Okay, let's break down these questions step-by-step, focusing on the logic and code references (based on the US National Electrical Code (NEC)).

Key Concepts Ampacity

The current-carrying capacity of a conductor (measured in Amps). This is determined by the conductor's size, insulation type, and installation conditions.

THHN

A common type of building wire with thermoplastic high heat-resistant nylon coating.

Terminal Temperature Rating

Electrical equipment (like circuit breakers) has a temperature rating for the conductors connected to them. This means the conductor's ampacity must be based on the temperature rating of the lowest rated termination point (the breaker or the equipment terminal).

NEC Table 310.16 (or similar depending on the NEC year)

This table is crucial. It lists the ampacities of various conductor sizes and insulation types at different temperatures (60°C, 75°C, and 90°C).

NEC 110.14(C) (or similar depending on the NEC year)

This is the general rule governing termination temperature limitations. It basically says: For equipment rated 100A or less, and for conductors size 14 AWG through 1 AWG, you must use the 60°C ampacity columns unless the equipment is specifically listed and marked for use with 75°C conductors. For equipment rated over 100A, use the 75°C ampacity columns.

Understanding the Question

The questions are asking for the smallest conductor size that is permitted to carry the stated current (70A or 50A) while also respecting the temperature rating limitations of the terminations.

Answering the Questions1. What's the minimum size THHN conductor permitted to terminate on a 70A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75C conductor sizing? Analyze

The breaker is rated 70A. The terminals are listed for 75°C.

NEC 110.14(C)(1)(a)(2) allows the use of the 75°C column for termination sizing Refer to Table 310.16 (75°C column)

2 AWG is rated for 95 Amps 4 AWG is rated for 85 Amps 6 AWG is rated for 65 Amps 8 AWG is rated for 55 Amps 10 AWG is rated for 40 Amps

Select

We need a conductor that can handle at least 70A based on the 75°C ampacity rating. 6 AWG is too small.

Answer



(b) 4 AWG2. What's the minimum size THHN conductor permitted to terminate on a 50A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 60C conductor sizing? Analyze

The breaker is rated 50A. The terminals are rated for 60°C.

NEC 110.14(C)(1)(a)(1) states that equipment rated for 100A or less shall use the 60 degree celcius column. Refer to Table 310.16 (60°C column)

2 AWG is rated for 95 Amps 4 AWG is rated for 70 Amps 6 AWG is rated for 55 Amps 8 AWG is rated for 40 Amps 10 AWG is rated for 30 Amps

Select

We need a conductor that can handle at least 50A based on the 60°C ampacity rating.

Answer



(c) 6 AWG3. What's the minimum size THHN conductor permitted to terminate on a 50A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75C conductor sizing? Analyze

The breaker is rated 50A. The terminals are rated for 75°C.

NEC 110.14(C)(1)(a)(2) allows the use of the 75°C column for termination sizing Refer to Table 310.16 (75°C column)

2 AWG is rated for 95 Amps 4 AWG is rated for 85 Amps 6 AWG is rated for 65 Amps 8 AWG is rated for 55 Amps 10 AWG is rated for 40 Amps

Select

We need a conductor that can handle at least 50A based on the 75°C ampacity rating.

Answer



(d) 8 AWGImportant Notes Specific NEC Edition

The exact table and section numbers may vary slightly depending on the edition of the NEC being used. Always refer to the specific code year applicable in your jurisdiction.

Derating

These calculations assume standard installation conditions. If you have more than three current-carrying conductors in a raceway, or the ambient temperature is high, you will need to "derate" the ampacity of the conductors, which means increasing the wire size further. The NEC provides tables and formulas for derating.

Local Codes

Always check with your local electrical inspector or authority having jurisdiction (AHJ) for any local amendments or interpretations of the NEC.

Safety First: This information is for educational purposes. Electrical work can be dangerous. Always consult a qualified electrician for any electrical installations or repairs. Flag for review