small little single phase fan motors in a commercial building. It usually has a 20 amp circuit and and 1 pole switch for a disconnect. If that is only pulling 3amps do they need to be fused or is that where the 800 or 250 percent kicks in.
When you say ahead do you mean a fused disconnect before the starter
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
We used KTK (200KAIC) ahead of small starters and FRS/TRS (before class J time delay) ahead of larger motors for short circuit protection. The European MCP's were just coming on the US market; before my employer sold to a larger outfit, we tried them with pretty good success.
Do you think this panel builder thought the overloads were adequate, or just missed the otherwise good practice specified by A-B?
- Location
- Massachusetts
It could be a fused disconnect but any type of fuse holder before the starter would work.
I am planning on adding class CC fuses in DIN rail fuse holders and wiring them between the main supply and each starter.
- Location
- Connecticut
- Occupation
- Engineer
Don, why is the 20A OCPD too big for the circuit "no matter what?" The 20A c/b is a motor feeder OCPD, if the fuses are the motor branch circuit OCPD. The motor feeder is #12 awg. A 20A c/b will protect the #12 feeder.
We have a winner.
Yeah this is a fan control panel and the HVAC guys did some work in it before I was called in.
To be clear I'm not arguing that the fuses aren't needed, but:
IF the O/L's had NOT been bypassed, I wonder if they would have survived as the extreme quick heating caused by the short circuits would have caused them to act and open the contactor. I know they are not intended to be short circuit protection, but in this case I'm guessing that if they were properly wired they would have possibly survived.
If the available fault current at the starters is high though, all bets are off.
I agree that IEC stuff is not as robust as NEMA...somewhere I had a guide wich compared life expectancy and max number of rated operating cycles and the IEC stuff was all short of what NEMA gear can take.
- Location
- Massachusetts
But ...... I will anyway. :grin: I know, it is what we do here.
I do not think so, I think this is the exact reason the fuses are required by AB and UL. IEC stuff cannot survive a short like NEMA stuf can.
Here is what I think happened here from the start.
This panel and it's fans where installed, it was powered up with no one checking the current draw on the motors or the airflow. The adjustable sheave on the motors where never adjusted and this overloaded the motors. At this point I further guess the OLs where bypassed to keep the motors on.
Not long after that the motor wiring shorted out in the motor j-box, I fund the wire nuts melted. At this point the OLs 'exploded' the first time.
The customer had the supplying contractor repale the OLs and I know they never checked the motors, the OLs blew again. Now is where I step in and offer to fix the problem for a price.
I replaced the OLs, the contactors and the motors, I get it all fired up and adjust the motor sheaves until the motors are drawing under their FLA.
All is good now except I will be going back to add fuses and disconnecting means at this control panel.
- Location
- Connecticut
- Occupation
- Engineer
I think the maximum permitted fuse for this application (assuming a 3 phase, 1.5HP, 460V motor) would be 10Amps (for a nontime delay fuse) UNLESS that rating was insufficient for the starting current, then a 12 A fuse could be used.
For a dual element fuse, 6A would be the maximum, 6-3/4A would be the max allowable fuse size, if the 6A was insufficient for the starting current.
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