I'm looking to buy a DIY mini-split system powered by 230 volts +/10%. My shop already has a 230 volt circuit (actually around 245) with 12 gauge wiring protected by a 20 amp breaker. I also have a disconnect box on the outside wall. The vendor is telling me I should use a 15 amp breaker to protect the equipment. My understanding has always been that a breaker is sized to protect the wiring. But the vendor makes it sound like it's sized to protect some internal component(s) in the outdoor or indoor units. Comments?
AC power for mini-split AC system
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The breaker is sized to protect the wiring (with the caveat that hvac wiring can be sized differently than general purpose loads, to make sure motors start w/o tripping). The disconnect is usually fused to protect the equipment, which also allows a machine with smaller rating to be attached to a circuit that's oversized (common when replacing old inefficient equipment.). also, note that a fair amount of HVAC equipment has a nameplate requirement for a fuse -- and that means an actual fuse, not a circuit breaker. the name plate will say 'max fuse 15A' (or whatever the actual rating is), if that's the case. If it says 'max overload protection 15A', you can protect with a breaker.
dave*99
Well-known member
I believe HACR rated breakers (most new breakers are rated HACR) are then norm. New equipment is unlikely to specifically require a fuse. It was commonplace years ago.
the super high seer mini split I just installed requres fuses. I don't know if that's just conservative design, or what, but it's still pretty common.
wyliesdiesels
Well-known member
the make and model of your equipment would be helpful
dcg9381
Well-known member
You use it to protect both. When I built our shop, I ran 10ga out to one of the AC disconnects. Turns out the 24K units spec a 20A breaker. Nothing wrong with using 10ga wire on a 20A breaker.
Pioneer WYS012GMFI22RL-10
Spec says 208-230, but somewhere else I found it listed as 230 +/- 10%, and a tech guy confirmed it. It's a pretty common spec. Same as my Puma air compressor.
wyliesdiesels
Well-known member
the voltage rating for circuit sizing doesnt matter
the stats i need to know are MCA and MOCP
for yours its 13a MCA (minimum circuit amps) which means #14 wire & 15a for the MOCP(maximum overcurrent protection)
for the MOCP it does state max fuse 15a so you will need a fusible disconnect at the unit with 15a fuses.
reference
EDIT: forgot that the 125% was already included in the MCA. my bad
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wyliesdiesels
Well-known member
not true even today. and the OPs unit does specify fuses..
wyliesdiesels
Well-known member
not always the case. Fuses react faster than breakers. and in the OPs case, they are required.
dave*99
Well-known member
I looked at the installation manual from the OP's Pioneer unit. Found this on page 16.
Looks like either a fuse or HACR breaker can be used.
The MCA already includes the additional 25% for the largest motor (compressor) plus 100% for any other smaller loads.
The MCA is the minimum ampacity requirement w/o further upsizing due to the load size or continous use. Distance and elevated temps can still increase the MCA wire size.
(MCA is just the old multiple motor feeder sizing calc = 125% of largest motor + sum of any othe misc loads each @100%.)
The MCA is the minimum ampacity requirement w/o further upsizing due to the load size or continous use. Distance and elevated temps can still increase the MCA wire size.
(MCA is just the old multiple motor feeder sizing calc = 125% of largest motor + sum of any othe misc loads each @100%.)
So, for that specific Pioneer mini-split, it looks like I'll need a disconnect with a 15 amp fuse on each hot leg? I'll check mine in the morning to see if it has a breaker or fuse(s). It was installed by the electrician who wired my shop, and it's been a while since I looked inside.
sparky 1971
Well-known member
It's probably just a cheap pull out style with neither breaker or fuses.
wyliesdiesels
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yeah whoops brain fart. forgot that the 125% was already included...
dave*99
Well-known member
I'm curious to see what the nameplate on your outdoor unit states. The installation manual for your unit states you can use a fuse or disconnect. Can you post a photo of the nameplate?
The way I interpret your situation is:
If your electrician already installed a FUSED disconnect then put two 15A fuses in it. Keep the 20A breaker in the panel.
If there is no fuse in the disconnect, then swap out the 20A breaker in your panel with a 15A breaker.
I just went through this in my house. Electrician prewired a 30A circuit on 10ga. wire. When the mini-split arrived I installed a simple disconnect and swapped the breaker out for a 20A because the installation manual called for 20A MOCP.
This article is old, but it discusses a number of the items covered in this thread.
