How is the required service amperage calculated here?

General lighting is estimated at 3 VA per square foot of dwelling floor area (NEC 220.12) and added to the small-appliance circuits (1500 VA each) and laundry circuit(s) (1500 VA each). The NEC Table 220.42 demand factor — 100% of the first 3000 VA, 35% of the remainder — is applied to that subtotal, and to that subtotal only. Fixed appliances are then added at 100% of their nameplate rating, because Table 220.42 does not cover them. The two together are divided by the service voltage to get amps.

Why is a demand factor applied instead of just adding up every load?

Not every lighting point and receptacle in a home is loaded at once, so NEC Table 220.42 allows that part of the load above the first 3000 VA to be counted at 35%. The diversity argument is specific to general lighting and receptacles, which is why the table applies to them and not to fixed appliances: a water heater or a range draws its nameplate load whenever it runs, so counting it at 35% would undersize the service. Applying the factor to everything is a common and consequential error — on a typical dwelling it understates the required service by around a third.

Does this include the heating and air conditioning load?

No, and that is the biggest thing missing. There is no field here for a furnace, heat pump, air handler or condenser. The fixed-appliance box is still not the place to put one, but not for the reason this answer used to give: appliances entered there are carried at 100%, exactly as a full calculation carries fixed electric space heating. What is missing is the noncoincident treatment. That calculation also treats heating and cooling as noncoincident — the two are unlikely to run at once, so it is generally permitted to carry the larger of the pair rather than both — but the one it carries goes in at full value. In an all-electric home an electric furnace, or a heat pump with resistance backup, can be worth more on its own than everything this page adds up, which is why HVAC is usually what settles the service size there. Read this number as the non-HVAC portion only, and have the heating and cooling added by whoever prepares the load calculation for the permit.

My outlets are 120 V — do I enter 120 or 240 for service voltage?

Enter 240 for a typical North American house. A 120/240 V split-phase service brings in two 120 V legs and is rated across both, so 240 is the divisor for the service as a whole even though most branch circuits run at 120 V. The field accepts anything from 120 to 240 and simply divides the demand VA by what you type, so entering 120 doubles the answer: on the default inputs, 240 V returns about 38 A and 120 V returns about 76 A from an identical connected load. Use 208 only for a unit fed from a 120/208 V system, which is common in apartment buildings.

Why is the answer so much smaller than the 100 A or 200 A service a house actually gets?

Two reasons, and the 35% demand factor is no longer one of them. That factor applies only to general lighting and receptacles, which is where NEC Table 220.42 puts it; the fixed-appliance figure you enter is carried at 100%, as the breakdown row says. What is genuinely absent is the heating and cooling load, which on an all-electric house is often the largest single item on the calculation and has no field here at all. The second reason is that a full standard-method calculation runs household cooking appliances and electric dryers through their own demand tables rather than at nameplate, which cuts them — so this page is conservative on those and silent on HVAC, and the two do not cancel in any predictable way. For scale: at 240 V with every input pushed to its maximum it returns about 193 A. A low figure here is not proof that an existing service has spare capacity — that question is settled by a full calculation, not by this one.

Materials & Quantities

Home Electrical Service Panel Amperage Sizing Calculator

Estimate the minimum electrical service amperage needed for a dwelling, using a simplified NEC 220 general load calculation.

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Dwelling Floor Area

The total livable floor area of the dwelling used for general lighting load.

Per NEC 220.12, this is the outside dimensions of the dwelling unit, not including open porches, garages, or unused/unfinished spaces.

Number of Small Appliance Circuits (Kitchen, Commonly 2)

20A kitchen/dining small-appliance circuits, each counted at 1500 VA.

NEC 220.52 requires at least two small-appliance branch circuits for the kitchen/dining area.

Number of Laundry Circuits (Commonly 1)

Dedicated 20A laundry branch circuits, each counted at 1500 VA.

NEC 220.52 requires at least one dedicated laundry branch circuit in a dwelling.

Total Fixed Appliance Nameplate Load (VA, Water Heater/Range/Dryer/etc.)

Sum of the nameplate VA for fixed appliances such as the water heater, range, oven, and dryer.

Add up the nameplate ratings (VA or watts) from each fixed appliance's data plate.

Service Voltage (V, Commonly 240 for US Dwellings)

The nominal service voltage supplying the dwelling.

Most US single-family dwellings are served by 240V (120/240V split-phase) service.

