Sizing the disconnect fuses for a 3ph. 208V. 1.5HP motor pump
- Thread starter zappy
- Start date
- Status
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
You start with the motor name plate amps
300 % is way to large, for TD fusescircuit breaker is 250% max
I would size about 125 % if it had a constant load
300 % is way to large, for TD fusescircuit breaker is 250% max
I would size about 125 % if it had a constant load
All the nameplate has is the horsepower and the voltage.
- Location
- Connecticut
- Occupation
- Engineer
Time delay fuse max rating would be 175%.
6.9A x 1.75 = 12.1A. A 15A TD fuse would be allowed (next size up.)
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What size OCPD is ahead of the disconnect?
SSDriver
Senior Member
- Location
- California
- Occupation
- Electrician
If the motor doesn't show amps I would go off the NEC chart 430.250. It shows 6.6 amps.
6.6 x 175%(TD fuse) = 11.55A
Next size up is a 15 amp time delay fuse.
6.6 x 175%(TD fuse) = 11.55A
Next size up is a 15 amp time delay fuse.
kwired
Electron manager
- Location
- NE Nebraska
Actually all motor calculations are supposed to use NEC table ampacity unless you have something that doesn't fit within the chart and the conditions mentioned on the chart. Only thing you use nameplate information for is selecting overload protection.
One flaw you have there (though you supposed to use NEC table values like I mentioned) is you didn't consider efficiency and power factor. 1.5 HP is the output shaft power which is 1119W, but the input to the motor will be higher because of these inefficiencies.
SSDriver
Senior Member
- Location
- California
- Occupation
- Electrician
Yes I agree, I was assuming that he was using the fuses as their source of overload. I could be wrong, the disconnect could be going to a starter but we don't know at this point
kwired
Electron manager
- Location
- NE Nebraska
Well sizing them at 300% doesn't exactly give any overload protection either.
SSDriver
Senior Member
- Location
- California
- Occupation
- Electrician
Lol I sized it at 175%. But I should have mentioned that he needs to size it at 115% or 125%(depending on motor type) for overload.Well sizing them at 300% doesn't exactly give any overload protection either.
30A
Yeah the fuses are just for overload protection
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Bussman and other fuse mfg have intermediate size fuses, I would look at a 10 amp TD fuse
I would size fuses at 125% to back up overloads that were 115%.
Often we see small motors protected with a circuit breakers, I feel fuses give better protection, but how much does it cost to replace motor?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Actual overloads provide better protection than fuses.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Yes, there were overloads as usual. The fuses were sized higher than the OLs setting
kwired
Electron manager
- Location
- NE Nebraska
If in your application this is the overload protection 30 amp fuses is not going to provide the protection. Neither is the 16 amps you calculated in OP. This be a motor that probably rated around 6-7 amps, you want it to trip if it goes over that level for very long and probably won't want over 7 or 8 amps fuses, exact selection will be dependent on actual nameplate amps.
ptonsparky
Tom
- Occupation
- EC - retired
Is this a three phase motor? We are assuming at the moment.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The fuses are for ground fault and short circuit protection. OL protection is often built into the motors of this size. The OL protects for over current.
ptonsparky
Tom
- Occupation
- EC - retired
Single phase, yes.
- Location
- Connecticut
- Occupation
- Engineer
The title of the thread says it's a three phase motor.
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