Which chart should I start with for a new circuit?

Start from the load. Convert watts to amps, size the overcurrent device with the breaker size calculator, then pick a conductor with the wire size chart. Finish by checking the run length in the voltage drop calculator and the raceway in the conduit fill calculator.

What is the difference between the charts and the calculators?

Charts are prebuilt tables you can scan or print for one common condition, such as copper at 75 °C and 30 °C ambient. Calculators take your actual numbers — material, insulation rating, ambient temperature, conductor count, length and load — and compute the result for that specific install.

Where do the numbers come from?

Conductor geometry comes from the standard ASTM B258 AWG progression, resistance from the copper and aluminium resistivity constants, and ampacity from a documented power-law fit against published allowable values. Conduit interior areas come from published trade-size dimensions. Every formula and constant is listed on the methodology page.

Can I use these results directly on a job?

Use them for planning, estimating and sanity checks. Final conductor, breaker and raceway selection must be verified against the code edition adopted in your jurisdiction and, where required, signed off by a licensed electrician. Local amendments and equipment listings can override a general calculation.

Why do the wire pages split into voltage drop, conduit fill and breaker size?

Each conductor size answers three separate questions on site: how far it can run before the drop exceeds 3 percent, how many of them fit in each conduit trade size, and which overcurrent device it can be protected by. Splitting them keeps each page focused on one decision instead of one long table.

All charts & calculators

Every reference on the site, grouped by what you're holding in your hand. All free, all formula-derived, all with the method shown.

Quick answer

This index lists every calculator and reference table on ElectricalCharts: 19 charts and calculators, 22 conductor sizes with a voltage-drop, conduit-fill and breaker-size page each, and 23 breaker ratings from 15 A to 400 A. Work load → breaker → conductor → run length → raceway, in that order.

How to work through a circuit

Most sizing questions on site resolve in five steps, and every step below has a page here. First convert the load to amps, because nameplate data arrives in watts, horsepower or VA far more often than in current. Second, size the overcurrent device for that continuous load. Third, pick a conductor whose allowable ampacity, after ambient-temperature correction and bundling adjustment, still covers the device rating. Fourth, check the run length so the voltage drop stays inside the 3 percent branch-circuit and 5 percent total recommendation. Fifth, confirm the conductors physically fit the raceway at the 40 percent fill limit for three or more wires.

The order matters. Sizing a conductor before you know the derating conditions is the most common cause of a rework — a 10 AWG copper conductor good for 40 A at 90 °C in free air drops well below that in a hot attic bundled with nine other current-carrying conductors, and the terminations are usually limited to 75 °C regardless of the insulation rating. Each calculator shows the intermediate factors, not just the final number, so you can see which condition is governing.

If you are troubleshooting rather than designing, start from the size you already have in the wall: open its conductor page below and read the ampacity, resistance and diameter, then follow the voltage-drop link to see the maximum length that size supports at your load.

Calculators

Interactive tools that take your own conditions and return a number for that install. Voltage drop works from length, load, voltage and phase; conduit fill works from wire type, count and trade size; breaker sizing works from continuous load and conductor; box fill works from the device, clamp and conductor allowances. Each one shows the intermediate factors — ambient correction, bundling adjustment, percentage of fill used — so you can see exactly which limit is binding before you buy material.

Wire & Conduit Charts

Scannable reference tables for the conditions that come up most: copper and aluminium ampacity by insulation temperature rating, conductor diameter and circular mil area by gauge, and the wire size that pairs with each common breaker. Use these when you want the answer at a glance or a page you can print and keep in the van; use the calculators when the ambient temperature, conductor count or run length is unusual.

Trade Size Charts

Adjacent trade references that electricians and mechanical installers reach for on the same job: sheet metal gauge to actual thickness, rebar bar sizes and weights, socket and nut sizes across imperial and metric, hex key dimensions, and pipe thread specifications. Same approach as the electrical charts — dimensions derived from published standards geometry, with the method stated rather than a table copied from a catalogue.

Wire size by breaker rating

Copper and aluminium conductor size, derating and maximum run length for every standard overcurrent device rating.

These pages answer the question the way it actually gets asked: you know the breaker you are installing and you need the conductor that goes with it. Each rating shows the copper and aluminium size side by side, the temperature column the size assumes, the derated ampacity under common attic and bundled conditions, and the run length at which a 3 percent drop forces you up one size. Aluminium is typically one to two sizes larger than copper for the same rating, which is why it shows up on feeders and services where the cost saving outweighs the extra conduit.

Every conductor size

Full specifications per size, plus the voltage-drop, conduit-fill and breaker-size breakdown for each.

Sizes run from 18 AWG control and fixture wire up to 750 kcmil service entrance conductors. Each specification page lists circular mil area, nominal diameter, resistance per thousand feet for copper and aluminium, and allowable ampacity at the 60, 75 and 90 °C insulation ratings. The three linked sub-pages take that same size further: maximum run length by load, how many fit each conduit trade size at the 40 percent fill limit, and the largest overcurrent device it can sit behind.

Frequently asked

Which chart should I start with for a new circuit?: Start from the load. Convert watts to amps, size the overcurrent device with the breaker size calculator, then pick a conductor with the wire size chart. Finish by checking the run length in the voltage drop calculator and the raceway in the conduit fill calculator.
What is the difference between the charts and the calculators?: Charts are prebuilt tables you can scan or print for one common condition, such as copper at 75 °C and 30 °C ambient. Calculators take your actual numbers — material, insulation rating, ambient temperature, conductor count, length and load — and compute the result for that specific install.
Where do the numbers come from?: Conductor geometry comes from the standard ASTM B258 AWG progression, resistance from the copper and aluminium resistivity constants, and ampacity from a documented power-law fit against published allowable values. Conduit interior areas come from published trade-size dimensions. Every formula and constant is listed on the methodology page.
Can I use these results directly on a job?: Use them for planning, estimating and sanity checks. Final conductor, breaker and raceway selection must be verified against the code edition adopted in your jurisdiction and, where required, signed off by a licensed electrician. Local amendments and equipment listings can override a general calculation.
Why do the wire pages split into voltage drop, conduit fill and breaker size?: Each conductor size answers three separate questions on site: how far it can run before the drop exceeds 3 percent, how many of them fit in each conduit trade size, and which overcurrent device it can be protected by. Splitting them keeps each page focused on one decision instead of one long table.