hvac disco question

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mayjong

Senior Member
hey all-
this sounds silly as i type it, but here goes.
at a hvac compressor disco, the name plate rating has a "minimum circuit ampacity" of 26A, and a "maximum fuse size" of 40A.

i understand that the wire size can be based on the min circuit amps. (don't want to start that argument!)

my question is about the maximum fuse size-
does the fuse size need to be 40A? could it be 35A? why not 30A?

i have always "called" it as "maximum overcurrent protection required at condenser" (per manufacturers instructions?)

is there anywhere (code/install instructions/etc?) that actually REQUIRES a 40A fuse, if the nameplate rating is "maximum 40A"???

thanks!
 
so-
just for kicks-
what would happen if i throw in a 30A fuse. (i know it would work for a while, but what are the potential problems?)

btw- i don't want to use a 30A, the contractor has installed it, and doesn't want to change it. i cannot justify it with code, and i cannot even FIND a manufacturer install guide (for ANY manufacturer) .

btw Augie, i had to look at where you come from, because this contractor did EXACTLY that about a month ago...funny stuff
 
mayjong said:
so-
just for kicks-
what would happen if i throw in a 30A fuse. (i know it would work for a while, but what are the potential problems?)

btw- i don't want to use a 30A, the contractor has installed it, and doesn't want to change it. i cannot justify it with code, and i cannot even FIND a manufacturer install guide (for ANY manufacturer) .

btw Augie, i had to look at where you come from, because this contractor did EXACTLY that about a month ago...funny stuff
Click to expand...

Post #2. Inrush current.
 
ya, i saw that

"issues" can mean anything...
the obvious is the fuse burning out, are there any other "not so" obvious problems?
thanks
 
It may not and probably will not be an issue as long as you use time delay fuses. The unit should not draw anything close to MCA. If the unit blows fuses in the future it is probably because something is not right with the unit.
 
mayjong said:
so-
just for kicks-
what would happen if i throw in a 30A fuse. (i know it would work for a while, but what are the potential problems?)

btw- i don't want to use a 30A, the contractor has installed it, and doesn't want to change it. i cannot justify it with code, and i cannot even FIND a manufacturer install guide (for ANY manufacturer) .

btw Augie, i had to look at where you come from, because this contractor did EXACTLY that about a month ago...funny stuff
Click to expand...

I'd say he slid under the wire, Code-wise, by 0.1 amps., with a 30 amp fuse I'm assuming a 30 amp disconnect. 440.12 requires the disconnect to be 115% of the selected current, or in this case, 29.9 amps.
Personal opinion: the 30 is asking for problems but not a violation.
 
I apologize, I should have been clearer or as my son loves to say more clearer. :grin: As the unit ages the compressor has a harder time starting and may even draw a few more amps then it did when it was new. If you are with .1amps as Gus says then I could see this breaker tripping years down the line when in fact it may do just fine if a 40 amp breaker were installed. I always use the max.
 
The MCA includes 125% of the RLA of the compressor plus the RLA of the fan motor. The MOP is 225% of the RLA of the compressor plus the RLA of the fan motor rounded down to the nearest fuse size. The MCA is also the continuous amp draw the internal overload is suppose to trip at.

EDIT; I should say the MCA is the maximum amp draw the overload should trip at. Some manufacturers use lower values for their overloads.
 
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augie47 said:
I'd say he slid under the wire, Code-wise, by 0.1 amps., with a 30 amp fuse I'm assuming a 30 amp disconnect. 440.12 requires the disconnect to be 115% of the selected current, or in this case, 29.9 amps.
Personal opinion: the 30 is asking for problems but not a violation.
Click to expand...
Aren't the rating of the disconnect switch and the rating of the fuse two different things? The fuse is designed to safely handle overcurrent..
 
mrd999 said:
Aren't the rating of the disconnect switch and the rating of the fuse two different things?
Click to expand...

Yes they are but once you go higher than a 30 amp fuse you will need a 60 amp disconnect.

Roger
 
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