About Watts Math

Watts Math is a set of free calculators for the electrical and heating-and-cooling questions that come up in a house: what gauge of wire and what breaker, how much voltage a long run loses, how many BTUs a room needs, what size mini split or generator, what an appliance costs to run, how many solar panels. Each one is built on a code table or a published sizing chart, and each page names it so you can check the number.

Where the numbers come from

Conductor resistance is NEC Chapter 9, Table 8; ampacity is NEC Table 310.16 at the 75°C column with the small-conductor breaker limits of 240.4(D) and the 125% continuous-load rule of 210.20(A). Voltage drop follows the 3% branch and 5% total recommendations in the 210.19(A) informational note. Cooling BTUs use the ENERGY STAR room air-conditioner chart and its published adjustments; heating uses the 30 to 60 BTU per square foot by climate zone that contractors quote, and the page says plainly that a Manual J calculation is the real answer. Generator watts come from manufacturer appliance tables. Electricity cost is watts times hours at your rate, with the US average from the Energy Information Administration as a default. Solar sizing uses NREL PVWatts' default system losses and regional peak sun hours.

What this site doesn't do

It doesn't store what you enter, require an account, or replace a licensed electrician, an HVAC contractor, or your local code and inspector. The tables here are the national code's general case; conduit fill, ambient temperature, bundling, terminal ratings and local amendments all change the answer, and an inspector will apply them. Use the numbers to plan and to understand a quote, not to skip the permit.

Corrections

If you are an electrician, HVAC technician or engineer and a table value or rule here is out of date -- code cycles change -- email hello@wattsmath.com. Sources are named on each page so a correction can be checked against them.