As shown in Table 310.104(A), conductors with THHN insulation are approved for use in dry and damp locations only and conductors with THWN, THHW, and THW insulation are recognized for use in dry and wet locations.

Section 310.15(B)(7)(1) permits one-family dwelling 120/240 volt, single-phase, service-entrance conductors to be sized at an ampacity of not less than 83% of the service rating. 

Therefore, 200 amperes x 83% = 166 amperes

Table 310.15(B)(16) indicates size 4/0 AWG THW aluminum conductors with an allowable ampacity of 180 amperes should be selected for this dwelling.

To find the full-load current rating of the single-phase transformer apply the single-phase current formula as shown:

I = (150 kVA x 1000) / 240 volts = 15,000/240 = 625 amperes
 

Table 240.6(A) illustrates the standard ampere ratings for fuses and circuit breakers. A circuit breaker with an ampere rating of 75 amperes is not a standard ampere rating.

As per Section 300.14 at least 6 in. of free conductor is to be left at each junction box for splices or the connection of luminaires or devices. The intent of this rule is to ensure enough conductor slack for the terminal connections or splices to be made easily.

Section 230.23(B) requires overhead service-drop conductors to be not smaller than 8 AWG copper or 6 AWG aluminum or copper-clad aluminum. Do not get service-drop conductors, the overhead conductors between the utility electric supply system and the service point, confused with the service-entrance conductors, the conductors from the service point from the utility company to the service disconnecting means.

The intent of Section 230.26 is to provide adequate vertical clearance of the conductors and prevent physical damage to the conductors and prevent persons from making accidental contact with the conductors which may create a shock hazard or electrocution. In no case shall the point of attachment of the service-drop conductors to be less than 10 ft. above finished grade.

Electrical continuity is required for metallic conduit to establish an effective ground-fault path and facilitate the operation of the overcurrent protective device. Reference Section 250.4(A)(4) and (5).

In compliance with Section 210.52(C)(5), receptacle outlets installed for countertop or work surfaces in dwelling units may be located on or above, but not more than 20 in. above the countertop or work surface. The intent of this rule is to eliminate the use of unsightly extension cords (which reduces the amount of usable countertop space) on the countertop or work surface to supply cord-and-plug connected appliances. Also, receptacles located more than 20 inches above countertops or work surfaces may interfere with the cabinetry to be installed.

Section 310.15(B)(3)(a)(2) clarifies that adjustment factors for more than three (3) current-carrying conductors in raceways shall not apply where the raceway is not more than 24 inches in length. Where more than 3 current-carrying conductors are installed in short lengths of conduit or tubing, the heating effect caused from the additional conductors is insignificant therefore, the allowable ampacity of the conductors is not greatly reduced.

Part VII of Article 440 outlines the special provisions for room air conditioners which are (1) rated not over 250-volts ac, single-phase and (2) which are cord-and-attachment plug connected. In order to prevent overloading the branch circuit, as per Section 40.62(C), the total rating of the A/C unit shall not exceed 50% of the rating of a branch circuit where lighting units, general-use receptacles or appliances are also supplied.

Part II of Article 240 explains the permitted location requirements for overcurrent devices. Section 240.24(F) tell us "overcurrent devices shall not be located over steps of a stairway." This requirement address a serious safety issue. No one should be required to stand on different treads on a stairway to replace, repair, maintain, install, troubleshoot, or reset overcurrent devices in a panelboard or switchboard located in a stairway.

The provisions in Part II of Article 680 apply to the installation of wiring methods and equipment in or adjacent to permanently installed swimming pools.

As confirmed by the requirements given in Section 680.22(A)(1), each permanently installed swimming pool is to be provided with at least one (1) single-phase, 15- or 20-ampere, 125-volt, receptacle placed not closer than six (6) feet from the pool and not more than twenty (20) feet from the pool.

This requirement allows ordinary appliances such as a radio, to be safely plugged in and used near the pool, but avoids the need for extension cords in the pool vicinity. Be advised the receptacles must be provided with GFCI protection. [680.22(A)(4)]

Part V of Article 250 covers the requirements for bonding of electrical installations. Section 250.104(A)(1) requires metal water piping systems to be bonded to any of (1) the service entrance enclosure, (2) the grounded conductor at the service, (3) the grounding electrode conductor.

Where mechanical and electrical connections are within equipment, a failure of electrical insulation can result in the connected piping systems becoming energized. For example, an insulation failure in an electrical circuit of a water heater could energize the metal water piping.

Metal water piping systems must be bonded so that when a fault occurs between an ungrounded (hot) conductor and the piping, the potential difference between the two is minimized, thereby reducing the risk of electrical shock.