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I haven't bought the A/C system yet, so can't post a photo of the nameplate. Turns out the disconnect is only a switch, so I guess my best bet is replacing the 20 amp breaker with a 15 amp. But I'll wait until I get the A/C system to decide what to do.
dave*99
Well-known member
That may be able to be replaced with a breakerI haven't bought the A/C system yet, so can't post a photo of the nameplate. Turns out the disconnect is only a switch, so I guess my best bet is replacing the 20 amp breaker with a 15 amp. But I'll wait until I get the A/C system to decide what to do.
I just installed a similiar minisplit, with a rebranded midea compressor. the manual says 'max overcurrent', the name plate says 'fuse', so I don't trust the manual. (the manual covers six or 10 different models in the line, which might be part of their confusion. ) Fuses are still specifiied in new designs, because the trip curves are different from breakers, and they can blow faster than a breaker trips, which helps protect electronics. A suitable disconnect and fuses costs less than $50, so it's not a big deal if he was to swap out the unfused disconnect for a fused one.
dave*99
Well-known member
Extra protection is good. Mini splits are used all over the world. How many fuses would you want it this was your power source? 
Been a while since I used one, but they may have a 10 AWG minimum wire size, and the wire entry needs a strain relief connector, not a anti-short bushing.
Gotta wonder how many of any type A/C compressors are wired properly. In my experience, it's been one of the installers who does the wiring. At my last house we replaced two very old 2.5 or 3-ton units with a single 5-ton zoned system. The installers used one of the existing power cables, which (as I recall) was 2 sizes too small. The only reason I found it was the existing breaker was also undersized and tripped a couple times. And I actually had to argue with the company's owner to get power cable and breaker upsized.
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theoldwizard1
Well-known member
This may be true, but fuses are still very slow !
You can run maybe 10%-25% OVER a fuses rating for MINUTES before it will blow !
dave*99
Well-known member
Looks like 14 AWG minimum, assuming this is the correct part number.
A QO breaker can be installed in place of the switch.
wyliesdiesels
Well-known member
can do the same with breakers... look at the trip curve chart for a breaker...
A QO circuit breaker will supply 120% load for nearly an hour before it trips, depending on the ambient temperature of the panel. The UL standard breakers are listed under allows 1800 seconds to trip at 200% of rated capacity. (Everything on the market today trips much faster).
American Locomotive
Well-known member
Do they? A Square-D QO breaker is can trip in less than 1 AC cycle in a short circuit situation. Can a fuse pop that fast?
dcg9381
Well-known member
On the DC solar side, we have fuses that are designed to quickly react and those that are designed with a much longer reaction curve. I've never used fuses AC side - are they all the same? I can't imagine that the ancient screw-in fuses that I've seen are the same as a modern AC fuse.
Here is Bussman's explanation of UL 1995 requirement. There are many types of fuses available with different time curves, interrupting capacity, voltage levels.
IMO, the mfrs should be spec'ing the fuse time curve when mentioning fuses only otherwise someone could put a long time delay style in or instantaneous trip. There are HACR named & rated fuses and others stating the equivalent to HACR rated.
A HACR rated cb and today's equivalent are understood to meet the expected time curve.
IMO, the mfrs should be spec'ing the fuse time curve when mentioning fuses only otherwise someone could put a long time delay style in or instantaneous trip. There are HACR named & rated fuses and others stating the equivalent to HACR rated.
A HACR rated cb and today's equivalent are understood to meet the expected time curve.
wyliesdiesels
Well-known member
were talking about overload protection for the equipment not short circuit protection...
desmopower
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I think the short answer is that the mini split needs either a 15A breaker or 15A fuse somewhere on the circuit. That was the same for the unit I installed. Also, I know your wiring is already done but 14ga should be acceptable for the load.
Seems like the best approach would be to either change out that switch for a breaker or change the disconnect to a fusible type, so you do not need to reduce the 20A supply to your entire shop.
Is that what you meant, that your entire shop is on a 240V / 20A?
Seems like the best approach would be to either change out that switch for a breaker or change the disconnect to a fusible type, so you do not need to reduce the 20A supply to your entire shop.
Is that what you meant, that your entire shop is on a 240V / 20A?
No, this circuit is dedicated to a mini-split. I had it installed when the shop was built. I went with 240V/20A, just to ensure I'd have a choice of mini-splits. Also, the unit will draw half as much current as its 120 volt brother. But I do have another 240V/20A indoor circuit to power an air compressor and MIG welder.
The shop also has two 120V/20A circuits for power tools, etc, and and two 120V/15A circuits for lighting and garage door opener. The slight overkill comes from dealing with too many places with one 15 amp outlet and a single light bulb socket in the garage.
Now I just need to move all my stuff from my attached garage to this one!
desmopower
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Ok, in that case I'd just change the breaker to a 15A and not worry about changing out the disconnect. You know the wiring can handle the 15A since it was installed for 20A.