Estimated required service amperage

38 A

Low confidence

This is a SIMPLIFIED illustrative version of the NEC 220 standard method, omitting several real-world factors (HVAC load, EV chargers, largest motor load addition, optional calculation method). Fixed appliances are carried at 100% because this page takes a VA total and not a count; 220.53 permits 75% where four or more fastened-in-place appliances are present, and a range or a dryer has its own table (220.55, 220.54) that this does not apply. It is NOT a substitute for a complete NEC Article 220 load calculation performed by a licensed electrician, which determines your actual required service size and is required for permitting. Always round up to the next standard breaker/service size (100A, 125A, 150A, 200A) and consult your electrician.

Total connected load (before demand factor): 13,030 VA
General lighting and receptacles, after the 220.42 factor: 5,110.5 VA
Fixed appliances, at 100%: 4,000 VA
Demand load: 9,110.5 VA

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Simplified NEC Article 220 standard method: general lighting load = floor area × 3 VA/ft² (NEC 220.12), added to the small-appliance circuits (1500 VA each) and the laundry circuit (1500 VA). The NEC Table 220.42 demand factor (100% of the first 3000 VA, 35% of the remainder) applies to THAT subtotal only — 220.42 governs general lighting and receptacle load. Fixed appliances are then added at 100% of nameplate, because they are not part of the 220.42 pool. Required amperage = total demand VA ÷ voltage

Inputs used

Dwelling Floor Area: 1510 sq ft
Number of Small Appliance Circuits (Kitchen, Commonly 2): 2
Number of Laundry Circuits (Commonly 1): 1
Total Fixed Appliance Nameplate Load (VA, Water Heater/Range/Dryer/etc.): 4000
Service Voltage (V, Commonly 240 for US Dwellings): 240

Intermediate steps

Total connected load (before demand factor): 13,030 VA
General lighting and receptacles, after the 220.42 factor: 5,110.5 VA
Fixed appliances, at 100%: 4,000 VA
Demand load: 9,110.5 VA

Final result37.96 A

Confidence note: This is a SIMPLIFIED illustrative version of the NEC 220 standard method, omitting several real-world factors (HVAC load, EV chargers, largest motor load addition, optional calculation method). Fixed appliances are carried at 100% because this page takes a VA total and not a count; 220.53 permits 75% where four or more fastened-in-place appliances are present, and a range or a dryer has its own table (220.55, 220.54) that this does not apply. It is NOT a substitute for a complete NEC Article 220 load calculation performed by a licensed electrician, which determines your actual required service size and is required for permitting. Always round up to the next standard breaker/service size (100A, 125A, 150A, 200A) and consult your electrician.

What this calculation does not cover

Amps are not spaces. This sizes the SERVICE and says nothing about the panel's physical capacity — a dwelling can sit comfortably inside 100 A of calculated demand and still need a new panel because every breaker position is taken, the busbar is rated below what is being added to it, or the enclosure will not accept the breaker type the new circuits require. Panels get replaced for spaces at least as often as for amperage.
The amperage is where the rest of the sizing starts, not where it ends. Service-entrance conductors, the grounding electrode conductor, the meter base, the mast or lateral and the utility's own drop or transformer all follow from this number and none of them fall out of it — and a dwelling service gets its own conductor allowance under NEC 310.12 rather than the general ampacity table, so the wire for a 200 A house is not the wire that table would give it.
One dwelling unit. A second unit on the same service, an ADU, or a detached garage or workshop on a feeder are not added by piling their loads into these boxes: a multifamily service runs through 220.84's own demand table, and a feeder to an outbuilding is calculated as its own load under 220.40. Stacking two dwellings into this page understates both of them.

Add the equipment this sizes

This result is a specification — 38 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Simplified NEC Article 220 standard method: general lighting load = floor area × 3 VA/ft² (NEC 220.12), added to the small-appliance circuits (1500 VA each) and the laundry circuit (1500 VA). The NEC Table 220.42 demand factor (100% of the first 3000 VA, 35% of the remainder) applies to THAT subtotal only — 220.42 governs general lighting and receptacle load. Fixed appliances are then added at 100% of nameplate, because they are not part of the 220.42 pool. Required amperage = total demand VA ÷ voltage

Which documents these citations point at

National Electrical Code (NFPA 70) — Article 220, 220.12, Table 220.42 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

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Called something else where you work? Consumer unit / breaker panel — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate home electrical service panel amperage sizing in 6 steps

Dwelling Floor AreaThe total livable floor area of the dwelling used for general lighting load.
Number of Small Appliance Circuits (Kitchen, Commonly 2)20A kitchen/dining small-appliance circuits, each counted at 1500 VA.
Number of Laundry Circuits (Commonly 1)Dedicated 20A laundry branch circuits, each counted at 1500 VA.
Total Fixed Appliance Nameplate Load (VA, Water Heater/Range/Dryer/etc.)Sum of the nameplate VA for fixed appliances such as the water heater, range, oven, and dryer.
Service Voltage (V, Commonly 240 for US Dwellings)The nominal service voltage supplying the dwelling.
Estimated required service amperageThe tool computes the estimated required service amperage from those figures and shows the formula, its sources, and a confidence rating alongside it.

