Ampora is an AI-powered electrical calculator and reference app designed for electricians and electrical engineers. It includes professional tools like voltage drop calculator, wire sizing calculator, conduit fill calculator, arc flash analyzer, load calculation tools, and a comprehensive NEC code reference database.

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Use Ampora's Voltage Drop Calculator to calculate voltage drop for single-phase and three-phase circuits. Enter your circuit voltage, current, wire gauge, and distance to get accurate NEC-compliant voltage drop calculations. The app uses the standard formula VD = (2 × K × I × D) / CM for single-phase and VD = (1.732 × K × I × D) / CM for three-phase circuits.

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How do I size electrical wire using NEC tables?

Wire sizing requires checking NEC Table 310.16 for conductor ampacity based on insulation temperature rating (60°C, 75°C, or 90°C). Apply temperature correction factors if ambient temperature exceeds 30°C, and conduit fill derating if more than 3 current-carrying conductors are in a raceway. Ampora's Wire Sizing Calculator handles all these factors automatically.

What is the NEC 3% voltage drop rule?

NEC 210.19(A) Informational Note recommends maximum 3% voltage drop for branch circuits and 5% total voltage drop (feeder + branch circuit combined). While these are recommendations, not requirements, many inspectors enforce them. For a 120V circuit, 3% equals 3.6V maximum drop.

Where is GFCI protection required by NEC code?

Per NEC 210.8, GFCI protection is required for 125V-250V receptacles in bathrooms, kitchens (countertop and within 6 feet of sink), garages, outdoors, basements, crawl spaces, laundry areas, and within 6 feet of any sink. The NEC 2023 expanded requirements to include 250V receptacles in many locations.

What is AFCI protection and where is it required?

Arc-Fault Circuit Interrupter (AFCI) protection detects dangerous arcing conditions that could cause fires. Per NEC 210.12, AFCI protection is required for 120V, 15A and 20A branch circuits in dwelling unit bedrooms, living rooms, kitchens, dining rooms, family rooms, hallways, closets, and similar rooms.

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Conduit fill is calculated using NEC Chapter 9 tables. One conductor can fill 53% of conduit area, two conductors can fill 31%, and three or more can fill 40%. Use Ampora's Conduit Fill Calculator to automatically determine the correct conduit size based on conductor types, sizes, and quantities.

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Residential service load calculations follow NEC Article 220. Start with 3 VA per square foot for general lighting, add small appliance circuits (1500 VA each), laundry circuit (1500 VA), and individual appliance loads. Apply demand factors per NEC Table 220.42 and 220.54. Ampora's Load Calculation tool automates this process.

What are NEC receptacle spacing requirements?

Per NEC 210.52, no point along the floor line of any wall space can be more than 6 feet from a receptacle outlet. This means receptacles must be spaced no more than 12 feet apart. Any wall section 2 feet or wider requires a receptacle. Kitchen countertops require receptacles so no point is more than 24 inches from an outlet.

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Sub-Panel Installation Guide: Sizing, Wiring & NEC Rules | Ampora

Sub-Panel Sizing: Determining Amperage Needs

Choosing the right sub-panel amperage requires calculating the total expected load on the panel, plus a reasonable allowance for future expansion. A sub-panel that is too small forces additional work later, while an oversized panel increases feeder cost unnecessarily.

Load Calculation Approach

Add up the anticipated connected load on the sub-panel. For residential sub-panels, use the NEC Article 220 standard calculation method or the optional calculation per 220.82 to determine demand. Key considerations:

General lighting and receptacles - 3 VA per square foot of the area served by the sub-panel
Dedicated circuits - Add full nameplate ratings for large appliances (EV charger, range, dryer, water heater)
Continuous loads - Must be rated at 125% for breaker sizing (NEC 210.20(A))
Motor loads - 125% of the largest motor plus sum of all other motors (NEC 430.24)
Future expansion - Allow 20-25% spare capacity for future circuits

Common Sub-Panel Sizes

Sub-Panel Size	Typical Use	Circuit Spaces
30A	Small shed, limited lighting and receptacles	4-6
60A	Detached garage, workshop, small addition	8-12
100A	Large garage, in-law suite, basement finish	12-24
125A	Large addition, workshop with heavy equipment	20-30
200A	Second dwelling unit, large commercial sub-feed	30-42

Pro Tip: The sub-panel amperage rating can never exceed the feeder breaker rating in the main panel. A 60A sub-panel fed by a 60A breaker is correct. But a 100A-rated sub-panel fed by a 60A breaker is also acceptable, as the feeder breaker limits the available current. Oversizing the panel bus gives room for future breaker expansion without replacing the panel.

