Service Panels & Load Center Install

The service panel is the heart of every residential electrical system. Everything - every receptacle, every lighting circuit, every piece of equipment - connects back to the panel's busbars through a breaker. A wrong service-sizing decision, a missed bonding jumper, or a cross-connected neutral in a sub-panel can burn a house down twenty years from now. This guide walks you through sizing, placement, grounding and bonding, sub-panel rules, breaker types, SPDs, and the torque values and labeling that separate inspection-ready work from a callback.

Service Sizing

The first question on every new build: what service size? Residential typical:

| Service Size | Typical Use                                    |
|--------------|------------------------------------------------|
| 100A         | Minimum new residential (small home, gas heat, no EV) |
| 125A         | Common midsize residential                     |
| 150A         | Larger homes with electric water heater/dryer  |
| 200A         | Default for most new homes today               |
| 320A (400A)  | Large homes, all-electric, EV + heat pump      |
| 400A+        | Custom, multiple sub-panels, heavy load        |

The service size is calculated per NEC Article 220 - a load calculation that adds up lighting, appliance, HVAC, and "other loads" with demand factors. On a typical 2,500 sq ft all-electric home with heat pump, range, dryer, water heater, and an EV charger, the calculation often lands at 180-220A, so a 200A service is the standard pick.

A 100A service is the legal minimum in most jurisdictions for a new single-family home, but it rarely fits modern loads. Most shops default to 200A even when a smaller service would calculate.

Meter Base to Panel

The service-entry sequence from the utility drop to the panel:

1. Utility service drop or lateral (overhead or underground, owned by the utility).
2. Weatherhead or lateral termination (the transition point where utility ownership ends and the homeowner's system begins).
3. Meter base (socket) - Where the utility meter plugs in. Outdoor, sealed, weatherproof.
4. Service entrance conductors (SECs) - The unfused conductors between the meter and the main disconnect. Any fault on these conductors must clear on the utility's transformer-side protection - so keep the run short and in a dedicated raceway.
5. Main disconnect (main breaker or main lug, inside or outside).
6. Panel busbars - Where the branch breakers snap in.

In most new installations the main breaker is inside the main panel - a "main breaker panel." In some markets and where the panel is set far from the meter, an outside disconnect (typically a 200A breaker at the meter) supplies a main-lug panel inside.

Service Entrance Conductor (SEC) Sizing

Sizing depends on ampacity, length, insulation type, and whether conductors are copper or aluminum. Residential SEC is almost always aluminum (cheaper for larger sizes). Typical pairings (at 75 deg C column, following 310.16 and service-rated provisions):

| Service Size | Copper SEC (typ)  | Aluminum SEC (typ) |
|--------------|-------------------|--------------------|
| 100A         | 4 AWG             | 2 AWG              |
| 125A         | 2 AWG             | 1/0 AWG            |
| 150A         | 1 AWG             | 2/0 AWG            |
| 200A         | 2/0 AWG           | 4/0 AWG            |
| 400A         | Parallel 250 kcmil| Parallel 350 kcmil |

Always confirm sizing against the current ampacity table and any local amendments. Some jurisdictions allow 310.12 (residential service) sizing that is smaller than strict 310.16 - that's why a 200A home often lands on 4/0 AL rather than 250 kcmil.

Torque every termination to the breaker or meter base manufacturer's spec. See torque section below.

Panel Placement - NEC 110.26 Working Space

The single most-violated rule on residential service work. NEC 110.26 requires clear working space in front of any electrical equipment you will service while energized:

• Width - The greater of 30 inches or the width of the equipment. You can have the wall at the edge of the panel as long as 30 inches of clear standing room exists in front.
• Depth - 36 inches from live parts (the panel face) to any obstruction. 42 inches in some higher-voltage categories - residential 120/240 V falls in the 36-inch category.
• Height - 6 feet 6 inches minimum headroom. A low basement ceiling that clips headroom at 6 feet is a violation - the panel has to relocate.
• Dedicated electrical space above the panel - A vertical zone from the top of the panel to the structural ceiling (or 6 feet above, whichever is lower) is reserved for electrical equipment only. No ducts, no plumbing, no sprinkler piping.

A panel in a closet stuffed with clothes, a panel with a sump pump discharge pipe running above it, a panel with a water heater sitting 6 inches to the side - all failures. Pick the panel location early and coordinate with every other trade before framing closes up.

Panel also cannot sit in a bathroom or a clothes closet (NEC 240.24).

Grounding Electrode System (GES)

The grounding electrode system is what ties the electrical system to the earth. It does NOT carry load in a healthy system - it is there for lightning, utility fault clearing, and stray-voltage drainage. Residential GES typically includes:

1. Ground Rods (Two, 6+ ft Apart)

Copper-clad or galvanized steel rods, 5/8 inch diameter, 8 feet long, driven so the top is at or below grade. NEC 250.53(A)(2) requires at least two rods if a single rod cannot achieve 25 ohms or less to earth (almost nobody tests - so everyone just installs two). Rods spaced at least 6 feet apart to avoid overlap of their effective ground spheres.

