RFS/MOCP

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rose1981

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The electrical data on a condenser unit indicates RFS -30Amp amd MOCP - 42/38Amp. Would it be ok if I go with a 30A fused disconnect switch with 30A fuse in it?
 
Based on 440.12 my guess would be "no" but I am not familiar with an electrical acronym "RFS".
Does the nameplate have "MCA" noted.
 
If manufacturer specify maximum recommended fuse size, he did work for us by calculating fuse per section 440.* i have other question:*
I read in some forum that reads: if nameplate doesn't specify the recommended size of c.b. it means manufacture didn't test the equipment with branch c.b. and we cannot use C.B. for branch circuit. what is your opinion for using HACR C.B. for branch circuit if manufacturer indicates only fuse size on his nameplate?*

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iwire said:
In my opinion that would be a violation.


As a side note, most, if not all molded case breakers are now HACR.

http://static.schneider-electric.us...n/Molded Case Circuit Breakers/0600DB0102.pdf
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I would agree. If the nameplate only mentions "fuse" and fails to mention "breaker" or "over current protection (non-specific)" then to meet mfg. specs you would need to protect the equipment by a fuse. I see that about on about 5% of my inspections, normally refrigeration equipment.
 
Forgot to mention in the post. The unit is rated at 208V, 3ph. MCA is 25amps, recommended fuse size (RFS) is 30Amps and MOCP is 42Amps.
Would it be ok if I go with a 30A fused disconnect switch with 30A fuse in it?
 
rose1981 said:
Forgot to mention in the post. The unit is rated at 208V, 3ph. MCA is 25amps, recommended fuse size (RFS) is 30Amps and MOCP is 42Amps.
Would it be ok if I go with a 30A fused disconnect switch with 30A fuse in it?
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I have never seen "recommended fuse size" on a piece of equipment. Make up your mind.

MCA is what size wire I use. MOCP is what I tie it to.
 
In my opinion, I would go for 40AF/60AS (i.e 40Amp fused, 60Amp switch)

Based on the given:
208V, 3-ph, MCA = 25Amps

1) To check for MOCP = (1.75 to 2.25)(motor compressor full load current) per NEC440.22(A)
Find full load current first,
MCA = (1.25) (full load current)
Full load current = (25A)/1.25 = 20A

MOCP = (2)(20A) = 40A fuse; (used 2 as multiplier in this calculation)

2) For minimum disconnect switch rating = (1.15)(full load current) per NEC 440.12(A)(1) = (1.15)(20A) = 23A (minimum)

But since the MOCP in the calculation is 40A fuse, I used 60A rated switch (40AF/60AS). Let us know your thoughts.
 
ActionDave said:
I have never seen "recommended fuse size" on a piece of equipment. Make up your mind.

MCA is what size wire I use. MOCP is what I tie it to.
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Mitsubishi for one does it. They will give you a MCA, a Recommended fuse size(or fuse/breaker size), and a MOCP. Personally I would size it to the recommended size.
 
ActionDave said:
That is so weird.

How does one decide to go above the RFS? Once someone decides to go over the RFS do they go straight to MOCP or just the next size up?
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As long as you are between the minimum required and the MOCP you should be good.

I have only come across "Recommended Fuse Size" on DC units. I guess it is a case of standards not keeping up with technology. Manufactures would rather fuse a unit to protect the DC converter than worry about nuisance trips.
 
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