Feeder Conductor Sizing (NEC 215 & 220)

Feeder conductors must be sized to carry the calculated load and must have an ampacity not less than the rating of the overcurrent device protecting them (NEC 215.2). When sizing feeders, also account for voltage drop, especially on longer runs.

Minimum Feeder Conductor Sizes

The following table shows minimum conductor sizes based on the feeder breaker rating, using 75°C termination ratings per NEC 110.14(C). Always verify conductor ampacity from NEC Table 310.16.

Feeder Breaker	Copper (THHN/THWN)	Aluminum (THHN/THWN)	Min. Ground (Cu)
30A	10 AWG	8 AWG	10 AWG
40A	8 AWG	6 AWG	10 AWG
50A	6 AWG	4 AWG	10 AWG
60A	6 AWG	4 AWG	10 AWG
100A	3 AWG	1 AWG	8 AWG
125A	1 AWG	2/0 AWG	6 AWG
200A	2/0 AWG	4/0 AWG	6 AWG

Voltage Drop: NEC 215.2(A)(4) Informational Note recommends feeder voltage drop not exceed 3%, and total branch circuit plus feeder drop not exceed 5%. For long runs (over 50 feet), you may need to upsize conductors. Use the formula: VD = (2 x K x I x L) / CM, where K = 12.9 for copper or 21.2 for aluminum, I = current in amps, L = one-way length in feet, and CM = circular mils of the conductor.

Conduit Sizing for Feeders

When running feeder conductors in conduit, size the conduit per NEC Chapter 9 Table 1 fill requirements. For four conductors (two hots, neutral, ground), the maximum fill is 40% of the conduit cross-sectional area.

Feeder Size	4 THHN Cu in EMT	4 THHN Cu in PVC Sch 40
6 AWG (60A)	3/4" EMT	3/4" PVC
3 AWG (100A)	1-1/4" EMT	1-1/4" PVC
2/0 AWG (200A)	2" EMT	2" PVC

Main Breaker vs Main Lug Sub-Panels

Sub-panels come in two configurations: main breaker and main lug only (MLO). Understanding when to use each is important for code compliance and safety.

Main Lug Only (MLO)

Feeder conductors land directly on the bus lugs. No main breaker in the sub-panel.

• Most common for sub-panels in the same building
• Protected by the feeder breaker in the main panel
• Less expensive
• No local disconnect at the sub-panel
• Requires feeder breaker in main panel for overcurrent protection

Main Breaker Sub-Panel

Has a main breaker that can disconnect all circuits in the sub-panel.

• Required for detached buildings (serves as disconnect per NEC 225.31)
• Provides local disconnect for maintenance
• Main breaker rating must match or exceed feeder breaker
• Slightly more expensive
• Can be used anywhere an MLO panel is used

NEC 225.31 - Disconnect Requirement

Each building or structure supplied by a feeder must have a disconnecting means installed at a readily accessible location nearest the point of entrance of the conductors. For detached buildings, this typically means a main breaker sub-panel or a separate disconnect switch ahead of an MLO panel. The disconnect must simultaneously disconnect all ungrounded conductors.

Grounding vs Bonding at Sub-Panels

This is the single most critical concept for sub-panel installations and the most common source of code violations. The rule is straightforward but often misunderstood:

The #1 Rule: Separate Neutral and Ground in Sub-Panels

In a sub-panel, the neutral (grounded conductor) and the equipment grounding conductor must be kept completely separate. The neutral bus and ground bus must NOT be bonded together. This is required by NEC 250.24(A)(5) and 250.142(B).