Bond the rods together with a single continuous grounding electrode conductor (GEC) - typically 6 AWG copper for 200A and under, 4 AWG for larger. GEC connects rods to the panel neutral/ground bus (service) or to an intersystem bonding terminal.

2. Water Pipe Electrode

NEC 250.52(A)(1): if the home has a metal underground water service (at least 10 feet of the pipe is in direct contact with earth), the water pipe is a required grounding electrode. Bond the GEC to the water pipe within 5 feet of where the pipe enters the building. Use a listed ground clamp (cast bronze, AC/DC listed).

The 5-foot rule matters because a plumber can replace any section of pipe upstream of your clamp without your involvement, so the bond has to sit close to the entry before any splice point.

Non-metallic water service (plastic PE pipe, which is most modern homes)? Water pipe is NOT an electrode. Ground rods plus any other electrode present cover the GES.

3. Concrete-Encased Electrode ("Ufer")

A new-construction concrete slab with a 20-foot length of 1/2 inch rebar or 4 AWG copper embedded in the footer becomes a concrete-encased electrode (CEE, aka "Ufer"). NEC 250.52(A)(3) requires a CEE in all new construction where a suitable rebar-in-footer is present.

Tie the GEC to the rebar with a listed connector (exothermic weld, listed rebar clamp, or listed connector) and bring it up through the foundation before the pour.

4. Other Electrodes

Metal water well casings, ground rings, plate electrodes, structural steel all count when present. On a typical wood-framed residential home, the triad is: ground rods + concrete-encased electrode + (metal water pipe if present).

All electrodes are bonded together into a single grounding electrode system. A panel does not get two separate GECs to two separate "grounds" - it gets one GEC that daisies through the whole system.

Bonding vs. Grounding - The Key Distinction

• Bonding - Intentionally joining metal parts together so they are at the same potential. A bonded system has no voltage between any two metal parts during a fault. Bonding keeps things safe to touch.
• Grounding - Connecting the system to earth via the grounding electrode system. Grounding gives lightning a path, drains static, and provides a reference.

Both are required; they do different jobs. A bonded system without a ground is still pretty safe to touch (though lightning will find a path anyway). A grounded system without bonding is a death trap (energized equipment will float at a dangerous voltage relative to other metal).

The Main Bonding Jumper

At the service disconnect only, the neutral bus is bonded to the panel enclosure by the main bonding jumper (MBJ). Usually a green screw, a green strap, or a listed bonding bar that ties the neutral bar to the panel can.

The MBJ creates the one and only path between neutral and ground at the service. Under normal operation, no current flows on that jumper - all return current stays on the neutral conductor. Under a fault (a hot shorted to a grounded enclosure), fault current flows back through the EGC, across the MBJ to the neutral bus, and back to the source, tripping the breaker.

Neutral and Ground are Separate in a Sub-Panel

This is the single most-violated rule by DIY homeowners and underqualified apprentices. In a sub-panel, the neutral bus and the ground bus are NOT bonded together. Different buses. No bonding screw. No jumper.

Why? If neutral and ground are bonded in the sub-panel, normal return current will flow on the EGC (the grounding conductor) - which means your copper ground wires and your bonded metal enclosures and metal piping are now carrying load current. That is dangerous (current on metal = shock risk) and violates 250.24(A)(5).

Every sub-panel install:

1. Remove the factory bonding screw or strap if one is shipped preinstalled.
2. Install a separate ground bar if the panel does not come with one, and land all EGCs on it.
3. Neutrals land on the isolated (floating, not bonded) neutral bar.

Sub-Panel Wiring

Sub-panel feeders in a modern single-dwelling installation are 4-wire:

• Two hots (L1, L2)
• One neutral
• One equipment grounding conductor (EGC)

Pre-1999 NEC allowed a 3-wire feeder (no separate EGC, with neutral bonded in the sub-panel) to a detached structure. Current NEC 250.32(B)(1) requires 4-wire to new detached structures too. Grandfathered 3-wire feeders exist in old houses - do not replicate them on new work.

Detached Structure Sub-Panels

If the sub-panel is in a detached garage, barn, or shed, NEC 250.32 requires its own grounding electrode system at that structure - typically two ground rods. That grounding electrode system at the detached building bonds to the sub-panel's ground bar, NOT to the neutral bar.

Breakers

Common breaker types in a residential panel:

• Single-pole (1-pole) - One hot, 120V. Feeds 15A or 20A lighting and receptacle branches.
• Double-pole (2-pole) - Two hots, 240V. Feeds range, dryer, water heater, sub-panel feeders, HVAC.
• Tandem / duplex / "slim" - Two single-pole 15A or 20A breakers in a single panel slot. Used when the panel has run out of spaces. Panel schedule must indicate tandem-allowed slots; some panels limit total tandems to prevent bus overload.
• GFCI - Integrated ground-fault circuit interrupter. Protects the full circuit downstream. Shows an ON/OFF/TRIP handle position and a TEST button.
• AFCI - Arc-fault circuit interrupter. Same form as a GFCI handle, with TEST button.
• DFCI (Dual-Function) - AFCI + GFCI in one breaker. Used where both are required (kitchen small-appliance, laundry).

