Determining Fuse Size

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brockbone

Member
Location
Glendale, CA
I need to replace 3 fuses in a disconnect adjacent to the switchgear behind a grocery store.
The disconnect is 3 phase 208V and feeds a large Evaporative Cooler /Condenser on the roof.
The ?spec? plate of the control cabinet on the roof is obliterated.
The disconnect feeders that connect to the roof cabinet are 1/0.
The cabinet feeds a:
20 HP Baldor Super E,
200V useable at 208V,
55.2 AMPS,
3PH,
NEMA NOM. EFF 93%,
40 degrees centigrade
....And a 3HP, 3PH, 208V, 8AMP pump motor

From the "prints" in the MCC room I have:
MCA. 395
MOPD 502
FLA 368.1
What size/type fuses should I install in the disconnect?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If you disconnect is feeding the entire cooler and it has a MCA of395 amps and your wire size is a 1/0 (not parallel) you have a bugger problem than detrmining fuse size.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If you disconnect is feeding the entire cooler and it has a MCA of395 amps and your wire size is a 1/0 (not parallel) you have a bugger problem than detrmining fuse size.

Why would a 20Hp motor need 395A?

I'd bet that the OP is reading the MCC print incorrectly or the print is wrong.
 

brockbone

Member
Location
Glendale, CA
Measured Amperage

Measured Amperage

The situation was that I was called at the request of the refrigeration tech to deal with a blown fuse on B phase.
By the time I got there he had replaced a 125 Fast-Acting fuse with a 150 timed delay.
He claims when he opened the disconnect A PH had a 150 Timed delay and C & D had the 125s.
He then proceeded to replace the last 150 with a 125.
The company that manufactured the equipment is no longer in business, and yes it is a variable speed motor, the 20HP one, and is feed on what looks like #4?s the pump on 12?s.
 

brockbone

Member
Location
Glendale, CA
Correction

Correction

The situation was that I was called at the request of the refrigeration tech to deal with a blown fuse on B phase.
By the time I got there he had replaced a 125 Fast-Acting fuse with a 150 timed delay.
He claims when he opened the disconnect A PH had a 150 Timed delay and C & D had the 125s.
He then proceeded to replace the last 125 with a150.
The company that manufactured the equipment is no longer in business, and yes it is a variable speed motor, the 20HP one, and is feed on what looks like #4?s the pump on 12?s.
 

brockbone

Member
Location
Glendale, CA
Measured Amperage cont..

Measured Amperage cont..

With a clamp on meter I get 18 to 50AMPS at the disconnect and on just the feed legs from the cabinet to the motor I get 10 to 40AMPS
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
So you have a 200 amp fused disconnect feeding 23 HP of total motor load?

It sounds like you need fuse reducers and a megger to me.

Art. 430 explains how to size motor feeder overcurrent protection. Art. 440 for HVAC equipment.
 

brockbone

Member
Location
Glendale, CA
125 amp max?

125 amp max?

This equipment, in this configuration, has been operating for many years and since I am not an electrical engineer I don?t want to redesign it.
And since the feeder wires from the disconnect to the roof are over 80' I wont be re-pulling them.
And since that wire is 1/0 I think 200's are out of the question, right?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
So you have a 200 amp fused disconnect feeding 23 HP of total motor load?

It sounds like you need fuse reducers and a megger to me.

Art. 430 explains how to size motor feeder overcurrent protection. Art. 440 for HVAC equipment.

why do you need fuse reducers? the overcurrent protection for the motor is provided by the drive.
 
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