Texas Journeyman Electrician Calculations Portion

Key Concepts

• Motor current comes from the tables, not the nameplate. Per NEC 430.6(A)(1), use Table 430.247–430.250 values (for 3-phase, Table 430.250) to size conductors, switches, and branch-circuit short-circuit/ground-fault protection. Nameplate current is used for overload protection and for low-speed (<1200 RPM), high-torque, canned-pump, and multispeed motors per 430.6(A)(2). 430.17 confirms the "highest rated" motor for 430.24 is based on table FLC.
• Single motor branch circuit: 125%. 430.22 — conductors supplying a single continuous-duty motor must have an ampacity of not less than 125 percent of the motor full-load current as determined by 430.6(A)(1).
• Feeder for multiple motors. 430.24 — 125% of the largest motor FLC plus 100% of all other motor FLCs.
• Short-circuit/ground-fault protection. Table 430.52 — inverse-time circuit breaker maximum is 250% of FLC; 430.52(C)(1) permits going up to the **next higher standard…

Key Concepts

• Service-entrance conductor ampacity. Service-entrance conductors shall have an ampacity of not less than the maximum load to be served; ampacity is determined from 310.14 and shall comply with 110.14(C) (230.42(A)).
• Continuous plus noncontinuous loads. Where service-entrance conductors supply continuous loads, or a combination, the minimum size shall have ampacity not less than the sum of the noncontinuous loads plus 125 percent of continuous loads (230.42(A)(1); parallel rule at 215.2(A)(1)).
• Adjustment/correction factors. Minimum size must have ampacity not less than the maximum load after applying any adjustment or correction factors (230.42(A)(2)). At 30°C ambient the correction factor is 1.00 (Table 310.15(B)(1)(1)).
• Termination temperature limit. Per 110.14(C)(1), for circuits rated 100 A or more, THHN (90°C) conductor ampacity is taken from the 75°C column unless equipment is listed and marked for higher-temperature terminations.
• **Minimum…

Key Concepts

Feeder minimum size — the 125% rule. Feeder conductors shall have an ampacity not less than the larger of 215.2(A)(1) or (A)(2) and shall comply with 110.14(C). Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load (NEC 215.2(A)(1)).

Exceptions to the 125% rule. If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the feeder conductors may be sized at the sum of the continuous load plus the noncontinuous load (215.2(A)(1) Exception No. 1). Grounded conductors not connected to an overcurrent device may be sized at 100 percent of the continuous and noncontinuous load (Exception No. 3).

Key Concepts

Continuous loads — conductors. Branch-circuit conductors must have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load (210.19(A)(1)).

Continuous loads — overcurrent devices. Where a branch circuit supplies continuous loads or a combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load (210.20(A)). Exception: where the assembly including the overcurrent devices is listed for operation at 100 percent of its rating, the rating may be not less than the sum of the continuous plus noncontinuous load. Working backward, a 20-ampere circuit is limited to 20 ÷ 1.25 = 16A continuous.

Calculations Portion

Key Concepts

• Raceway fill is a two-table lookup. Conductor cross-sectional areas come from NEC Chapter 9, Table 5; the permitted raceway fill areas come from Chapter 9, Table 4. Selection rule: total conductor area must be ≤ the raceway's listed fill area (Chapter 9, Table 4; Table 5).
• Percent fill depends on conductor count. More than two conductors in a conduit = 40% maximum fill; exactly two conductors = the 31% column (Chapter 9, Table 1). Data cabling in conduit follows the same 40% limit for more than two cables (Chapter 9, Table 1).
• Rounding for maximum conductor count: divide the 40% area by the conductor area and round down unless the decimal is 0.8 or greater (Chapter 9, Note 7). Example: 0.213 ÷ 0.0211 = 10.09 → 10 conductors in 3/4" EMT. -…

Key Concepts

Which current do you use? Per 430.6(A)(1), the values in Table 430.247 through Table 430.250 — not the nameplate current — are used to determine conductor ampacity, switch current ratings, and branch-circuit short-circuit and ground-fault protection ratings. Per 430.6(A)(2), the motor nameplate current rating is used for separate motor overload protection.

Motor overload protection (430.32(A)(1)). A separate overload device for a continuous-duty motor rated more than 1 hp shall be selected to trip or be rated at no more than the following percent of nameplate full-load current: service factor 1.15 or greater = 125%; marked temperature rise 40°C or less = 125%; all other motors = 115%. Where that setting will not start the motor or carry the load, 430.32(C) permits 140% / 140% / 130% respectively.

Key Concepts

• Which current to use. Per 430.6(A)(1), the values in Table 430.247 through Table 430.250 are used instead of nameplate current to determine ampacity of conductors, current ratings of switches, and ratings of branch-circuit short-circuit and ground-fault protection. Per 430.6(A)(2), the nameplate current rating is used for separate motor overload protection, and for motors built for low speeds (less than 1200 RPM), high torques, canned pumps, or multispeed motors.
• Branch-circuit conductors (430.22). Conductors supplying a single continuous-duty motor shall have an ampacity of not less than 125 percent of the motor full-load current as determined by 430.6(A)(1). Special multipliers: wye-start/delta-run conductors between controller and motor, 72 percent (430.22(C)); part-winding, 62.5 percent (430.22(D)).
• Feeder conductors (430.24). Feeder ampacity = 125% of the largest motor FLC + 100% of all other motor FLCs. Per 430.17, the highest…

Key Concepts

Use table FLC, not nameplate, for circuit sizing. Per 430.6(A)(1), the values in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 are used instead of the nameplate current to determine ampacity of conductors, current ratings of switches, and current ratings of branch-circuit short-circuit and ground-fault protection. Nameplate current ratings are used for separate motor overload protection (430.6(A)(2)). The rating of an inverse time circuit breaker must be based on the table FLC, not nameplate amperage (430.52(B)).

Disconnecting means (Article 430 Part IX). Per 430.110(A), the disconnect must be rated at least 115 percent of the motor full-load current from Table 430.250.

Key Concepts

Area classification — Class I, Division 1. Class I locations are those in which flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures (500.5(B)). A Class I, Division 1 location is one in which ignitible concentrations exist under normal operating conditions, exist frequently because of repair or maintenance operations or leakage, or where breakdown/faulty operation could release ignitible concentrations and simultaneously cause failure of electrical equipment so that the equipment becomes a source of ignition (500.5(B)(1)). The informational note lists transfer of volatile flammable liquids between containers and interiors of spray booths as typical Division 1 locations. Each room, section, or area is classified individually (500.5(A)(1)).

Key Concepts

• Maximum circuit current (690.8(A)). PV source circuit current = the sum of the short-circuit current ratings of the PV modules connected in parallel × 125 percent (690.8(A)(1)(a)). A licensed PE's stamped industry-standard calculation is permitted for systems with inverter generating capacity of 100 kW or greater, but the value shall not be less than 70 percent of the 690.8(A)(1)(a)(1) value. DC-to-dc converter circuit current = sum of parallel-connected converter continuous output current ratings. Inverter output circuit current = the inverter continuous output current rating.
• Conductor ampacity (690.8(B)). Conductors shall have ampacity not less than the larger of (B)(1) the maximum current of 690.8(A) × 125% without adjustment and correction factors, or (B)(2) the maximum current with adjustment and correction factors. Exception: an assembly with its OCPD listed for continuous operation at 100 percent of rating…