Match the breaker to the panel manufacturer. A Square D QO panel takes QO breakers only. A Siemens panel takes Siemens or QP. Mixing brands voids the panel listing and will be flagged by an inspector.

Surge Protective Device (SPD)

NEC 230.67 (2020 and later) requires a Type 1 or Type 2 SPD on every new residential service. Type 2 integrates at the panel - either a plug-in SPD in two adjacent slots, or a wire-in side-mount unit.

Install per manufacturer: one hot leg from each phase, neutral, and EGC short and direct to the SPD terminals. Minimize lead length (every inch of lead adds impedance, defeating the fast clamping response). Typical 6-inch lead per terminal, tightly bundled.

The SPD clamps transient surges (from lightning, utility switching, downstream equipment) to safe levels, protecting the panel and everything plugged in. Expected lifetime is years, not decades - the indicator light on the SPD is what tells the homeowner it's still alive.

Torque Specs

Every conductor termination on a breaker, lug, meter base, or neutral bar has a manufacturer's torque spec. Over-torque crushes the conductor; under-torque creates a high-resistance joint that heats, oxidizes, and eventually fails. Always use a torque screwdriver or torque wrench.

Typical values (always verify against the actual label on the equipment - they are stamped on breakers and often on the deadfront):

| Terminal Size            | Typical Torque          |
|--------------------------|-------------------------|
| #14-#10 AWG branch breakers | 20-25 in-lb          |
| #8 AWG                   | 25-35 in-lb             |
| #6 AWG                   | 45-60 in-lb             |
| #4-#1 AWG                | 75-110 in-lb            |
| 1/0-4/0 AWG (main lug)   | 200-275 in-lb           |
| 250-500 kcmil            | 275-450 in-lb           |

NEC 110.14(D) (2017 and later) explicitly requires torque verification by a torque tool or similar. "I tightened it until it felt right" is no longer acceptable.

Labeling Circuits

Every circuit on the panel schedule gets a clear, useful, homeowner-readable label. Bad labels ("Kitchen," "Upstairs") frustrate the next electrician called out. Good labels:

• KITCHEN SA 1 (east counter, dishwasher)
• KITCHEN SA 2 (west counter, microwave)
• MBED RECEP + LIGHT
• LAUNDRY 20A
• DRYER 30A
• RANGE 40A
• HVAC COMPRESSOR 30A
• FURNACE 15A
• EV CHARGER 40A (dedicated, garage wall)
• BATH 1 & 2 RECEP (shared)
• OUTSIDE RECEP (front + rear GFCI)

Printed labels (Brother P-touch) look professional. Handwritten is acceptable for a well-organized schedule; illegible Sharpie-on-masking-tape is not.

Pre-Energize Checklist

Before you flip on the main:

• Every breaker matches the panel schedule.
• Every conductor is torqued to spec.
• The MBJ is installed (service panel only).
• Neutral bus is bonded to the can (service only); neutral bus is floating (sub-panel).
• Ground bus has all EGCs landed.
• SPD installed and wired per instructions.
• Deadfront in place, screws snug.
• Panel door closes fully.
• Meter seal undisturbed - you do NOT pull a meter without the utility's authorization.

Common Failures

• No working space in front of the panel. Clear 30-in by 36-in zone, check headroom.
• Neutral and ground bonded in a sub-panel. Remove the bonding screw; separate buses.
• Missing MBJ at the service. Install the green screw or jumper.
• GEC run through metal conduit without bonding the conduit. Metal conduit carrying a GEC must be bonded at both ends or it becomes a choke coil.
• SPD on a 400A service panel missed. Required on every service dwelling unit, regardless of size.
• Torque marks absent. Many AHJs now require visible torque marks (paint pens) to confirm verification.

Day 1 Checklist

• Panel and service plans reviewed
• Panel grounding kit (lugs, bonding screw, ground bar)
• Torque screwdriver and torque wrench, calibrated
• Ground rods (2), clamps, GEC
• SPD unit
• Label maker or legible Sharpie + masking
• Tools: linemans, side cutters, nut drivers, cable cutters, appropriate strippers

Expert Tips

• "The MBJ is the difference between a safe service and a fault-hunter's nightmare. Install it. Check it."
• "Sub-panel neutrals and grounds stay apart. Every time. No exceptions."
• "Torque every lug. Mark every torque. Photograph the torque marks for the field file."
• "Run the GEC continuous - no splices on a grounding electrode conductor." (NEC 250.64(C).)
• "Plan the panel location before the framer arrives." Reworking a panel wall after drywall is a two-day headache.