Do all fuses have to be the same size?

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overkill94

Member
Location
California
In either a fused disconnect or a regular set of branch-circuit fuses, occasionally I'll see fuses of different sizes (i.e. two 25 Amp fuses and one 30 Amp fuse). As long as they're the same type, is there any reason this isn't allowed? On the one hand, as long as the maximum fuse size properly protects the wiring and/or motor then it shouldn't matter, but then I guess there could be issues with one leg tripping before another being an issue. Is there anything in 508A or the NEC that specifically forbids this or does it just fall under the generic 110.3 requirements?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
As long as the conductors and loads are properly protected by the largest fuse, I don't see any code issues. Even having the same size fuses does not mean that more than one fuse will open if there is an overcurrent condition.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In either a fused disconnect or a regular set of branch-circuit fuses, occasionally I'll see fuses of different sizes (i.e. two 25 Amp fuses and one 30 Amp fuse). As long as they're the same type, is there any reason this isn't allowed? On the one hand, as long as the maximum fuse size properly protects the wiring and/or motor then it shouldn't matter, but then I guess there could be issues with one leg tripping before another being an issue. Is there anything in 508A or the NEC that specifically forbids this or does it just fall under the generic 110.3 requirements?
I don't think it's a problem. I have seen discos on 240V high leg services where the high leg fuse was a lot smaller than the other two.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
There is no code issue as long as the fuses are not oversized.

Whenever I see different size fuses, or those from different manufacturers, I think of 'cheapest maintenance' being performed. Someone installed the cheapest fuse they could find or whatever they had on hand, usually without any thought for carefully selected coordination or any other issue dependent on the specifics of each fuse.
 

overkill94

Member
Location
California
Thanks guys, it's been something that I've taught to be frowned upon at the least but wasn't sure if it was straight-up wrong.

Is there anything wrong with mixing time-delay with non-time delay fuses?
 

magoo5150

Member
Location
Mississippi
Is there anything wrong with mixing time-delay with non-time delay fuses?[/QUOTE]



Yes. Fuse selection is based on application. You can not just grab any fuse as long as it fits and is the same amperage. This dramatically changes arc flash duration and skews any calculations that may have been done.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
One fuse rep suggested the same brand, not just type and size be installed. No matter whose you used.

Try doing a FOP across different fuse types, mfgs, etc. All with the same load. I quit using one brand because they consistantly ran hotter and seemed to give me more grief.
 

magoo5150

Member
Location
Mississippi
One fuse rep suggested the same brand, not just type and size be installed. No matter whose you used.

Try doing a FOP across different fuse types, mfgs, etc. All with the same load. I quit using one brand because they consistantly ran hotter and seemed to give me more grief.
I agree. Trip curves are different from brand to brand.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
One fuse rep suggested the same brand, not just type and size be installed.

Just had a situation: 200A RK5 fuses 250V - load current 302A for 8 minutes.
Brand 1 takes 5 minutes to trip
Brand 2 takes 10 minutes to trip

Both brands perform within the UL tolerances for this 150% overload.
 

Carultch

Senior Member
Location
Massachusetts
Pie is spelled with an "e" everyone knows that. I can't do math but at least I can spell three letter words.


Pi the number doesn't have an e in it. The value of pie as a number would be 8.53972..., as in Archimedes' circle ratio number of pi multiplied by Euler's number e . I don't know of any applications of it.

The Greeks pronounce the letter pi the same way we pronounce the letter p. Obviously we need a way to distinguish the two, which is why we say the Greek letter pi the same as pie.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
Pi the number doesn't have an e in it. The value of pie as a number would be 8.53972..., as in Archimedes' circle ratio number of pi multiplied by Euler's number e . I don't know of any applications of it.

The Greeks pronounce the letter pi the same way we pronounce the letter p. Obviously we need a way to distinguish the two, which is why we say the Greek letter pi the same as pie.

WHAT!:? I am stunned at this news.:rotflmao:
 
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