Choosing the right wiring for a new custom home is one of the most important decisions a builder or homeowner makes — it affects safety, performance, future flexibility, and the cost of upgrades down the road. Modern custom homes must support not just lighting and appliances but whole-house HVAC, dedicated electric vehicle chargers, home automation, high-speed networks, and sophisticated entertainment systems. Because wiring is hidden once walls are closed up, selecting durable, code-compliant materials and planning for future needs up front can save thousands and avoid disruptive retrofit work.
For everyday branch circuits in North American custom residential construction, nonmetallic sheathed cable (NM-B, commonly known by brand names such as Romex) with copper conductors remains the most common choice. NM-B is economical, easy for electricians to install, and well-suited to typical 15–20 amp lighting and receptacle circuits. In higher-end custom homes or areas where future flexibility and ruggedness are priorities, installers often use conduit systems (electrical conduit with individual THHN/THWN conductors). Conduit is more expensive to install but makes it far easier to pull new cables later, allows for larger-capacity feeders, and can be required in certain exposed or high-humidity locations. Armored cable (MC/BX) and direct-burial-rated wiring appear in some applications as well, while service feeders and large subpanel runs sometimes use larger-gauge copper or aluminum conductors per local code.
Beyond power wiring, nearly every new custom home includes a structured low-voltage plan: Category-rated data cabling (Cat5e/Cat6/Cat6A or fiber), RG6 coax for video, speaker and multiroom audio wiring, security and doorbell systems, and conduit runs for future low-voltage upgrades. Modern electrical codes (NEC in the U.S.) also impose safety devices such as GFCI and AFCI protection, dedicated circuits for major appliances, and requirements for surge protection and grounding. Ultimately the “best” wiring approach depends on local code, budget, desired longevity and flexibility, and the complexity of the home’s systems. In the rest of this article we’ll compare the pros and cons of the common wiring types, highlight where contractors typically use each, and offer practical guidance for planning wiring in a new custom build.
NM‑B (Romex) versus THHN in conduit (common branch‑circuit wiring types)
NM‑B (Romex) is the bundled, sheathed cable assembly you see in the majority of residential work: insulated conductors plus a bonding/ground conductor inside a flat plastic jacket. It’s designed for dry, concealed locations (inside walls, ceilings, and floors) and is quick and inexpensive to install because it’s a single product you staple, fish or run through framing cavities. THHN is an individual insulated conductor that is intended to be run inside conduit or raceway. THHN (often supplied as THHN/THWN dual‑rated) is the typical choice where wires need mechanical protection, where runs are exposed, or where the installer or owner wants the ability to pull, replace or upgrade conductors later without opening walls.
The practical differences affect cost, labor and future flexibility. NM‑B is materially cheaper per circuit and faster to install for most interior branch circuits, but once it’s stapled into walls it’s harder to modify later; NM‑B is also limited to dry, concealed locations and must be protected where exposed. THHN in conduit raises upfront material and labor costs (conduit, fittings, pull boxes, and more labor for pulling), but conduit protects conductors, allows easier future swapping or upsizing, and can be required by local code or by design choices (e.g., firewalls, garages, basements, or accessible raceways). There are also ampacity and installation considerations: multiple individual conductors in a conduit can require ampacity derating when there are many current‑carrying conductors grouped together, and termination temperature ratings at devices can limit the use of the higher temperature ratings of THHN unless the terminations are rated accordingly.
In new custom home construction the most common approach for general branch circuits is still NM‑B for typical interior, concealed runs because it minimizes time and cost while meeting code in most jurisdictions. That said, many custom or high‑end builds use a hybrid strategy: NM‑B for most branch circuits and THHN in conduit for selected runs where future expansion, mechanical protection, or aesthetic concerns matter — for example conduit to service panels/subpanels, to exterior/outdoor boxes, home theaters, workshops, or any location where the owner wants a “pullable” system for upgrades. Final choice should be guided by local code requirements, the electrician’s recommendations, and the owner’s priorities for cost, durability and future flexibility.
