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oversized fuses? #13008 08/22/02 08:57 PM
Joined: Dec 2001
Posts: 444
Sandro Offline OP
Had a service call today....a customers baling machine at a clothing factory had blown a fuse. The main 600
V 60A fuses at the disconnect were fine. I opened up the control box of the unit and found original factory installed 3 - 600V td dual element 35Amp fuses and one was blown. My question is, the nameplate on the machine says max 21.5amps, why and what protection does 35amp fuses provide? Why didn't the factory use 22-25 amp fuses?

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Re: oversized fuses? #13009 08/22/02 10:54 PM
Joined: Apr 2002
Posts: 7,209
HotLine1 Offline
Let's throw out some suggestions:
1: They are not original factory fuses"
2: Some maintenance guy, or ??? only had 35 amp fuses??
3: What size is the fuse block??? 30/60 amp?
4: Does this machine have more than 1 motor?
5: What's the nameplate for? Whole machine??
6: Why are there 60 amp fuses in the main, if the plate says 21.5?
If the "factory" intended 21.5 amp max, usually a 30 amp fuse block would be installed. If the fuse block is 60 amp, then maybe the nameplate is incorrect.

Or perhaps the machine was modified, rebuilt, butchered, and none of the info you read was correct. Did you determine the cause of the blown fuse??

Further info or discussion at your convenience.


Re: oversized fuses? #13010 08/22/02 11:46 PM
Joined: Jul 2002
Posts: 175
electric-ed Offline
Does the machine have a motor? Motor circuits are permitted to be fused at 175% of rated amps (time-delay fuse) and 300% (non-time-delay).


Re: oversized fuses? #13011 08/25/02 08:07 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and

I'm thinking Motor also!


The listed load of 21.5 Amps is what the combined equipment will draw and is used in most part to figure the size of the branch circuit feeding it. This would also be the load VA shown on a Panel Schedule.

With a Minimum Circuit Amperes listing of 21.5, this places the branch circuit into the "No Less Than 30 Amps" range.

The 35 Ampere OCPD rating is either including LCL, the previously mentioned Motor stuff, or both. I'm sure you will see this listed on the nameplate under the "Max Breaker Or Fuse Size" listing

The 35 Amp fuse size will be for the Running Protection which is at the equipment [most likely via an EXO type fusable disconnect switch]. If this is printed on the nameplate, then this will be the Minimum Branch Circuit size to feed it - which translates into #8 cu, protected by no less than a 35 amp breaker - or no higher than a 40 amp breaker [without pushing into the 50 amp range and the associated discussions which accompany a termination for >60C on sizes less than #1].

Anyhow, without making things more corn-fuse-ee'n, the equipment has been figured to draw a maximum of 21.5 Amps at whatever voltage is listed, during normal operation and with "Normal Enviromental And System Ratings" [AKA not running the equipment in a closet where the ambient temperature is no less than 150C and the system voltage wanders so much that the KVA drawn is always changing].
This is listed to give the designer / Engineer a KVA load reference, plus to give the Installer a load reference to use for circuitry [like branch circuit size, voltage drop, and if you are the designer / EE - SCA calcs and Feeder / Transformer size].

The 35 Amp rating is to provide proper running protection. It includes LCL and / or Motor derating factors. This is running protection for THE EQUIPMENT AS A UNIT based on the FLA of all items in the assemblage.
This is the Highest fuse rating to be used AT THE EQUIPMENT.

The branch circuit could be #2's, protected by a 90 amp breaker in the subpanel or MSB, but the fuses in the disconnect switch at the equipment need to be no larger than 35 amp [and probably no smaller than 35 amp either!]

Clear as mud, huh? [Linked Image]

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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