TN LLE HVAC and Air-Conditioning Circuits 2 — Questions and Answers
Question 1: According to NEC Article 440, what is the maximum allowable voltage drop for the branch circuit supplying an air-conditioning compressor?
- 3%
- 5%
- No specific limit in Article 440 (Correct answer)
- 10%
Correct answer: No specific limit in Article 440
NEC Article 440 does not specify a maximum voltage drop for branch circuits supplying compressors. Voltage drop recommendations are found in informational notes throughout the NEC.
While the NEC provides voltage drop recommendations in informational notes (suggesting 3% for branch circuits and 5% total), Article 440 specifically governing air-conditioning and refrigeration equipment does not mandate a specific voltage drop limit. The voltage drop recommendations are advisory rather than enforceable requirements.
Question 2: What determines the minimum branch-circuit conductor size for a single hermetic refrigerant motor-compressor per NEC 440.32?
- The nameplate rated-load current (Correct answer)
- 125% of the rated-load current
- The nameplate minimum circuit ampacity
- 115% of the locked-rotor current
Correct answer: The nameplate rated-load current
Per NEC 440.32, the branch-circuit conductor size for a single motor-compressor must not be less than 125% of the rated-load current OR the branch-circuit selection current, whichever is greater.
NEC 440.32 states that conductors supplying a single motor-compressor shall have an ampacity not less than 125% of either the rated-load current or the branch-circuit selection current, whichever is greater. The rated-load current is the primary value found on the equipment nameplate and serves as the basis for conductor sizing calculations.
Question 3: When an air-conditioning unit has both a compressor motor and a fan motor, how is the overcurrent protection device sized?
- Based on the largest motor only
- Sum of all motor currents plus 25%
- Based on the compressor rated-load current plus the full-load current of other motors, with OCPD sized per the largest motor's table value (Correct answer)
- Based on the locked-rotor current of all motors combined
Correct answer: Based on the compressor rated-load current plus the full-load current of other motors, with OCPD sized per the largest motor's table value
The OCPD is sized using the largest motor's maximum rating from the applicable table, plus the sum of the rated-load or full-load currents of all other motors in the circuit.
For multimotor equipment like an A/C unit with compressor and fan motors, NEC 440.22 requires the overcurrent device to be rated based on the compressor motor's applicable table value (which accounts for starting current) plus the full-load current ratings of the additional motors. This ensures the OCPD can handle the highest starting surge while protecting against overload.
Question 4: What is the maximum distance a disconnecting means for an air-conditioning unit can be located from the equipment per NEC 440.14?
- Within 25 feet
- Within 50 feet
- Within sight from and readily accessible (Correct answer)
- Adjacent to the unit only
Correct answer: Within sight from and readily accessible
NEC 440.14 requires the disconnecting means to be within sight from and readily accessible from the air-conditioning or refrigerating equipment.
NEC 440.14 specifies that the disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. 'Within sight' is defined by the NEC as visible and not more than 50 feet distant. This ensures maintenance personnel can verify the disconnect is open while working on the equipment.
Question 5: A room air conditioner connected to a 120-volt, 15-ampere branch circuit must not exceed what percentage of the circuit rating per NEC 440.62(B)?
- 50%
- 80% (Correct answer)
- 100% if no other outlets exist
- 60%
Correct answer: 80%
Per NEC 440.62(B), a room air conditioner rated 120 volts shall not exceed 80% of the branch-circuit ampere rating if other outlets are also supplied.
NEC 440.62(B) limits a cord-and-plug-connected room air conditioner to 80% of the branch-circuit rating when other outlets are supplied by the same circuit. If the air conditioner is the only load on the circuit, it may be rated up to 50% of the branch-circuit rating for a 15A or 20A circuit. This ensures adequate capacity for other connected loads.
Question 6: Per NEC 440.65, what additional protection is required for single-phase residential HVAC equipment rated at 250 volts or less?
- Surge protection
- AFCI protection
- HACR-type circuit breaker
- Listed LCDI or AFCI protection cord set (Correct answer)
Correct answer: Listed LCDI or AFCI protection cord set
NEC 440.65 requires cord-and-plug-connected single-phase HVAC equipment rated 250 volts or less to be provided with one of several specified protection methods including LCDI or AFCI protection.
NEC 440.65 mandates that single-phase cord-and-plug-connected room air conditioners, household refrigerators/freezers rated 250V or less shall be provided with a factory-installed LCDI (leakage-current detection and interruption) or AFCI protection device. This addresses the fire risk from damaged cords that can develop series arcing faults.
TN LLE HVAC and Air-Conditioning Circuits 2
According to NEC Article 440, what is the maximum allowable voltage drop for the branch circuit supplying an air-conditioning compressor?