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[quote=Anonymous][quote=Anonymous]Thank you, electrician friend! So our current setup is hydronic, heat-only, and we have ducts and 3 separate air handlers. The air handlers will be replaced 1-for-1. The tankless water heater that heats the water for the hydronic system runs on gas.

The HVAC guys are proposing various things, but the least ridiculous proposal involves 35A + 30a + 30a for 84,000 BTU total across 3 units (adding AC coil for each air handler). So same 84,000 BTU as you’re calculating with a high SEER unit. Now I’m wondering if this design is also constrained by the 3 separate air handlers and duct setup. The house is 2-stories but mostly 1 room deep so it’s pretty stretched out. One of the guys was adamant that higher BTUs and less air handlers/AC units would not work and would be less efficient.[/quote]

You’re welcome.

I would say that you’re definitely constrained by your ductwork and air handler layout to requiring three compressors. Just so you have it, the power consumption of an AC system is dependent on the thermal load rating in BTUs and the seasonal energy efficiency ratio (SEER) rating. The minimum allowable SEER rating is 14; higher is better.

An 84,000 BTU (14 SEER) system would draw 84,000 / 14 watts of power = 6 kilowatts. At 240V, that is a current draw of 6000 / 240 = 25A. This is the actual steady state current draw, but most AC units will have some safety buffer and accommodation for fans, electronics, and other non-compressor components. This is captured as Minimum Circuit Amps (MCA) which is usually a little under 2X the typical current draw. About 48A for an 84,000 BTU (14 SEER) system. This will determine wire gauge. Then, HVAC systems are special since they draw a tremendous amount of current when first starting up (locked rotor amps) and this translates to use of a larger sized breaker (typically ~50% greater than the MCA rating for the system). The actual breaker size used for an HVAC is specified by this and is indicated as MAX CKT BRK AMPS on your HVAC nameplate.

95A still seems a bit high, TBH, unless you’re getting really inefficient systems – should be more like 75A across three breakers. Definitely don’t even worry about upgrading to a 400A service though. Even if all three systems are running at the same time, you’re only going to pull about 30A maximum steady state. The real issue is when a particular AC system initially kicks on and draws massive current for a fraction of a second, but you have three systems operating independently, so likelihood of any 2 or 3 kicking on at the exact same time is close to zero.

An alternative to using a breaker with a higher amperage rating would be to use a Type C (5-10X rated current) or Type D  (10-20X rated current) industrial breaker specifically designed to withstand a higher inrush current, as often seen with HVAC compressors and large motors. More than you wanted to know, OP. [/quote]

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