When planning the electrical system for a new custom home, one of the first technical questions is “how big should the electrical service be?” The electrical service size — expressed in amperes (amps) and delivered at the service voltage — determines how much total power the utility will supply to the home and sets the capacity available for lighting, outlets, major appliances, HVAC systems, electric vehicle (EV) charging, and future additions. In North America the typical residential service is split-phase 120/240 V and common main breaker sizes are 100 A, 150 A, and 200 A; larger homes or those with heavy electrical demands may require 400 A or more. In regions that use single-phase 230 V residential supply, the same principle applies: the ampere rating of the service governs total available power.
Selecting the right service size depends less on a single rule-of-thumb and more on anticipated loads and future plans. Key factors include the square footage and number of living zones, whether heating and water heating are electric or gas, central air conditioning capacity, kitchen appliances (induction ranges, double ovens), laundry equipment, EV charging needs, workshops with heavy machinery, pools or hot tubs, and any home automation or whole-house battery systems. A modest custom home that relies on gas heating and has no EV might be fine with a 100–150 A service; a modern family home with multiple HVAC units, an electric range, a heat pump, and an EV charger commonly moves to a 200 A service; high-end estates, large properties with workshops, or properties with large solar-plus-storage systems often need 400 A or a divided service arrangement.
Code requirements and formal load calculations govern the official answer. Electricians and engineers use the National Electrical Code (NEC) (or local equivalents) and a calculated load worksheet to size the service and panel(s), accounting for continuous loads, demand factors, and motor starting currents. Other practical considerations include physical space in the meter/panel location, the need for subpanels, potential generator or transfer-switch integration, the local utility’s capacity and allowance for larger services, and the cost jump for larger meter sets, conduit, and conductors. Because the difference between undersizing (frequent tripped breakers, inability to use appliances simultaneously) and oversizing (higher upfront equipment and utility hookup costs) can be significant, thoughtful planning is important.
For anyone building a custom home, the best path is to involve your electrician or electrical engineer early in the design phase. They’ll perform a proper load calculation, discuss likely future needs (EVs, electrification trends), and coordinate with the utility so you get a service sized to meet both current and foreseeable demands without unnecessary expense. This planning ensures the electrical backbone of the house supports comfort, safety, and flexibility as technology and household needs evolve.
Common electrical service sizes (100A, 150A, 200A, 400A)
Common residential service sizes refer to the maximum continuous current that the service equipment and meter are rated to supply: typical nominal values are 100A, 150A, 200A, and 400A. These are normally delivered as single‑phase 120/240 V services for homes. A 100A service is common in older or very small dwellings and can be limiting once you add central air, electric heat, or multiple major appliances. A 150A service is a step up for modestly sized homes or renovations. The 200A service has become the de facto standard for new builds because it provides comfortable headroom for modern load demands (range/oven, clothes dryer, central HVAC, kitchen appliances, and some EV charging). A 400A service (or paired 200A services) is used for large custom homes with multiple high‑capacity systems, large workshops, multiple HVAC systems, heavy EV charging needs, or when whole‑house backup and battery storage are integrated.
Which size is appropriate depends on the expected loads and the results of a formal load calculation (per the applicable electrical code and local utility rules). The load calculation accounts for fixed appliances, HVAC loads, electric water heating, and lighting, applying demand factors that reduce the total assumed simultaneous load for many circuits. Because those calculations and code interpretations vary, designers often choose a 200A service as the most cost‑effective baseline for a new custom home and then upsize to 400A or add dedicated high‑capacity feeders if the calculation or future plans (multiple EVs, an all‑electric home, large shop equipment) indicate more capacity is needed. Physical equipment considerations — panel space, conductor size, meter and service entrance equipment — also influence the choice and installation cost.
So, what is the typical electrical service size for a new custom home? In most cases today a 200A, single‑phase 120/240 V service is the standard and is sufficient for the majority of custom homes when planned correctly. If you expect unusually large loads now or in the near future (several EV chargers, whole‑house electric heating, large shop motors, extensive battery + PV systems), a 400A service or dual 200A arrangements are commonly chosen to future‑proof the installation. Final selection should follow a qualified electrician’s load calculation, local code requirements, and coordination with the utility (availability of three‑phase, meter locations, and service lateral capacity) to ensure safe, compliant, and cost‑effective service sizing.