Estimated required service amperage by dwelling floor area

Page defaults, not your figures above.

Dwelling Floor Area	Estimated required service amperage (A)
1,000 sq ft	35.7
1,500 sq ft	37.9
2,000 sq ft	40.1
2,500 sq ft	42.3
3,000 sq ft	44.5

Frequently asked questions

How is the required service amperage calculated here?: General lighting is estimated at 3 VA per square foot of dwelling floor area (NEC 220.12) and added to the small-appliance circuits (1500 VA each) and laundry circuit(s) (1500 VA each). The NEC Table 220.42 demand factor — 100% of the first 3000 VA, 35% of the remainder — is applied to that subtotal, and to that subtotal only. Fixed appliances are then added at 100% of their nameplate rating, because Table 220.42 does not cover them. The two together are divided by the service voltage to get amps.
Why is a demand factor applied instead of just adding up every load?: Not every lighting point and receptacle in a home is loaded at once, so NEC Table 220.42 allows that part of the load above the first 3000 VA to be counted at 35%. The diversity argument is specific to general lighting and receptacles, which is why the table applies to them and not to fixed appliances: a water heater or a range draws its nameplate load whenever it runs, so counting it at 35% would undersize the service. Applying the factor to everything is a common and consequential error — on a typical dwelling it understates the required service by around a third.
Is this result enough to size and permit my actual service panel?: No. This is a simplified illustrative version of the NEC 220 standard method that omits real-world factors such as HVAC load, EV chargers, and the largest-motor load addition. It is not a substitute for a complete NEC Article 220 load calculation performed by a licensed electrician, which is required for permitting — always round up to the next standard breaker/service size (100A, 125A, 150A, 200A) and consult your electrician.
Does this include the heating and air conditioning load?: No, and that is the biggest thing missing. There is no field here for a furnace, heat pump, air handler or condenser. The fixed-appliance box is still not the place to put one, but not for the reason this answer used to give: appliances entered there are carried at 100%, exactly as a full calculation carries fixed electric space heating. What is missing is the noncoincident treatment. That calculation also treats heating and cooling as noncoincident — the two are unlikely to run at once, so it is generally permitted to carry the larger of the pair rather than both — but the one it carries goes in at full value. In an all-electric home an electric furnace, or a heat pump with resistance backup, can be worth more on its own than everything this page adds up, which is why HVAC is usually what settles the service size there. Read this number as the non-HVAC portion only, and have the heating and cooling added by whoever prepares the load calculation for the permit.
My outlets are 120 V — do I enter 120 or 240 for service voltage?: Enter 240 for a typical North American house. A 120/240 V split-phase service brings in two 120 V legs and is rated across both, so 240 is the divisor for the service as a whole even though most branch circuits run at 120 V. The field accepts anything from 120 to 240 and simply divides the demand VA by what you type, so entering 120 doubles the answer: on the default inputs, 240 V returns about 38 A and 120 V returns about 76 A from an identical connected load. Use 208 only for a unit fed from a 120/208 V system, which is common in apartment buildings.
Why is the answer so much smaller than the 100 A or 200 A service a house actually gets?: Two reasons, and the 35% demand factor is no longer one of them. That factor applies only to general lighting and receptacles, which is where NEC Table 220.42 puts it; the fixed-appliance figure you enter is carried at 100%, as the breakdown row says. What is genuinely absent is the heating and cooling load, which on an all-electric house is often the largest single item on the calculation and has no field here at all. The second reason is that a full standard-method calculation runs household cooking appliances and electric dryers through their own demand tables rather than at nameplate, which cuts them — so this page is conservative on those and silent on HVAC, and the two do not cancel in any predictable way. For scale: at 240 V with every input pushed to its maximum it returns about 193 A. A low figure here is not proof that an existing service has spare capacity — that question is settled by a full calculation, not by this one.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.