Calculations for the Texas electrician exam
The calculations part of the TDLR exam is where most people fail: about 20% pass it. Every topic here has unlimited problems with fresh numbers, a worked solution and the 2026 NEC section to open.
Mixed practice: every topic
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Dwelling load calculations
Services and feeders for houses and apartments under Article 120.
Dwelling optional load calculation
The optional method adds every general load at nameplate (2 VA/ft² lighting, 1,500 VA per small-appliance and laundry circuit, range, dryer, water heater and other appliances), takes the first 8 kVA at 100% and the rest at 40%, then adds the heat or AC load from 120.82(C) and any EV charger at 100%.
Dwelling standard load calculation
The standard method adds the dwelling loads one group at a time, each with its own demand factor: general lighting at 2 VA/ft² plus 1,500 VA per small-appliance and laundry circuit through Table 120.45, 75% of four or more fastened appliances, the dryer, the range from Table 120.55 and the larger of heat or AC.
Fastened appliance demand load
In a dwelling standard calculation, add the nameplate ratings of the fastened-in-place appliances such as the water heater, dishwasher, disposal, trash compactor and built-in microwave.
Commercial load calculations
Lighting, receptacles and kitchen equipment in other occupancies.
Commercial lighting load
Multiply the floor area by the Table 120.42(A) unit load for the occupancy, such as 1.3 VA/ft² for an office or 1.9 VA/ft² for retail.
Receptacle load
In non-dwelling buildings, count 180 VA for each receptacle yoke (120.14(I)) and add any multioutlet assembly load (120.14(H)).
Conductors and overcurrent
Ampacity, derating, voltage drop, continuous loads and breaker sizes.
Ampacity correction and adjustment
To derate a conductor, take its Table 310.16 ampacity (the 90 °C column for 90 °C wire), multiply by the ambient temperature correction factor from Table 310.15(B)(1)(1) and by the Table 310.15(C)(1) adjustment factor for more than three current-carrying conductors.
Conductor sizing
To size a conductor, find the smallest size in Table 310.16 whose ampacity covers the load, reading the column your terminations allow under 110.14(C): 60 °C for circuits of 100 A or less unless the equipment is marked 75 °C, and 75 °C above 100 A.
Continuous loads
A continuous load runs at its maximum current for 3 hours or more.
Overcurrent device sizing
An overcurrent device must protect the conductor at its ampacity.
Voltage drop
Voltage drop is the voltage lost in the conductors.
Motors
Full-load current, conductors, overloads, branch-circuit and feeder protection.
Motor branch-circuit conductors
Branch-circuit conductors for one continuous-duty motor must have an ampacity of at least 125% of the motor's table full-load current (NEC 430.22).
Motor feeders
A feeder for several motors needs conductors rated at least 125% of the largest motor's table full-load current plus the full-load currents of the other motors (NEC 430.24).
Motor full-load current
Motor full-load current (FLC) for sizing conductors and short-circuit protection comes from NEC Table 430.248 (single-phase) or 430.250 (three-phase), found by horsepower and voltage, not from the nameplate.
Motor overload protection
A separate motor overload relay is sized from the nameplate full-load current, not the NEC table.
Motor short-circuit protection
A motor's branch-circuit breaker or fuse is sized by multiplying the table full-load current by the percentage in Table 430.52(C)(1) for the device type, such as 250% for an inverse time breaker or 175% for a dual-element fuse.
Boxes and raceways
Box fill, conduit fill and pull box sizing.
Box fill
Box fill is the minimum volume a box needs for the conductors, devices, clamps and fittings inside it.
Conduit fill
Conduit fill limits how much of a raceway's cross section the conductors may take up: 40% for more than two conductors.
Pull box sizing
Pull box sizing under NEC 314.28 applies to conductors 4 AWG and larger.
Grounding and bonding
Equipment grounding and grounding electrode conductor sizes.
Equipment grounding conductor sizing
The equipment grounding conductor is sized from the rating of the overcurrent device protecting the circuit, using Table 250.122, not from the load or the circuit conductors.
Grounding electrode conductor sizing
The grounding electrode conductor (GEC) is sized from the largest ungrounded service-entrance conductor, or the combined area of one phase of parallel sets, using Table 250.66.
Transformers
Primary and secondary current and overcurrent protection.
Transformer current
A transformer's full-load current on either side is its kVA rating times 1,000 divided by that side's voltage, and also divided by 1.732 for a three-phase transformer.
Transformer overcurrent protection
For a transformer rated 1000 V or less, NEC Table 450.5(B) (Table 450.3(B) in the 2023 NEC) sets the largest overcurrent device as a percentage of rated current.
Electrical theory
Ohm’s law, power, circuits, three-phase and power factor.
Ohm's law and power
Ohm's law says voltage equals current times resistance (E = I × R), and power equals voltage times current (P = E × I).
Power factor and efficiency
True power in watts equals apparent power in volt-amperes times the power factor, so kW = kVA × PF.
Series and parallel circuits
In a series circuit the resistances add and the same current flows through each one.
Three-phase power
For a balanced three-phase load, current equals volt-amperes divided by the line-to-line voltage times 1.732 (the square root of 3).