A calculator with no code values inside it
This tool runs the arithmetic of a service load calculation and nothing else. It contains no NEC table values, no unit loads, no demand factors, and no tier breakpoints. Every code-derived number is one you type in after reading it out of the edition of NFPA 70 your jurisdiction has actually adopted. That is deliberate: those values change between editions and adoption lags by state and by city, so a hardcoded number would be silently wrong for a large share of the people using it. What the tool contributes is bookkeeping — keeping eight load categories, their demand factors, and a tiered lighting calculation straight while you work.
What you enter and what happens to it
Connected loads go in as volt-amperes across eight categories: general lighting, small appliance and kitchen circuits, laundry, range or oven, dryer, water heater, HVAC, and other fixed appliances. Five of those — range, dryer, water heater, HVAC, and other fixed appliances — get their own demand factor field, and the tool computes demand as connected VA × (percentage ÷ 100). Each field carries a hint naming the code section you should be reading for that value.
General lighting, small appliance, and laundry are handled differently. They are summed into a single combined figure first, and the tiered lighting demand structure is applied to that combined number rather than to each one individually. Total demand VA is the lighting demand plus the five factored categories. Calculated amps are total demand VA ÷ service voltage, where voltage is a dropdown of 120, 208, 240, or 480 V and defaults to 240.
The final step rounds the calculated amps up to the next entry in the tool’s list of common service ratings: 100, 125, 150, 200, 225, 320, 400, 600, 800, 1000, and 1200 A. As the page notes, those are equipment rating increments in ordinary use, not code values.
How the lighting tiers are applied
The combined lighting, small appliance, and laundry figure is sliced into three parts. The portion up to the Tier 1 limit is multiplied by the Tier 1 factor. The portion spanning from the Tier 1 limit up to the Tier 2 limit is multiplied by the Tier 2 factor. Anything above the Tier 2 limit is multiplied by the remainder factor. The three products are added together.
The following numbers are arbitrary placeholders chosen only to show the arithmetic. They are not code values and must not be carried onto a job. With a combined figure of 10,000 VA, a Tier 1 limit of 3,000 VA at 100%, a Tier 2 limit of 7,000 VA at 40%, and a remainder factor of 20%: the first 3,000 VA contributes 3,000, the next 4,000 VA (from 3,000 up to 7,000) contributes 1,600, and the remaining 3,000 VA contributes 600, for a lighting demand of 5,200 VA. Add that to your factored range, dryer, water heater, HVAC, and other fixed appliance demands, divide by voltage, and round up.
Two defaults that will bite you
Every load field and every tier field opens at 0, while the five demand factor fields open at 100%. An untouched form therefore returns 0 VA, 0 A, and a 100 A recommendation that means nothing except that 100 is the smallest number in the list. Separately, if both tier limits are left at 0 the tiering is bypassed and the whole combined lighting figure is multiplied by the Tier 1 factor alone — which at its default of 0 contributes nothing. Fill in all five tier fields before you trust the lighting line.
Limitations
This is an estimating and planning aid. It performs arithmetic on values you supply. It does not interpret the code, does not know which NEC edition your authority having jurisdiction has adopted, does not verify that you selected the right section for your occupancy type, and does not check conductor sizing, ampacity derating, voltage drop, overcurrent protection, service equipment ratings, grounding and bonding, or available fault current. It implements a standard-method structure only; the optional dwelling method is organized differently and is not what this page computes. Nothing here replaces an engineered design, a code interpretation, or the determination of a licensed electrician or licensed professional engineer. Verify every input against your adopted code edition and have the final load calculation reviewed before you size equipment, submit for permit, or order a service.
For related work, the conduit fill calculator takes the same user-supplied-values approach to raceway fill, the HVAC load calculator gives a rough starting point for the mechanical load you enter here, and the copper price page tracks the commodity behind most of your conductor budget.
Frequently Asked Questions
Why does a blank form show 100 A?
Because every connected load field opens at 0 and both lighting tier limits open at 0, an untouched form produces a total demand of 0 VA and 0 calculated amps. The tool then rounds up to the smallest entry in its list of common service ratings, which is 100 A. That is a rounding artifact, not a recommendation. An empty form is not an answer, and 100 A is not a default service size.
Why do small appliance and laundry not have their own demand factor fields?
The tool folds general lighting, small appliance, and laundry into one combined figure and applies the tiered lighting demand structure to that combined number. Those three categories therefore share a single set of demand factors, which are the five tier values you enter. Range, dryer, water heater, HVAC, and other fixed appliances each get their own percentage field instead.
Does the building type selector change the calculation?
No. The dropdown is a label carried into the copied summary so you know which calculation you were running. It does not alter any arithmetic, does not swap in a different demand structure, and does not restrict which fields apply. Choosing the right code section for your occupancy is your job, not the tool.
What happens if I leave the tier limits at zero but enter a tier factor?
The tool skips the tier logic entirely and multiplies the whole combined lighting, small appliance, and laundry figure by the Tier 1 factor alone. That is a deliberate fallback for a flat, single-percentage calculation, but if you meant to apply a tiered structure and only filled in the percentages, your lighting demand will come out wrong. Enter all five tier fields, or none.