This HVAC system we recently installed has been causing quite a bit of trouble for us with inspectors. If you look at the label, you will see that the breaker size is 25 amps, and the minimum circuit ampacity (MCA or wire size) is 29 amps. The MOCP (maximum overcurrent protection, max fuse, or max breaker is 49—what? Who has ever heard of a 49A breaker?! A closer look at the data tag indicates that the ratings are for a 49A time delay fuse and a 25A HACR breaker. As counterintuitive as it may seem, we decided to do what the manufacturer said and used #10 wire with a 25A breaker.

We failed the inspection. The inspector said we needed a 30A breaker. Okay, okay, that makes a bit more sense since it's above the MCA, so we went ahead and replaced the 25A breaker with a 30A breaker. The next inspector failed us AGAIN. Their reason this time? Breaker is oversized. What gives?

To be fair to the inspectors, the data plate confused them, too. One of our techs, Elliot, reached out to Mitsubishi Electric to see what they had to say so that we could get a straight answer (and something we could show an inspector when they come by next time). It turns out that the manufacturer does indeed recommend 25A, but the breaker just needs to be between the recommendation and the 49A max fuse. 

That might make zero sense, but it turns out that the newest UL standard, UL 60335-2-40, partially changed the way manufacturers determine their MCA and MOCP. Some of these newer systems, especially those with ECMs and inverter-driven compressors, might have inflated MCAs due to the testing and certification methodology, and manufacturers could recommend a breaker size lower than the MCA. Mitsubishi Electric explains this phenomenon in Application Note 1044, particularly as it relates to their equipment, but we’ll go over the apparent contradiction and how you can plead your case to an inspector if you come across this same issue. Application Note 1044 is not currently on MyLinkDrive, but we received it from our local Mitsubishi Electric representative, so we'd recommend reaching out to yours if you want to read it.

UL Ratings

Among the many changes that have come with the rise of A2L refrigerants, electrical testing standards and certification are a less often discussed but still critical change. Specifically, there has been a shift from UL 1995 to UL 60335-2-40 that took effect last year. 

A Brief History of UL 1995 and UL 60335-2-40

UL Solutions sets the electrical safety standards for all types of equipment, parts, and machinery, including heating and air conditioning units. Manufacturers have been required to meet these safety criteria, have their equipment certified, and label their units accordingly. The test conditions set by the standard also dictate values like minimum circuit ampacity (MCA, for wire sizing) and maximum overcurrent protection (MOCP, MOP, or max fuse/breaker, for breaker and fuse sizing).

UL 1995 has been the electrical safety standard for heating and cooling equipment since 1995. It has undergone a few revisions, the most recent being in 2022, ranging from minor technical changes to logo removals. 

That said, the UL 1995 standards were designed to ensure safety when working with A1 refrigerants. While A2Ls are really only slightly flammable, extra caution and a more conservative electrical design approach are required for safety reasons. UL Solutions published a new standard, UL 60335-2-40, to address the industry changes and provide more rigorous testing for equipment certification. 

What About Equipment Certified Based on UL 1995?

UL 60335-2-40 was first published all the way back in 2012 and has since been revised a few times. However, it officially took effect on January 1, 2024, and most manufacturers certified their equipment for UL 1995 in the meantime.

In cases where a product was UL 1995-certified prior to the official adoption of UL 60335-2-40 in 2024 and has not been modified, the UL-1995 certification remains valid. However, manufacturers can (and often do) recertify equipment launched within the last couple of years to meet the latest applicable standard (which, in this case, is UL 60335-2-40), even if there are no design modifications. Systems and individual components may be tested and certified (this seems like a small detail, but this part will be important later).

How Does UL 60335-2-40 Affect Wire and Breaker Sizing?

UL 1995 used a formula based on worst-case scenario amp draw under normal operating conditions (i.e., the same conditions that give us the ratings like compressor RLA) to develop the MCA and MOCP ratings. In those formulas, the loads are assigned multipliers to account for anomalous overcurrent conditions. For MCA, this number is 1.25 on the primary load (usually compressor RLA) and the resistive loads (like base pan heaters and heat strips). For MOCP, this multiplier is 2.25 on the primary load.

UL 60335-2-40 explores a wider range of operating conditions, not just the rated ones; in heat pumps, power is measured in heating AND cooling modes during the minimum and maximum rated ambient conditions, and then there’s a 6% voltage tolerance (both over and under) to account for fluctuations in power. As a result, the UL 60335-2-40 testing and certification protocols cover much more ground than UL 1995. 

Systems with ECMs are just one example of how this change affects MCA and MOCP ratings. Since most new equipment contains ECMs, which can (but won’t necessarily) draw higher current than PSC motors, we’ll have higher amp draw values to plug into those equations (and higher MCAs and MOCPs as a result). 

What Do ECMs Have to Do With This?

Steve Rogers from TEC joined the podcast last year to discuss some of TEC’s findings about power factor and watt draw for ECMs. It was in the context of ACCA Standard 310, but it’s still a great resource that you can listen to HERE. We also have a tech tip explaining the differences between PSC and ECM power consumption in detail, but here’s the long and short of it:

Historically, HVAC systems have used PSC (permanent split capacitor) motors, which are AC motors. When there’s alternating current (AC), the voltage and current change between positive and negative based on the hertz (60 in the US, so that’s 60 changes per second). Motors are magnetic loads that produce inductive reactance (a form of resistance found in magnetic loads). Inductive reactance causes the current to lag a bit behind the voltage, and voltage and current must move in the same direction (positive or negative) to do work. 

