Hey Jack,Good question! It definitely seems odd to see such high MOPs compared to the actual current draw. Let's break down why this might be the case and how to approach it.

Understanding MOP (Maximum Overcurrent Protection) and its Purpose



It's NOT just about protecting the circuit against overloads. While overload protection is a factor, the MOP is also designed to protect the equipment itself. Specifically, it's intended to allow the furnace to start without nuisance tripping. Motor Starting Characteristics

Furnaces typically have motors (blower motors, draft inducer motors, etc.). Motors draw significantly higher current during startup (inrush current, also sometimes called locked rotor amps (LRA)). The MOP must be large enough to accommodate this inrush current without tripping the breaker.

Manufacturer's Responsibility

The MOP rating on the nameplate is the manufacturer's specification. They've likely tested the furnace extensively and determined the highest overcurrent protection needed to reliably allow the furnace to start and operate without damage. This is why you should

ALWAYS follow the manufacturer's instructions when it comes to the MOP.

Why the Discrepancy Seems Large

The 9A and 11.4A furnaces with 20A MOPs have high values because of the inrush current of the motors, particularly the blower motor. While the running current is low, the starting current can be significantly higher, even exceeding 20A for a short duration.

Why You Should Stick to the Nameplate MOP


Warranty

Using a breaker smaller than the nameplate MOP could void the warranty. If the furnace fails and it's determined that the overcurrent protection was undersized, the manufacturer likely won't cover the repair.2.

Nuisance Tripping

While a 15A breaker might work most of the time, it's very likely to trip occasionally, especially during colder weather when the motor might require more effort to start or after power outages. This will lead to call-backs, frustrated customers, and potential damage to the furnace from repeated, sudden starts and stops.3.

Safety

The MOP is also related to the integrity of the internal wiring and components of the furnace. The manufacturer has specified the MOP based on their assessment of what the unit can safely handle. Undersizing the overcurrent protection could potentially lead to overheating and damage to the furnace components under certain (possibly rare) conditions.

How the MOP is Determined (Generally)

While you won't be calculating the MOP yourself (you're relying on the manufacturer), here's a general idea of how it's determined:1.

Full Load Amps (FLA)

The manufacturer starts with the full load amps (the Total Current on your nameplate).2.

Locked Rotor Amps (LRA)

They then determine the Locked Rotor Amps (LRA) of the largest motor in the furnace (usually the blower motor). LRA is the current drawn when the motor is initially started.3.

Code Requirements

The NEC (National Electrical Code) has specific rules (often in Article 430 for motors) for sizing overcurrent protection based on FLA, LRA, and the type of motor.4.

Testing

The manufacturer tests the furnace extensively to ensure that the selected overcurrent protection allows for reliable starting and operation under various conditions.5.

Safety Factors

They often add a safety factor to account for variations in components, operating conditions, and the age of the motor.

Addressing Your Concerns



210.21(B)(2)

While 210.21(B)(2) deals with receptacle ratings and maximum loads on branch circuits, it doesn't directly apply to equipment nameplate MOP. It is a different consideration that helps ensure the breaker and recptacle are adequate for general use. This table primarily refers to general purpose circuits, not dedicated equipment circuits with specific MOP requirements.

Wire Size

Make sure that the wire size you use is appropriately sized for the MOP and meets the NEC ampacity requirements. Use Table 310.16 in the NEC as a reference. For example, if you are using a 20A breaker, you'd need to use 12 AWG copper wire (assuming it's rated for 75°C).

In summary



Always adhere to the manufacturer's specified MOP. The MOP is not solely based on the running current of the furnace; it's primarily designed to accommodate motor starting currents. Undersizing the overcurrent protection can lead to nuisance tripping, warranty issues, potential damage to the furnace, and potentially unsafe conditions.* Verify that your wiring is appropriately sized for the MOP.I hope this helps clarify things. Let me know if you have any more questions!Disclaimer: Always consult with a qualified electrician for any electrical work. Flag for review