Plain-English answers to the most common questions about NEC electrical code, construction math, and estimating. Every answer links to the free calculator that does the arithmetic for you.
Electrical Code Questions
What wire size do I need for a 20-amp circuit?
12 AWG copper (or 10 AWG aluminum) is the NEC minimum for a 20-amp branch circuit at 60 degrees C per NEC Table 310.12. At 75 degrees C (the standard commercial termination rating) 12 AWG copper is still the answer from Table 310.16 with a 20-amp ampacity of 25 amps. Apply temperature and bundling correction factors when required. Use the free NEC Wire Size Calculator to check your specific conditions.
What is the NEC voltage drop limit?
The NEC recommends – but does not mandate – keeping voltage drop to 3% or less on branch circuits and 5% or less total (feeder plus branch) per NEC 210.19(A) Informational Note 1. These are design targets; the AHJ may enforce them. Use the free Voltage Drop Calculator to verify any conductor selection.
How many wires fit in 3/4-inch EMT?
For 12 AWG THHN conductors, NEC Annex C Table C1 allows up to 9 wires in 3/4-inch EMT at the 40% fill limit. For 14 AWG THHN, the same table allows up to 12 wires. Use the free Conduit Fill Calculator to check any combination of sizes and insulation types in any conduit trade size.
What is the NEC box fill rule?
NEC 314.16 requires the total cubic-inch volume of all conductors, devices, internal clamps, and EGCs in a box must not exceed the listed box volume. Table 314.16(B) gives per-conductor allowances by AWG (12 AWG = 2.25 cubic inches). Use the free Box Fill Calculator to sum the allowances and check against your box size.
How is a residential service sized per NEC 220?
NEC Article 220 standard method: 3 VA per square foot for general lighting, 1,500 VA per 20-amp small-appliance circuit (minimum two), 1,500 VA for laundry, demand factors from NEC 220.42, range demand per NEC 220.55 column C, dryer per NEC 220.54, and the larger of A/C or heat. Round up to the next standard service size. Use the free Electrical Load Calculator to run the full NEC 220 calculation.
What is the EGC size for a 20-amp circuit?
Per NEC Table 250.122, the equipment grounding conductor for a 20-amp OCPD is 12 AWG copper or 10 AWG aluminum. The EGC is sized by the OCPD rating, not the load current. Use the free Grounding Conductor Sizer for any OCPD rating.
Construction Math Questions
How do I calculate concrete for a slab?
Length (ft) x width (ft) x thickness (ft) / 27 = cubic yards. Add 10% waste for standard pours. Example: 10×10 slab at 4 inches thick = 10 x 10 x 0.333 / 27 = 1.23 cubic yards, or 1.36 yards with waste. Use the free Concrete Volume Calculator for slabs, footings, and round columns.
What is the difference between markup and margin?
Markup is the percentage added to cost to set price. Margin is the percentage of the final price that is profit. A 25% markup produces a 20% margin. Confusing the two is one of the most common estimating errors in construction. Use the free Construction Markup Calculator to see both figures side by side with a full bid breakdown.
How many drywall sheets do I need?
Measure total wall and ceiling area, add 10% waste, divide by the sheet coverage (32 sq ft for a 4×8 sheet). Use the free Drywall, Mud & Tape Estimator to get sheet count, mud weight, tape footage, and screws per room in one step.
How do I read roofing squares?
One roofing square = 100 square feet of roof surface. Divide roof area by 100, multiply by 3 bundles per square for standard shingles. Add 10% waste for simple gable roofs; 15% for hip or valley roofs. Use the free Roofing Material Estimator to compute area from footprint plus pitch automatically.
Have a question that is not answered here? All 14 calculators with their full code references are at the Tools hub. For NEC article-by-article quick reference, see the NEC Code Lookup Hub.
