Systems calculators

What these tools have in common

Every calculator in this category answers one question: what size does it need to be? Not what it costs, not how much to order — what size. A conductor, a conduit, a piece of equipment, a duct. These are the numbers that get decided before anything is bought, and the ones an inspector will look at afterwards.

They also share something more important. None of these sizes is a matter of preference or of buying the biggest thing you can afford. Each one is governed by a published table — the NEC for electrical work, ACCA Manual J, S and D for HVAC, the plumbing code for drainage and supply — and those tables exist because the consequences of getting the number wrong are physical. A conductor sized too small does not simply underperform; it heats up inside a wall where nobody can see it. A duct sized too small does not just move less air; it drives static pressure up until the blower struggles and the coil freezes.

Getting it wrong is a safety and inspection problem, not a budget problem

This is the difference between this category and the cost calculators. If a remodel estimate is twenty percent light, you are annoyed and you find the money. If a conductor is one size light, the circuit will pass a visual inspection, run apparently fine for years, and cook its insulation slowly. If an air conditioner is one ton oversized, it will short-cycle, never dehumidify, and wear its compressor out early while the homeowner blames the brand.

Under-sizing and over-sizing both fail, and they fail differently. Undersized electrical is a heat problem. Oversized HVAC is a humidity and equipment-life problem. Undersized duct is a noise and airflow problem. In none of those cases does the failure announce itself on the day of installation, which is precisely why the tables exist and why the inspector cares.

Which tool to reach for

Reach for the wire size calculator when you know the load and the distance and need the conductor — it runs both the ampacity and the voltage drop checks and tells you which one governs, which on a long run is nearly always voltage drop. Reach for the conduit fill calculator once you know the conductors and need to know what raceway they legally fit in. The two go together: fill and derating are related, because the same conductor count that fills the pipe also reduces what each conductor can carry.

On the mechanical side, start with the BTU calculator to get the cooling load and the tonnage band, then take the resulting airflow into the duct size calculator at 400 CFM per ton. Sizing duct before you know the load is doing the work in the wrong order.

What these numbers can and cannot tell you

They can tell you whether a proposal is in the right band, catch the contractor who is sizing an air conditioner by square footage and rounding up, and give you the vocabulary to ask a useful question. They follow published code tables and standard engineering formulas, and the explainer on each page names the table the value came from so you can check it.

They are not a substitute for a licensed electrician's or mechanical contractor's design. They size one thing in isolation. A real electrical design also selects overcurrent protection, sizes the grounding conductor, checks box fill and bend count, and applies continuous-load factors. A real HVAC design runs Manual J room by room, selects equipment with Manual S, and lays out the whole duct system with Manual D against the blower's actual static pressure. Code editions differ by jurisdiction and are amended locally. Use these to understand and to check. Have the design signed by someone licensed to sign it.