For a HVAC mini split condenser rated for 240v and minimum of 26Amp and max fuse 40 AMP what should be the disconnect sizing 30 or 60 AMPs,
I am looking to hookup the mrcool 27k condenser which is rated as above, all the disconnect i see on the market is either 30 or 60, would 30AMP disconnect suffice ? or should i search for 40 AMP disconnect for this in particular? 60 AMP seems oversizing.
You will need a 60A disconnect since fuses larger than 30 will not fit in a 30A disconnect. 60A is the next size larger fuse chassis.
NEC 440.12 requires the minimum rating of the disconnect to be 115% of nameplate, so a 30 disconnect would meet the bare minimum, but blowing fuses would be a real risk when brown-out or other low voltage events occur.
Edit: Online I do not find a 27k unit that matches 26A, but on the 27k Gen 3 MrCool the instructions say switches are required to be rated at 1.5x the rating of the unit. The instructions are part of the UL (or other testing lab) approval, and the NEC requires installing as instructed. Check your instructions to see if similar size requirement is given.
Also beware the instructions may give a chart that specifies minimum wire size. This size may be adequate to comply with NEC 240.4(G) if using THWN wires in a raceway, but the NM "Romex" section of the NEC limits current to the 60°C column of 310.16, so running NM in the interior of your house for a 21A rated minimum circuit ampacity would require #10 NM cable even if the instructions say minimum wire size of #12.
A disconnect is not a protective device for the wires or equipment. It can act in that way, but the primary purpose is so that you can definitively turn off the equipment within sight of the equipment for safety purposes during repairs and maintenance.
A disconnect also serves as a way to disconnect power during an unrelated emergency such as a fire so that there are no live wires in a dangerous area. That is the disconnect for an entire service as required in the latest NEC. Although generally for other reasons that is done using a meter main, it can also be done using a disconnect.
Just like the disconnect for your entire service can't be a 60A disconnect if your service is larger than 60A (which is nearly always the case), you can't use a 30A disconnect for equipment on a 40A circuit.
So the real question is: What size is the circuit for this equipment?
26A can be the maximum current draw or it can be the continuous current draw. If it is the maximum current draw then, in theory, you could use a 30A circuit breaker and 10 AWG wire. However, I suspect it is the continuous current draw, in which case you have to size the circuit to be at least 32.5A, which means at least a 35A breaker. 35A breakers are a thing. However, for a bunch of reasons including ampacity ratings on copper NM cable and aluminum wire and the ability to allow for future larger equipment, a 40A circuit is very common, and the equipment ratings allow that.
Which means most of these installations will be done with a 40A breaker and appropriate cable/wires (e.g., 8 AWG NM or UF cable). Which means the disconnect must be at least 40A. Disconnects are available in only a few sizes because the primary protection, with a very specific rating to match the wire and equipment, is provided by breakers in the panel and not by the disconnect.
Because these disconnects are so standard - only 2 sizes (30A or 60A), needed for every HVAC installation (and sometimes for other types of equipment), they are really very inexpensive. Looking at Home Depot, I see a range of ~ $ 15 to $ 30, depending on brand, 30A vs. 60A and some other factors. Not a lot of money compared to the HVAC equipment or even wire costs. The only ones that cost more are ones with a convenience GFCI receptacle included. That is actually a nice way to also take care of the requirement (relatively new) of having an available 120V receptacle within a certain distance (I believe 25') of HVAC equipment, which can be used for refrigerant recovery or other servicing needs.
This is actually the exact same issue as "main panel breaker size" when using a "main panel" as a subpanel for an outbuilding. The main breaker in the panel is only functioning as a disconnect. So you might have a 60A or similar feed, depending on how much power you need in the outbuilding and on how much power is available in the service after doing a Load Calculation on the main building, but the panel in the outbuilding can easily be a 200A panel with a 200A main breaker. And that's just fine.