Copper versus aluminum conductor choices
Copper and aluminum differ mainly in electrical conductivity, mechanical behavior, and cost. Copper has higher conductivity per cross‑section and greater tensile strength, so a smaller copper conductor carries the same current as a larger aluminum conductor. Aluminum is lighter and usually cheaper by weight, but because of its lower conductivity you must use a larger aluminum conductor to achieve the same ampacity. Aluminum also has greater thermal expansion and tends to creep under sustained load and heat cycles; that behavior, along with a reactive oxide layer that is electrically insulating, historically led to connection failures on branch circuits when improper terminations or materials were used. Modern practice uses specific aluminum alloys (AA‑8000 series) and approved connectors, plus anti‑oxidant compounds and correct torque values, to mitigate those issues.
Practical installation and code considerations determine where each metal is appropriate. For branch circuits in new homes, copper is overwhelmingly preferred because it’s easier to terminate reliably at switches, receptacles and breakers, occupies less conduit volume, and reduces long‑term maintenance and insurance concerns. Aluminum is still commonly used for larger feeders and service conductors (main service and subpanel feeders) where the weight and cost advantages are most significant — for example, utility service entrance cables or large ampacity feeders run from the meter to the panel. When aluminum is used, installers must follow NEC ampacity tables, use terminations and lugs rated for aluminum (or AL/CU), apply recommended torque values, and often apply antioxidant compound to mitigate oxidation and loosening.
For new custom home construction the most common wiring practice is copper branch‑circuit wiring (typically NM‑B/”Romex” for in‑wall residential runs) with copper THHN/THWN in conduit used selectively where the design calls for conduit, exposed runs, commercial‑style wiring or future‑proofing. Feeder and service conductors are frequently aluminum in many builds to save cost and weight, though high‑end custom builds will often specify copper feeders or full copper service for maximum reliability and lower voltage drop. In short: copper is the default and best practice for day‑to‑day branch circuits in new custom homes; aluminum remains a cost‑effective and code‑acceptable choice for larger feeders and service conductors when installed correctly.
NEC requirements and local code compliance (AFCI/GFCI, ampacity)
The National Electrical Code (NEC) and the local authority having jurisdiction (AHJ) set mandatory requirements that govern where and how protective devices and conductor sizing must be applied in a dwelling. AFCI protection has been progressively expanded by successive NEC editions to cover most outlets in living areas (initially bedrooms and now many other rooms), while GFCI protection is required for locations with increased shock risk such as kitchens, bathrooms, garages, outdoors, basements, and laundry areas. Ampacity rules determine the minimum conductor size for a circuit based on expected load and must account for continuous loads (requirement to size at 125% of continuous current), conductor material, and installation conditions defined by the NEC (temperature rating, bundling/raceway derating, etc.). Because jurisdictions adopt different NEC editions and may add local amendments, the exact scope of AFCI/GFCI coverage and allowable installation methods can vary — so code research and permit inspections are part of every new build.
Those code mandates directly influence how electricians plan and install wiring. Circuit layouts are developed from a load calculation to ensure feeders and panel capacity are adequate; branch-circuit conductor sizes are chosen to meet ampacity requirements (for example, common practice uses 14 AWG copper for 15 A and 12 AWG copper for 20 A branch circuits), and continuous loads are accounted for in breaker selection. Where many cables are run together (in attics, chases, or conduit), NEC derating factors reduce allowable ampacity and may force upsizing conductors. Similarly, the need for AFCI and GFCI protection affects device selection — protection can be provided at the breaker or at the receptacle, and some installations use combination AFCI/GFCI devices where the code allows. Proper grounding, neutral handling (especially with multi-wire branch circuits), labeling, and adherence to box fill and conduit fill rules are all part of demonstrating compliance at inspection.
As for the wiring type most common in new custom home construction: NM-B (commonly called “Romex”) with copper conductors is still the default for most concealed branch-circuit wiring in U.S. homes because it is cost-effective, easy to install, and permitted by code for standard concealed residential applications. That said, many custom or high-end homes (and some jurisdictions) use alternatives such as THHN/THWN conductors pulled in conduit or metal-clad (MC) cable for feeders, exposed runs in basements/garages, or where future flexibility, mechanical protection, or higher heat ratings are desired. The choice is driven by local code, builder and owner preference, inspection requirements, and specific installation conditions (e.g., long runs, high loads, or finished basements where surface raceways are preferred), so verifying local requirements and consulting a licensed electrician/inspector before specifying wiring is essential.
