Exam Cheat Sheet · Quick Reference

Texas - Master Electrician - Calculations Portion

Texas · PSI Services Contractor

Verified, not estimated. Every figure below is drawn from the official exam structure we maintain — question counts, passing standard and topic weighting. Practice questions are grounded in the source law with statute citations. We omit any figure we can't verify rather than guess at it.

Total questions

Passing score

70%

Exam time

170 min

Administered by

PSI Services Contractor

Format

Reference materials allowed

Texas State Portion 33 questions

Calculations and Theory 2 Q · 7%

Electrical Services, Service Equipment, and Separately Derived Systems 8 Q · 27%

Electrical Feeders 3 Q · 10%

Branch Circuit Calculations and Conductors 4 Q · 13%

Electrical Wiring Methods and Electrical Materials 2 Q · 7%

Electrical Equipment and Devices 2 Q · 7%

Motors and Generators 6 Q · 20%

Electrical Control Devices and Disconnecting Means 1 Q · 3%

Special Occupancies, Equipment, and Conditions 1 Q · 3%

Renewable Energy Technologies 1 Q · 3%

Key Distinctions

Single-Phase Voltage Drop FormulavsThree-Phase Voltage Drop Formula

Single-phase uses a multiplier of 2 (VD = 2×K×I×D/CM) because current travels out and back, while three-phase uses 1.732 (VD = 1.732×K×I×D/CM).

Chapter 9 Table 8, Voltage drop formula

Single-Phase Transformer Current FormulavsThree-Phase Transformer Current Formula

Single-phase uses I = kVA×1000/E (line-to-line voltage only), while three-phase uses I = kVA×1000/(E×√3).

NEC Article 250.30, transformer formula I = kVA × 1000 / E

Motor Branch-Circuit Conductor Sizing (NEC 430.22)vsMotor Overload Protection Sizing (NEC 430.32)

Branch-circuit conductors are sized at 125% of the Table 430.250 FLC value, while overload devices are sized at 125% (SF ≥1.15) of the nameplate current, not the table value.

NEC Table 430.250, Section 430.22, Section 430.6(A)(1)

Table 430.250 FLC (Motor Tables)vsNameplate Current

NEC 430.6(A)(1) requires Table 430.250 FLC for sizing branch-circuit conductors and short-circuit protection, while nameplate current is used for sizing overload devices per 430.32.

NEC Table 430.250, Section 430.22, Section 430.6(A)(1)

Continuous LoadvsNon-Continuous Load

Continuous loads (energized 3+ hours) must be multiplied by 125% when sizing conductors and feeders per NEC 215.2(A)(1) and 230.42(A)(1), while non-continuous loads are calculated at 100%.

NEC 230.42(A)(1)

Grounding Electrode Conductor (GEC) — Table 250.66vsEquipment Grounding Conductor (EGC) — Table 250.122

The GEC is sized from Table 250.66 based on the size of the largest service-entrance conductor, while the EGC is sized from Table 250.122 based on the rating of the overcurrent protective device.

NEC Table 250.66

Motor Feeder Ampacity (NEC 430.24)vsGeneral Feeder Ampacity (NEC 215.2)

Motor feeders (430.24) require 125% of the largest motor FLC plus 100% of all other motor FLCs, while general feeders (215.2) require 125% of continuous loads plus 100% of non-continuous loads.

NEC Table 430.250, Section 430.24

Straight Pull Box Minimum Length (NEC 314.28(A)(1))vsAngle Pull Box Minimum Length (NEC 314.28(A)(2))

For straight pulls the minimum box length = 8 × the largest conduit trade size, while angle pulls use 6 × the largest conduit trade size plus the sum of other conduit sizes on the same wall.

NEC 314.28(A)(1)

Time-Delay (Dual-Element) Fuse — Motor Branch-Circuit ProtectionvsNon-Time-Delay Fuse — Motor Branch-Circuit Protection

Per Table 430.52, time-delay fuses are limited to 175% of motor FLC for branch-circuit short-circuit protection, while non-time-delay fuses are permitted up to 300% of motor FLC.

NEC Table 430.250, Table 430.52, Section 240.6(A)

40% Conduit Fill (More Than 2 Conductors)vs53% Conduit Fill (1 Conductor) / 31% (2 Conductors)

When three or more conductors are installed in a raceway, Chapter 9 Table 4 limits fill to 40% of the conduit's cross-sectional area; one conductor uses 53% and two conductors use 31%.

Chapter 9 Table 5, Chapter 9 Table 4

Temperature Correction Factor (Table 310.15(B)(2)(a))vsConduit Fill Adjustment Factor (Table 310.15(B)(3)(a))

The temperature correction factor adjusts conductor ampacity for ambient temperatures above or below 30°C based on insulation rating, while the fill adjustment factor reduces ampacity when more than 3 current-carrying conductors share a raceway.

NEC Table 310.16, Table 310.15(B)(2)(a), Table 310.15(B)(3)(a)

THWN Insulation (75°C Rating)vsTHHN Insulation (90°C Rating)

THWN conductors are rated 75°C and use the 75°C column of Table 310.16 for ampacity, while THHN conductors are rated 90°C and use the 90°C column, which yields higher base ampacity values.

NEC Table 310.16, Table 310.15(B)(2)(a), Table 310.15(B)(3)(a)

Key Terms

NEC 430.24 — Motor Feeder Ampacity Rule NEC Table 430.250, Section 430.24

Feeder conductors supplying multiple motors must have ampacity equal to 125% of the largest motor's Table 430.250 FLC plus 100% of all other motor FLCs.