Also, note where installed in or attached to a building or structure, all metal piping systems, including gas piping, that is likely to become energized shall be bonded to a grounded source. [250.104(B)]

The installation specifications for armored cable, Type AC, are addressed in Part II of Article 320. Section 320.23(B) makes it explicit, where the armored cable is installed parallel to the sides of rafters, studs or floor joists, neither guard strips nor running boards shall be required to protect the cable in accessible attics.

As per Section 320.23(A), if the cables run across the top of the floor joists in an attic that is not accessible by permanent studs or ladders, protection of the cable is required where within six (6) feet of the nearest edge of the scuttle hole or attic entrance.

The requirements of Article 520 apply buildings used as a theater or for a similar purpose. Section 520.8 mandates the control of emergency systems in theaters to comply with the rules set forth in Article 700; Part V of Article 700 applies to the control of emergency lighting circuits. Section 700.21 does not permit a manual control switch for emergency lighting to be placed in a motion picture projection booth or on a stage or platform; the switch shall be located in the lobby or at a convenient accessible location.

Emergency lighting systems are designed and installed to maintain a specific degree of illumination for means of egress of the building in the event of fire, or similar emergency, or if the normal power supply fails.

Part IV of Article 690 covers the wiring methods specified for use on solar photovoltaic (PV) systems arrays. Section 690.31(C)(1) indicates open, single-conductors such as Type USE-2 and single-conductor cable listed and identified as PV wire are permitted for
use in PV source circuits within the PV array.

These conductors are evaluated and listed for use where exposed to sunlight and wet conditions.

Section 210.8(A)(7) confirms that no matter what the location or room in a dwelling, where the 15- or 20-ampere, 125-volt eceptacles are installed within six (6) feet from the top inside edge of the bowl of the sink, GFCI protection must be provided to prevent a ground-fault shock hazard.

Part VII of Article 250 covers the general requirements for methods of equipment grounding.

Exception No.2 to Section 250.142(B) permits the meter enclosure to be grounded by connection on the load side of the service disconnect where (1) no ground-fault protection is installed, (2) the meter enclosure is adjacent to the service disconnecting means, and (3) the grounded service conductor is not smaller than the size specified in Table 250.122.

Because of the presence of water and grounded surfaces that contributes to a potential shock hazard, Section 210.8(A)(7) confirms any 125-volt, 15- or 20-ampere receptacle installed within six (6) feet of a sink is required to be GFCI protected. Section 210.12(A) verifies AFCI protection is required for all 120-volt, 15- or 20-ampere branch-circuits that supply outlets in kitchens in dwellings.

AFCI devices are designed to determine resistance to unwanted tripping due to the presence of arcing that occurs in utilization equipment under normal operating conditions or to a loading condition that closely mimics an arcing fault.

Article 210 provides the general rules regarding branch circuits. Branch circuit and receptacle ratings are addressed in Part II of Article 210. To prevent overheating receptacles, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 21(B)(2), where connected to a branch circuit supplying multiple receptables or other outlets. This table shows a 15-ampere rated receptacle may carry a maximum cord-and-plug-connected load of 12 amperes under this condition. 

Part II of Article 368 governs the permitted installation of busways. For busways in industrial establishments only, providing overcurrent protection at the point where the busway size is changed is not required if the busway having the smaller ampacity does not exceed fifty (50) feet in length and has an ampacity of at least one-third the rating of the overcurrent protection of the larger usway.
 
1/3 of 600 amperes = 200 amperes

Reference Section 368.17(B), exception

As explained in Section 330.5, the inside of all raceways and enclosures located underground is classified as a wet location. The conductors and cables installed in underground installations must be of a type listed for use in wet locations.

Restrictions of the use of non-metallic sheathed cable is addressed in Part II of Article 334. Section 334.10(B)(1) indicates Type NMC is permitted in dry, moist, and damp locations, but wet locations are not included in this list. Section 334.12(B)(4) prohibits Types NM and NMS cables installed in wet or damp locations.

As defined in Article 100, a continuous load is a load expected to continue for three (3) hours or more.

The installation requirements for fixed outdoor electric deicing and snow melting equipment are given in Article 426. As per Section 426.4, fixed outdoor electric deicing and snow melting equipment shall be considered a continuous load.

When sizing conductors and overcurrent protection for the fixed outdoor equipment you must take into consideration the equipment can operate over 3 hours, so it is considered a continuous load.

To solve this problem first locate the conductor size on the left side of Table 310.15(B)(16) and look under the copper THWN column with a temperature rating of 75°C and note the ampacity of the conductors is 115 amperes. The table is based on an ambient temperature of 86°C and not more than 3 current-carrying conductors in the raceway.

In this situation there are 4 current-carrying conductors in the raceway therefore, we must apply the correct adjustment factor (derating value) as shown in Table 310.15(B)(3)(a) as follows:

2 AWG THWN ampacity = 115 amperes (before derating)
115 amperes x 80% (adjustment factor) = 92 amperes