Low‑voltage/structured cabling (Ethernet, coax, alarm, smart‑home)
Low‑voltage or structured cabling covers the non‑power wiring that carries data, video and control signals: Ethernet (network) cable, coax for video, telephone/alarm conductors, speaker runs, doorbell and many smart‑home sensor/control cables. The most common choices for new homes are twisted‑pair Ethernet (Cat6 or Cat6a) for network and IP devices and RG‑6 coax for cable/antenna/video; alarm panels and sensors typically use multi‑conductor cables sized for the device (for example 18/4 or 22/4), and speaker runs are usually 16/14‑gauge. Installers also consider cable jacket ratings (CM/CMR/CMP) depending on where a cable is routed (in‑wall, riser, plenum), shielded vs. unshielded pairs for high‑EMI environments, and whether to add singlemode/multimode fiber for long runs or future bandwidth needs.
Best practice for structured cabling in a custom home focuses on centralization, future proofing and neat, testable terminations. Run home‑runs from each outlet back to a single structured media enclosure or patch‑panel location so signals can be managed and re‑patched as needs change. Leave spare conduits or raceways and pull strings for future upgrades, avoid bundling low‑voltage with AC power runs (maintain separation to reduce interference and meet code where required), and use proper wall plates and punch‑down/RJ45 terminations. Label and test each run with a certifier or network tester at installation, plan for PoE requirements (power budgets for cameras, access points, and sensors), and provide proper grounding/bonding for coax shields and any active equipment to minimize noise and surge damage.
If you mean “What wiring is most common in new custom home construction?” the answer splits by class: for mains and branch circuits the most common residential choice is NM‑B (Romex) cable because it’s economical and easy to install in stud cavities; THHN in conduit is used where conduit routing or higher protection is preferred, or by builders who want a conduit backbone for future changes. For low‑voltage systems in new custom homes, Cat6 (or Cat6a for higher bandwidth/future proofing) and RG‑6 coax remain the most common installs today, with homeowners increasingly asking for at least one or two fiber runs or empty conduits to support fiber later. In a custom build, plan the low‑voltage layout early, install higher‑category cable where practical, and include spare pathways and a centralized media enclosure so the infrastructure can adapt as technologies change.
Grounding, bonding, service panel sizing and subpanels
Grounding and bonding are foundational for electrical safety. Grounding provides a low‑impedance path to earth via the grounding electrode system (ground rods, Ufer concrete-encased electrode, metal water service, etc.), while bonding electrically connects metal parts and piping to the grounding system so they stay at the same potential. At the main service the neutral is bonded to the grounding electrode conductor with the main bonding jumper; downstream subpanels must have the neutral isolated from the panel enclosure and ground bus so the neutral current does not flow on grounding conductors. Proper bonding of gas, water, and structural metal, plus an adequate grounding electrode conductor, reduces shock risk and limits voltages on metallic systems during faults or lightning events.
Service panel sizing and the use of subpanels are planned from a load-calculation and convenience perspective. Modern custom homes commonly use a 200 A main service as a baseline because it covers typical HVAC, kitchen, electric range, dryer, and future EV chargers; larger or luxury homes may need 400 A or split services. The service and feeder conductors must be sized to the calculated load and protected by appropriately rated overcurrent devices; feeders to remote subpanels are sized for the loads they will serve and must include an equipment grounding conductor. Subpanels are useful to distribute circuits closer to loads (garage, workshop, master suite) and to segregate high‑amp circuits; remember to isolate neutral and bond ground only at the service equipment when installing subpanels.
As for the type of wiring most commonly used in new custom home construction: NM‑B (Romex) is the dominant choice for interior branch‑circuit wiring because it is economical, quick to install, and permitted in most residential locations. THHN/THWN conductors pulled in conduit are commonly used for feeders, exterior or underground runs, and in homes that want added future flexibility or higher mechanical protection; high‑amp circuits (stovetops, ranges, EV chargers) and service entrance conductors are also often copper or aluminum conductors in conduit or SER cable. The practical approach in many custom builds is a hybrid: NM‑B for the bulk of interior branch circuits, with THHN/THWN in conduit or specialty cable for feeders, exterior wiring, and any circuits where future upgrades or added protection are a priority.