NEC & ACCA methods · rule citations on every result · nothing uploaded
Free trade calculators — built for the job site
Voltage drop, wire size, conduit fill, conduit bending marks, duct sizing, load calcs — the math electricians and HVAC techs run daily, with every rule citation shown so you can defend the number at inspection. Start with the voltage drop table below, or pick a calculator from the shelf.
System
V
A
ft
Drop limit
Smallest size ≤ 3%
8 AWG
3.13 V · 2.6%
12 AWG reference
7.9 V · 6.58%
fails both targets
Volts lost on the pick
3.13 V
120 V source
| Size | Drop (V) | Drop (%) | vs 3% |
|---|---|---|---|
| 14 | 12.55 | 10.46% | ✗ over |
| 12 | 7.9 | 6.58% | ✗ over |
| 10 | 4.97 | 4.14% | ✗ over |
| 8 | 3.13 | 2.6% | ✓ pass |
| 6 | 1.97 | 1.64% | ✓ pass |
| 4 | 1.24 | 1.03% | ✓ pass |
| 3 | 0.98 | 0.82% | ✓ pass |
| 2 | 0.78 | 0.65% | ✓ pass |
| 1 | 0.62 | 0.51% | ✓ pass |
| 1/0 | 0.49 | 0.41% | ✓ pass |
| 2/0 | 0.39 | 0.32% | ✓ pass |
| 3/0 | 0.31 | 0.26% | ✓ pass |
| 4/0 | 0.24 | 0.2% | ✓ pass |
| 250 | 0.21 | 0.17% | ✓ pass |
Within half a point of the limit, treat it as a fail: the K-factor method assumes ~75 °C conductor temperature and doesn’t see terminations. Aluminum runs roughly two trade sizes behind copper — the selector above applies K = 21.2 automatically.
K-factor method (K = 12.9 Cu / 21.2 Al) at 75 °C conductor temperature. 3% branch / 5% total are NEC informational targets — estimates, not enforceable limits.
All 18 calculators
Every tool shows its source table — 310.16, Chapter 9, 314.16, 240.4(D), 210.20(A), 220.83 — next to the answer.
Electrical Calculators
View category →Voltage Drop Calculator
Enter voltage, load amps and run length — get the voltage drop in volts and percent for every wire size, with the smallest size that holds 3%.
Wire Size Calculator
Enter load current and terminal temperature rating — get the minimum AWG by ampacity, with the 240.4(D) small-conductor caps already applied.
Conduit Fill Calculator
Pick conduit type and trade size, list your wires — get the fill percentage against the NEC 40% limit and the max count of that wire.
Box Fill Calculator
Enter the box volume and everything that lands in it — wires, clamps, devices, grounds — and see the 314.16 cubic-inch tally with pass/fail.
Breaker Size Calculator
Enter watts, voltage and whether the load is continuous — get the breaker size, the 125% design current and the minimum copper at 75°C.
Residential Load Calculator
Enter square footage and major appliances — get the demand load in VA and the service size (100/125/150/200 A) the numbers support.
Generator Size Calculator
List what must run in an outage with each item's starting watts — get running watts, peak demand and the suggested nameplate kW.
Conduit Bending Calculator
Pick a bend — 90° stub, offset or 3-point saddle — enter the rise or obstruction, and get every pencil mark plus the shrink.
Stub-Up Calculator
Enter the stub height and conduit size — get the take-up, the exact mark from the end of the conduit, and the full take-up table.
Offset Bending Calculator
Enter the offset rise and pick an angle — get the distance between marks from the multiplier table and the shrink to add back.
3-Point Saddle Calculator
Enter the obstruction's width and height — get the saddle depth, center-to-side mark spacing, and the total shrink for each angle combo.
Conduit Bending Multipliers Chart
The whole hand-bender cheat sheet in one table: take-ups per size, offset multipliers, shrink per inch of rise, and saddle combos.
HVAC Calculators
View category →Duct Size Calculator
Enter CFM and friction rate — get the round duct diameter, the standard size to order, velocity, and the full size chart.
HVAC Load Calculator
Square footage, climate zone and house details in — get the estimated BTU, tonnage and a size breakdown you can defend.
CFM Calculator
Room airflow three ways: share of the whole-home CFM by area, air changes per hour, or tonnage × 400 — all in one place.
Room BTU Calculator
Room size, sun, occupants and kitchen use in — get the BTU and the mini-split or window unit size to buy.
Ton to BTU Converter
Convert either direction between tons, BTU/h and kW — with the common system sizes pre-mapped in a table.
SEER Savings Calculator
Old SEER, new SEER, annual kWh and your rate in — get the new consumption, kWh saved and dollars per year.
Guides
What Is Voltage Drop?
What voltage drop is, why it matters, the 3%/5% targets, and how to fix a run that fails.
NEC Wire Ampacity Chart
Table 310.16 at 60/75/90°C for copper, with the 240.4(D) caps and circular-mil areas in one printable table.
What Is a Ductulator?
The HVAC slide rule explained — what it does, the equal-friction math inside, and why the free web version works.
Manual J Explained
What ACCA Manual J is, what it measures, when permits require it, and where the simplified estimate fits.
CFM per Ton Explained
Why 400 CFM per ton is the standard, when 350 is right, and what happens when airflow misses the target.
How to Size a Breaker
The five-step branch-circuit sizing method — load, 125% rule, standard sizes, wire pairing, and the mistakes inspectors catch.
How to Bend Conduit
Hand-bender basics: the three marks, stub-ups with take-up, offsets with multipliers and shrink, 3-point saddles — with worked examples.
What this site does — and what it does not
It does
- Run the NEC and ACCA tables — 310.16, Chapter 9 fill, 314.16, the ductulator’s equal-friction math — and cite which rule produced each number.
- Show the full table, not just the answer: every AWG’s drop, every trade size’s fill, every CFM’s diameter.
- Keep every calculation in your browser; nothing you type is stored or sent.
It does not
- Replace a licensed electrician, HVAC designer, or your local AHJ — permits and engineered designs follow the code cycle in force where you work.
- Cover motor and HVAC nameplate sizing — those carry their own MCA/MOCP math.
- Substitute for ACCA Manual J/D on jobs that require certified calculations.