As a result, not all volt-amps (VA) are consumed as power (watts); we have something called power factor that determines how much power performs work (0 indicates a total waste and no work; 1 or unity indicates that all VA are converted to watts, as is true of resistive loads that completely convert power to heat). If a motor has a power factor of 0.9, that indicates that 90% of those volt-amps are working, and about 10% return to the source. Conductors are sized to handle all of the amperage, not just the amps that get converted to work.

ECMs (electronically commutated motors) are a completely different animal. ECMs are controlled electronically, and their current does not form that same sinusoidal pattern as PSC motors. There are electronic pulses rather than a constant reversing of direction from positive to negative.

ECMs have a lower power factor as a result; the current merely pulses while the system receives incoming AC voltage. For example, you might read 7 amps on an ECM, whereas you’ll read 5 amps on a PSC motor under the same conditions, but less power is being consumed by the ECM due to its lower power factor and how it works. The power consumed is what people pay the utility company for, not the volt-amps that don’t do any work, so that’s why ECMs are more efficient.

ECMs can draw higher amperages when they have to ramp up, such as if a blower needs to overcome higher static pressure. In the case of PSC motors, the power draw drops under those conditions. THAT is a completely different way of handling a worst-case scenario, which is what the MCA and MOCP math is based on.

However, despite the differences in ECMs and PSC motors, the actual formula that manufacturers use to determine MCA and MOCP has remained the same between UL 1995 and UL 60335-2-40. 

The Formula

When thinking about MCA and MOCP, we’re interested in the amp draws of all components, including the largest load, all other magnetic loads, and resistive loads, so the variables in the formulas (per Article 440 of the National Electrical Code/NEC) are as follows:

• LOAD1 = current of the largest motor or branch circuit selection current (usually compressor)
• LOAD2 = sum of the electrical current in all other motors
• LOAD3 = current of electrical resistance heaters

The formulas are as follows (note that UL has other formulas for other use cases, but these are the most common):

• MCA = (1.25 x LOAD1) + (1.25 x LOAD 3) + LOAD2
• MOCP = (2.25 x LOAD1) + LOAD2 + LOAD3

Where UL 60335-2-40 Matters

Traditionally, when we’ve had systems with PSC motors, the compressor RLA was LOAD1. Multipliers of 1.25 and 2.25 accounted for anomalies. Inverter-driven equipment and ECMs are different, and the full range of operation (even extremes) is demonstrated in UL 60335-2-40 testing and certification. 

Remember when I said that both components and systems may be certified? If a system uses components that have been UL 60335-2-40 certified by a third-party, then we can plug the compressor RLA and fan FLA into the equations above. However, if the individual components are NOT UL 60335-2-40 certified by a third party, the manufacturer must use the entire system’s worst-case scenario amp draw instead of compressor RLA. As a result, when you’re working with inverter-driven equipment, you may notice that the condenser’s data plate says “Inverter Input” in lieu of compressor RLA and LRA. That inverter input is the worst-case scenario amp draw.

However, some systems also have controls that monitor current and work with internal overcurrent protection (i.e., not the breaker). Sometimes, the manufacturer opts to use this maximum limit instead of the RLA or inverter input for LOAD1. Since this is an upper limit, the load variables in the equations above will be even higher… while being subjected to the same multipliers in the formula. Thus, you get higher MCAs and MOCPs than expected, particularly with the ECMs and inverter-driven equipment that have taken over new equipment.

In other words, we’re taking worst-case scenarios based on internal equipment monitoring and protection and THEN plugging them into a formula with multipliers for an even worse worst-case scenario (worser case scenario?). The math will yield an inflated MCA that the system will likely never achieve.

When MCA > Recommended Breaker Size

Manufacturers are legally required to publish the MCA and MOCP on each condenser data plate. Typically, HVAC contractors must select a breaker between the MCA and MOCP that is suitable for the wire size selected. However, some manufacturers, like Mitsubishi, also print breaker size recommendations on their labels as a courtesy. In cases like our recent situation, these recommendations can be confusing when they’re below the MCA. 

The main thing to remember here is that residential HVAC equipment is equipped with internal safeties, such as motor overloads, and the breaker is a bit of a last resort. Because HVAC systems are equipped with these other safeties, the breakers aren’t really there to protect the conductor in the same way as most traditional circuits, and safety isn’t really the concern with small breakers; nuisance trips are.

Therefore, when you come across a unit like the one we’ve been dealing with, it seems like using the recommended breaker is a recipe for nuisance trips. However, neither the NEC nor Intertek (which provides UL testing and certification) explicitly requires HVAC breakers to be greater than the MCA.

What Do We Do?

Because the ratings are determined by math, not necessarily by real-world data, the manufacturers’ engineers make their recommendations based on real-world data to maximize safety and prevent nuisance trips. For the unit in our recent situation, Mitsubishi recommends a 25A breaker with the following reasoning:

In the case of Mitsubishi Electric’s equipment that contains inherent overcurrent protection, our engineers feel confident that the MCAs calculated via UL60335-2-40 [latest edition] would never be physically achievable and as such, sizing the overcurrent device below the listed MCA is a viable option. While this approach goes against conventional electrical service sizing it doesn’t present a violation of Intertek’s rating criteria. (Mitsubishi Application Note 1044, pg. 5)

There it is—straight from the manufacturer.

The verdict here is that the breaker size needs to be suitable for the conductor and below the MOCP, just not necessarily above the MCA. Mitsubishi’s recommendation is to choose a breaker between the recommendation and the MOCP, not to exceed the latter (which is definitely against code, no matter how you slice it). 

This situation is one of those cases that shows us that there’s more nuance in field application than the math formulas in a code book… especially formulas designed for motors that work completely differently from the ones in most new installs nowadays.