My new split system air conditioner (not a heat pump) has a 40amp breaker in the main box (Note: these are all standard breakers, not the new ones). A 10 gauge wire runs to a subpanel with a 25amp breaker for the outside condenser unit and a 15amp breaker for the air handler (in the attic). The outside condenser unit has a rating of 25 overcurrent amps. But the air handler has a rating of 2.3 overcurrent amps (from the chart in the manual). So I'm thinking this 15amp breaker is overkill. But the 25 and 15 switches add up to the 40amp on the main panel (does that mean they couldn't put anything less?).

But since I've got a 10 gauge wire running between the subpanel and the main panel breaker, the main panel should have a 30amp instead of a 40amp breaker (I think). But since the total overcurrent amps on the condenser and air handler add up to 27.3 then that's less than 30 so the 10 gauge wire should be safe. So are the 25 and 15 breakers in the subpanel going to protect me from worrying about too much current flowing on that 10 gauge wire back to the main panel 40amp breaker (which is apparently too large otherwise) if they malfunction?

Also, should I worry about that 40amp main panel breaker allowing too much current to pass through from main to subpanel if the only two machines on the subpanel should never draw more than 27.3 overcurrent amps?

Thanks for reading,
Morty

To help you visualize:
Machine______________________________|subpanel| 10 gauge wire |mainpanel
A/C condenser (25 overcurrent amps) >>>>>> 25amp \ _____________ 40amp
Air handler (2.3 overcurrent amps) >>>>>>>> 15amp /

If the installation matches the electrical installation instructions, there is nothing wrong.

Amperage is not additive. You cannot put a 25 amp breaker and a 15 amp breaker on the same circuit and call it a 40 amp circuit. It just does not work that way.

If you have a 40 amp circuit breaker that is the feed for a secondary panelboard that has a 25 amp breaker and a 15 amp breaker, which I think is what you are describing, and the calculated load for those circuits is 40 or less amps you will be fine. Now to the cable between the two panel boards, depending on the type of cable and the temperature of the terminals a 10 AWG cable could be correct.

The equipment your are describing requires dedicated circuits by the manufacturer. So, nothing can be added to those circuits anyway.

There are a great many considerations when dealing with motors but I'll just hit the two most applicable in your situation.

1) When a motor starts a great deal of current is drawn. This is because of the physics behind electromagnetism and current flow and well beyond the scope of this reply. Anyway, when designing a motor circuit, you have to make sure that the inrush current to the motor does not exceed the circuit breaker's rating otherwise the breaker would trip every time you started the motor.

2) Breakers work on an "inverse time" process. Roughly speaking the higher the current passing through the breaker the faster the breaker will trip. So if you ran 16 amps through a 15 amp breaker it would take longer for the breaker to trip than say 2500 amps.

The 15 amp breaker in your system is protecting against either a "Ground Fault" or a "Line Fault" condition. Either of these events will cause a extremely high current to pass through the breaker which will cause it to trip almost immediately.

should I worry about that 40amp main panel breaker allowing too much current to pass through from main to subpanel.
It only passes through what is being used. You don't have 40 amps just sitting in the line waiting to rush on out. You can draw 1 Amp off of a 40 Amp circuit.

The code may have an issue with the #10 copper supplying 40 amperes. See 240.D(7).
As far as circuit breaker sizes and connections the manufacturers installation instructions are required to be followed by the code. See 110.3(B).

Frequently with these kind of units the inside unit is supplied by/from the outside unit. Again, the manufacturers instructions need to be followed.

Has this equipment already been installed? Did a licensed contractor install it?

The code referenced above is a typo. The correct cite is 240.4(D)(7) - 10 AWG copper, 30 Amps.

However the opening paragraph of 240.4 states "Protection of conductors... shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)

Table 310.15(B)(6) specifies 30 amps at 60(C) - 35 amps at 75(C) and 40 amps at 90(C).

unless otherwise permitted or "required" in 240.4(A) through (G)...

so is 240.4(D)(7) a requirement that limits 10 AWG to 30 amperes?

"Has this equipment already been installed? Did a licensed contractor install it?"
Yes, it's been installed and worked fine over the summer. Yes, licensed electricians, one with 40 years and the other with 10 years experience, installed it. I watched the inspector unscrew the subpanel cover and check the new breakers and he saw the 40amp in the main panel but nobody verbally pointed out the 10 gauge wire.

By the way, the whole subpanel is brand new, replaced from what had been a 30 non-bus fuse box for the old unit. The new 25 and 15 breakers are HACR but not AFCI or GFCI labelled. The main panel 40amp is a standard thermal breaker.

I've got a very similar 2nd split A/C system that needs replacing on a house where there's a subpanel with a 10 gauge wire and currently an old fuse box but in its main panel there's a 30amp breaker (not 40amp like the 1st system). The difference from the 1st system in above posts which is 2-ton, this 2nd system is 2 1/2 tons but I'm thinking about replacing it with the same make and model split system as the 1st one I was explaining above, thus downgrading it to 2-ton.

In this 2nd A/C system if it has the same overcurrent amps for both the air handler (2.3) and condenser unit (25), then the 30amp in its main panel box should be exactly right, shouldn't it?
Because before I was getting confused when the 25 and 15 added up to the 40amp on the 1st system and thought on this 2nd system I would need to upgrade the 30amp to a 40amp. I guess my real question on the 2nd A/C system is: I understand a subpanel or branch panel can have breakers that add up to more amps than the one breaker in the main panel is rated for because generally not everything is going to be "on" or drawing current at the same time. However, in the case of an A/C system, both machines would always be drawing current at the same time and so if the 25 and 15 because they're drawing through the same 10 gauge wire add up to 40 then a 30amp breaker on the main panel I thought wouldn't be enough--but it's the total overcurrent amps possible that's more important than the numbers rated on the breakers?

Not being a code scholar, it appears t me that 240.4 is deferring to 310 for "specified" ampacities. As in the 75(C) and 90(C) columns of 310.15(B)(16) but limiting to 30 amps for the 60(C) column.

You are using the term "Overcurrent amps" which is incorrect. The correct term is "Overcurrent Protection."

For example, you can have a 25 amp breaker protecting a 20 amp circuit with no problems. However, if for any reason the amperage increases to 30 amp, the breaker will trip and shut the circuit down to protect the downstream circuit components.

Case in point, you have a 15 amp breaker protecting a circuit that is only drawing 2.3 amps.

The 25 amp breaker more than likely is protecting a circuit that actually draws less than 25 amps when it is active.

These are the breakers in the secondary panel. The feed from the main panel to the secondary panel is being protected by a 40 amp breaker. It will trip if for some reason the draw on the secondary panel becomes greater than 40 amps.

ZinscoMan,
You should start by reading the installation instructions for the equipment
you purchase. The instructions will tell you what size overcurrent devices you can and cannot install to protect the equipment. (and protect your warranty)

As for the panel circuit breaker protecting the split system outside and inside units with FLA (full load amps) of 25 and 2.3, according to NEC 440.33 the total load is calculated by increasing the load of the largest motor by 125% and adding the other motor(s) - 25 * 1.25 + 2.3 = 33.55 amperes. So a 40 ampere overcurrent device for this would be appropriate. In my opinion a 30 ampere circuit breaker does not satisfy the code.

The 2014 NEC has 909 pages of rules for most situations and conditions. We all do our best to interpret these rules and without exception learn something new every day....!