Load calculation and NEC demand factors
Load calculation starts by inventorying every electrical load in the home — general lighting and receptacles, small-appliance and laundry circuits, fixed appliances (ranges, ovens, clothes dryers, water heaters), HVAC and heat pumps, motor loads, and any special loads such as pool equipment, hot tubs, EV chargers, or workshops. The National Electrical Code (NEC) defines methods to convert those connected loads into a calculated demand using standard values (for example, a VA-per-square-foot allowance for lighting and fixed VA values for small-appliance circuits) and then applies demand factors that recognize not all loads operate at full capacity simultaneously. For sizing feeders and the service, continuous loads (loads expected to run for 3 hours or more) must be considered at 125% of their continuous current, and the total calculated demand is used to select conductor ampacity and overcurrent protection.
NEC demand factors and tables are the key tools that reduce the sum of connected loads to a realistic service requirement. There are specific provisions for dwelling units (lighting load VA per square foot, required small‑appliance and laundry circuit allowances), and separate demand factor tables for fixed cooking appliances, dryers, and water heaters that progressively reduce the contribution of multiple similar appliances. Motor loads and HVAC equipment are handled differently because of inrush/start-up currents and nameplate ratings; manufacturers’ data and NEC guidance determine whether the full motor nameplate load, locked-rotor current, or a reduced demand allowance is used in the calculation. Local code amendments and the authority having jurisdiction (AHJ) can affect which tables and rounding rules apply, so a formal calculation by a licensed electrician or electrical engineer is usually required for permit approval.
Typical starting points for new custom homes reflect the outcomes of those load calculations. For modern single‑family custom homes, 200-amp, 120/240 V single‑phase service is the most common baseline choice because it accommodates typical lighting, appliances, and central HVAC for many floor plans. Smaller homes or very modest builds may be served with 100–150 A, while larger luxury homes with multiple HVAC systems, electric heat, EV chargers, whole‑house electric vehicle infrastructure, large workshops, pools, or extensive electric cooking might require 400 A or more (sometimes split into multiple service meters or a 400/600 A service entrance). The only reliable way to determine the correct service size is to perform the NEC-compliant load calculation for the specific home (including expected future expansions) and discuss provision for future loads with the electrician and the utility — many builders choose a 200 A service as a practical compromise or step up to 400 A if significant electric heating, multiple EV chargers, or high-end electric loads are planned.
Impact of major appliances and HVAC on service size
Major appliances (ranges, ovens, electric clothes dryers, water heaters, electric ovens, and large electric cooking appliances) and HVAC equipment are often the largest continuous and motor loads in a home and therefore exert the biggest influence on the required main service size. Typical nameplate ampere draws: electric ranges/ovens often demand 30–50 A circuits (single range may be rated 40–50 A), electric dryers 30 A, water heaters 30 A or more, and dishwashers/microwaves 15–20 A each. While not every appliance runs continuously, the National Electrical Code (NEC) load-calculation methods apply demand factors and treat some loads as continuous (requiring 125% sizing) and others with diversity, so the combined impact on service size depends on which loads can reasonably be expected to operate at once. Also consider motor starting (inrush) currents from pumps and compressors — a compressor may draw several times its running current at startup — which affects feeder capacity and voltage-drop planning even if it doesn’t change the steady-state ampacity requirements.
HVAC systems—central air conditioners, heat pumps, furnaces with electric heat strips, and multi-zone systems—can dominate the load calculation. A typical modern 3-ton central air conditioner/heat pump may have a running current in the 20–40 A range on a dedicated 30–60 A circuit depending on efficiency and motor design, but starting currents can be much higher; electric resistance furnaces or strip-heat add large continuous loads that substantially increase service requirements. Homes using all-electric heating (baseboard or electric furnaces) can require much larger main services than homes that use gas or fuel for heat. Multi-unit HVAC setups (separate systems for different zones or multiple compressors) multiply the effect and often push custom homes from a standard service into a larger capacity service.
For new custom homes the typical service size nowadays is a 200 A, 120/240 V single‑phase service — this is the common baseline because it accommodates central HVAC, kitchen appliances, laundry, and most modern plug loads with headroom for normal diversity. Smaller 100 A or 150 A services are generally inadequate for most new custom homes, while 400 A (or parallel/dual 200 A services) is chosen for very large homes, homes with multiple large HVAC systems, all-electric heating, simultaneous EV charging, substantial workshop loads, or other heavy demands. The correct choice should come from a formal NEC-compliant load calculation done by a licensed electrician or electrical engineer in coordination with the utility; that calculation will factor in appliance ratings, HVAC nameplate data, continuous-load rules, demand factors, and reasonable future expansion so the service is safe and appropriately sized.