NEC 430.22 — Motor Branch-Circuit Conductor Ampacity NEC Table 430.250, Section 430.22, Section 430.6(A)(1)

Branch-circuit conductors for a single motor must have ampacity not less than 125% of the motor's full-load current from Table 430.250.

NEC 430.32(A)(1) — Motor Overload Protection NEC 430.32(A)(1)

For motors with a service factor of 1.15 or greater, the maximum overload device trip setting is 125% of the motor's nameplate full-load ampere rating.

NEC 314.28(A)(1) — Straight Pull Box Sizing NEC 314.28(A)(1)

For straight pulls, the minimum length of a pull box must be at least 8 times the trade size diameter of the largest conduit entering the box.

NEC 215.2(A)(1)(a) — Feeder Conductor Ampacity for Continuous Loads NEC 215.2(A)(1)(a)

Feeder conductors must have ampacity not less than the non-continuous load plus 125% of the continuous load when the overcurrent device is not rated for 100% continuous use.

NEC 230.42(A)(1) — Service Entrance Conductor Ampacity NEC 230.42(A)(1)

Service entrance conductors must have ampacity not less than the non-continuous load plus 125% of the continuous load served.

NEC 240.6(A) — Standard Overcurrent Device Ratings NEC Table 430.250, Table 430.52, Section 240.6(A)

When a calculated overcurrent device rating does not correspond to a standard size, NEC 240.6(A) lists the standard ratings to which the next higher size may be selected, including 100, 110, 125, 150, 175, 200, 225A, etc.

NEC Table 250.66 — Grounding Electrode Conductor Sizing NEC Table 250.66

The GEC is selected from Table 250.66 based on the size of the largest ungrounded service-entrance conductor; for example, 4/0 AWG copper service conductors require a minimum 2 AWG copper GEC.

NEC Table 250.122 — Equipment Grounding Conductor Sizing NEC Table 250.122

The EGC is sized from Table 250.122 based on the rating of the overcurrent protective device; a 400A OCPD requires a minimum 3 AWG copper EGC.

NEC Table 220.55, Note 1 — Range Demand Load Adjustment NEC Table 220.55, Note 1

For a single electric range rated over 12 kW, the Column C demand (8 kW for 1 range) is increased by 5% for each kilowatt (or major fraction) above 12 kW.

NEC Table 430.52 — Motor Branch-Circuit Short-Circuit Protection NEC Table 430.250, Table 430.52, Section 240.6(A)

Specifies the maximum rating of branch-circuit short-circuit and ground-fault protective devices as a percentage of motor FLC; time-delay fuses are limited to 175% and non-time-delay fuses to 300%.

Full-Load Current (FLC) vs. Nameplate Current NEC Table 430.250, Section 430.22, Section 430.6(A)(1)

Per NEC 430.6(A)(1), Table 430.250 FLC values (not nameplate current) govern conductor and short-circuit protection sizing, while nameplate current governs overload protection sizing.

Circular Mils (CM) Chapter 9 Table 8, Voltage drop formula

The cross-sectional area of a conductor in circular mils, used in voltage drop calculations; found in NEC Chapter 9 Table 8 (e.g., 4 AWG copper = 41,740 CM, 1 AWG = 83,690 CM, 2 AWG = 66,360 CM).

K Factor (Resistivity Constant) Chapter 9 Table 8, Voltage drop formula

A constant used in voltage drop formulas representing conductor resistivity: K = 12.9 for copper and approximately 21.2 for aluminum.

Service Factor (SF ≥ 1.15) NEC 430.32(A)(1)

A motor nameplate rating indicating the motor can handle overloads above full-load; when SF ≥ 1.15, NEC 430.32(A)(1) allows overload protection at 125% of nameplate current.

Separately Derived System NEC Article 450, Section 250.30

A premises wiring system whose power is derived from a source of electric energy (such as a transformer secondary) having no direct connection to the supply conductors of another system, governed by NEC Article 250.30.

Formulas to Know

Single-Phase Transformer Secondary CurrentI = (kVA × 1000) / E

Three-Phase Transformer Secondary CurrentI = (kVA × 1000) / (E × √3) [√3 ≈ 1.732]

Single-Phase Voltage DropVD = (2 × K × I × D) / CM [K=12.9 copper; D=one-way distance ft; CM from Ch.9 Table 8]

Three-Phase Voltage DropVD = (1.732 × K × I × D) / CM

Motor Branch-Circuit Conductor Ampacity (NEC 430.22)Minimum conductor ampacity = FLC(Table 430.250) × 1.25

Motor Feeder Ampacity (NEC 430.24)Feeder ampacity = (Largest motor FLC × 1.25) + Σ(all other motor FLCs × 1.00)

Adjusted Conductor Ampacity (Temp + Fill Derating)Adjusted ampacity = Base ampacity(Table 310.16) × Temp correction factor × Fill adjustment factor

Conduit Fill — Total Area (>2 conductors, 40% fill)Total fill area = Σ(n × area per conductor); Select conduit where 40%-fill capacity ≥ total fill area

Service / Feeder Current from VA (Single-Phase)I = VA / E

Service / Feeder Current from VA (Three-Phase)I = VA / (E × √3)

Minimum Ampacity for Continuous + Non-Continuous LoadsMin ampacity = (Continuous load × 1.25) + (Non-continuous load × 1.00)

Maximum Continuous Load from Known Conductor AmpacityMax continuous load = Conductor ampacity / 1.25

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