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Tech Tips:

Silicon Carbide vs. Silicon Nitride Hot Surface Ignitors (HSIs)

The ignitor of a furnace (or a gas pool heater or other gas-fired appliance) is the heat source, and it can come in one of four varieties: standing pilot, direct spark (DS), intermittent spark (ISI), and hot surface (HSI). We have a tech tip explaining each of these ignitor types in detail, but we’re going […]

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Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

In this essential episode, Bryan Orr sits down with Elliot, the residential install supervisor at Kalos Services, to unpack a critical issue that’s causing confusion among HVAC technicians, electricians, and inspectors alike: the new standards for breaker and conductor sizing on inverter-driven equipment. The conversation was sparked by Elliot’s frustrating experience of having two inspectors in the same county fail the same installation for opposite reasons—one for an oversized breaker and another for an undersized breaker. This contradiction led to a deep dive into recent changes in UL standards and how they affect everyday HVAC installations.

The heart of the issue stems from the transition to low-GWP refrigerants and the updated UL 60335-2-40 Edition 3 standard, which replaced the 1995 certification approach. This new standard introduced more conservative calculations for electrical characteristics, particularly affecting equipment using A2L refrigerants. The result? Data tags now show higher Minimum Circuit Ampacity (MCA) ratings than before, even though the equipment itself hasn’t changed—only the math used to calculate these values has shifted. This has created a puzzling situation where the MCA can be higher than the recommended breaker size, which seems counterintuitive to anyone familiar with traditional electrical principles.

Bryan and Elliot clarify the fundamental rule that still applies: size your wire to the MCA and your breaker to the MOCP (Maximum Overcurrent Protector). The confusion arises because manufacturers like Mitsubishi are now including “recommended breaker” sizes on data tags that are lower than the MCA—a courtesy to contractors, not a code requirement. The higher MCA reflects conservative safety margins that account for extreme operating conditions, but in practice, inverter-driven systems have multiple built-in protections that prevent them from ever actually reaching these calculated amperage levels. The key takeaway is that contractors can safely install breakers at the recommended size without safety concerns, as long as the breaker’s lugs are rated to accept the wire size required by the MCA.

The episode also explores how inverter-driven equipment fundamentally differs from traditional PSC motors, particularly regarding locked rotor amps (now more accurately termed “inverter input”) and voltage drop considerations. Unlike conventional motors that simply run slower with reduced voltage, inverter-driven compressors and ECM motors compensate by drawing more current to maintain performance, creating a potential compounding effect with voltage drop that installers need to understand—even though voltage drop itself isn’t an enforceable NEC code requirement.

Topics Covered:

New UL 60335-2-40 Edition 3 standards and their impact on electrical calculations for HVAC equipment
The relationship between MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protector) and why they can now seem contradictory
Recommended breaker sizes on modern data tags and why they may be lower than the MCA
Handling inspector conflicts and failed inspections related to breaker sizing
Differences between inverter-driven equipment and traditional PSC motors in electrical behavior
The transition from “locked rotor amps” to “inverter input” terminology for modern equipment
Voltage drop considerations with inverter-driven systems (NEC 210.19A and 215.2A)
Why inverter-driven equipment draws more current at lower voltages compared to traditional motors
Proper wire and breaker sizing for A2L refrigerant equipment (454B systems)
NEC Section 440 requirements specific to air conditioning and refrigeration equipment
Breaker lug ratings and ensuring they can accept the required wire size
Practical advice for communicating with inspectors and resolving code disputes

Read the tech tip on this topic HERE.

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Sizing Breakers and Conductors (for HVAC) – Have Things Changed?

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