Future-proofing: EV charging, solar PV, battery storage, and expansions
Future-proofing an electrical system means planning the service and distribution to accommodate foreseeable additions without costly rework. For EV charging, that commonly means ensuring the main service and panel have enough capacity and spare breaker spaces for one or more dedicated 240 V circuits (Level 2 chargers are the common expectation), and running conduit or a 240 V stub to garages or carports so a charger can be added later with minimal disruption. For rooftop or ground-mounted solar PV and battery storage, plan physical space for inverters, disconnects and batteries, and allow panel space or a spare conduit for combiner/inverter wiring and for the battery system’s interconnection equipment. In practice this also includes siting equipment for ventilation, code-required clearances, and accessible locations for meter and service equipment to simplify future utility or inspector access.
Practical strategies to future-proof include sizing the service and main panel with headroom, providing subpanels where high-load areas (workshop, ADU, pool equipment) are likely, and leaving multiple spare breaker slots. Include dedicated raceways/conduits from the service or panel to likely expansion locations (garage, roof edge, mechanical room) so long cable pulls or trenching aren’t required later. If you plan solar and batteries, coordinate locations so inverters and battery modules can be mounted near the main panel or a service-side interconnection point; if you plan whole-house backup or a generator, allow for space to install an automatic transfer switch or a manual transfer arrangement. All of these measures reduce future labor and increase the chance that an upgraded system will meet local code and utility interconnection requirements without major change to the service entrance.
What is typical for a new custom home: today a 200-amp single-phase service is the norm and a practical minimum for most new custom homes because it provides room for HVAC, electric ranges, clothes dryers, an EV charger, and some future additions without immediate upgrade. Smaller houses or lightly equipped properties sometimes use 150 A, but that’s less future-ready; very large homes, estates with electric heating, multiple EV chargers, workshops, or commercial loads commonly step up to 400 A or larger, and three-phase where required. The correct choice depends on a detailed load calculation (per the National Electrical Code and local amendments) that accounts square footage, heating/cooling, major appliances, EVs, pools/spas, and planned solar/battery systems. The best approach is to have a licensed electrician or electrical engineer perform the load calculation and specify service size and layout while incorporating the future-proofing items (conduit runs, spare panel capacity, inverter and battery locations) so the installed system meets current needs and is ready for foreseeable upgrades.
Service entrance types and panel configurations (single‑phase vs three‑phase, meter, main breaker)
Service entrance refers to how electric service is brought to the building — commonly overhead conductors on a mast or underground service lateral and conduit — and the first piece of equipment you encounter: the meter and the main disconnect. Residential service is usually single‑phase, 120/240 V, delivered through a meter that the utility owns or seals; the meter feeds a service disconnect (main breaker) and the service panel (load center). Larger or special installations may use three‑phase service (120/208 V or 277/480 V depending on configuration) when the expected loads — commercial equipment, large HVAC compressors, elevators, or heavy machine tools — justify it and when the utility can supply it. The service entrance also includes the service conductors, weatherhead or underground splice, grounding electrode system (ground rods, water pipe, Ufer ground) and bonding back to the neutral at the service disconnect as required by code.
Panel configurations vary based on space, expandability, and code requirements. Typical residential setups include a meter-main (meter and main breaker in one enclosure) or separate meter and main panel; panels can be “main breaker” type (single main disconnect) or less commonly “split-bus” or main-lug-only panels where a separate main disconnect is provided elsewhere. Most new homes require ample breaker spaces, AFCI/GFCI protection on specified circuits, and provision for subpanels if you plan to add workshops, outbuildings, or EV chargers. Conductor ampacity, bus rating, proper labeling, accessible working clearances, and coordinated grounding/bonding are all important elements installers must size and arrange to comply with the National Electrical Code and local amendments.
For a new custom home, the typical electrical service size is a 200 amp, single‑phase, 120/240 V service — it offers a practical balance of capacity and cost and is chosen to accommodate modern demands like multiple HVAC units, large kitchens, and EV charging. Homes with very large square footage, several electric heat or HVAC systems, whole‑house electric loads (e.g., multiple ovens, large pools/spas, heavy duty workshops), extensive solar-plus-battery systems, or commercial‑type equipment may require 400 amp service or three‑phase service; utilities sometimes provide 320A/400A metering or twin 200A panels as alternatives. Final sizing should be based on a formal load calculation using current code demand factors, coordination with your utility for available service types, and consultation with a licensed electrician or electrical engineer to allow for future expansion and proper panel configuration.