At the main service panel, neutral and ground are bonded together via the main bonding jumper. This is the ONLY point where they should be connected. If you bond neutral and ground at a sub-panel, you create parallel return paths for neutral current, which puts current on equipment grounding conductors, metallic raceways, and equipment enclosures. This is a shock hazard and will cause GFCI devices to nuisance trip.

What This Means in Practice

Remove the bonding strap/screw - Most panels ship with a green bonding screw or bonding strap connecting the neutral bus to the enclosure. When used as a sub-panel, this MUST be removed.
Separate bus bars - The neutral bus must be isolated (floating) from the panel enclosure. The ground bus is bonded to the enclosure.
Neutral conductors on the neutral bus only - White/gray wires go to the isolated neutral bus.
Grounding conductors on the ground bus only - Bare or green wires go to the grounding bus that is bonded to the enclosure.

Main Panel (Service)

• Neutral and ground ARE bonded
• Main bonding jumper installed
• Grounding electrode conductor connected here
• Neutral current returns through neutral bus

Sub-Panel (Downstream)

• Neutral and ground are NOT bonded
• Bonding screw/strap REMOVED
• Neutral bus floats (isolated from enclosure)
• Ground bus bonded to enclosure

4-Wire Feeder Requirement

Because neutral and ground must be separated at the sub-panel, you need four conductors in the feeder:

1. Line 1 (Hot) - Ungrounded conductor, typically black or red
2. Line 2 (Hot) - Second ungrounded conductor for 240V, typically black or red
3. Neutral - Grounded conductor, white or gray insulation
4. Equipment Ground - Bare copper, green, or green with yellow stripe

The old practice of using a 3-wire feeder (two hots and a neutral, with neutral bonded to ground at the sub-panel) has not been permitted for new installations since the 2008 NEC. Even for existing 3-wire sub-panel feeds, any modification or upgrade requires conversion to a 4-wire system.

Exception - NEC 250.32(B) Exception: An existing 3-wire feeder to a separate building is permitted to remain if: (1) there is an existing installation with no continuous metallic paths between buildings (no metal water pipe, no metal conduit, etc.), (2) ground-fault protection is not installed on the supply side, and (3) the grounded conductor is used for grounding the equipment. However, best practice for any new or upgraded installation is always a 4-wire feeder.

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Grounding Electrode Requirements

Whether a sub-panel requires its own grounding electrode system depends on its location:

Same Building as Main Panel

A sub-panel in the same building as the main panel does NOT require its own grounding electrode system. The equipment grounding conductor in the feeder provides the ground fault return path back to the main panel's grounding electrode system.

Separate/Detached Building

Per NEC 250.32(A), a sub-panel in a separate or detached building MUST have its own grounding electrode system. This includes ground rods, concrete-encased electrodes, or other electrodes per 250.52. The grounding electrode conductor connects the sub-panel's ground bus to the local grounding electrode(s).

Grounding Electrode Conductor Sizing at Sub-Panel

For a detached building sub-panel, size the GEC per NEC Table 250.66, based on the largest ungrounded feeder conductor:

• 6 AWG copper feeder (60A): 8 AWG copper GEC
• 3 AWG copper feeder (100A): 6 AWG copper GEC (but 6 AWG max to ground rods)
• 2/0 AWG copper feeder (200A): 4 AWG copper GEC

Remember: GEC to ground rods never needs to be larger than 6 AWG copper per NEC 250.66(A).

Working Space Clearances (NEC 110.26)

Sub-panels must meet the same working space requirements as any other electrical equipment. NEC 110.26 specifies minimum clear working space:

Dimension	0-150V to Ground	151-600V to Ground
Depth (Condition 1)	36 inches	36 inches
Width	30 inches or width of equipment, whichever is greater	30 inches or width of equipment, whichever is greater
Height	78 inches (6.5 feet)	78 inches (6.5 feet)

Dedicated space - NEC 110.26(E) requires dedicated space above and below the panel extending from floor to ceiling (or 6 feet above, whichever is lower). No piping, ducts, or other equipment unrelated to the electrical installation is permitted in this space.
Illumination - NEC 110.26(D) requires illumination for all working spaces around service equipment, switchboards, and panelboards in non-dwelling locations. Good practice for all installations.
Panel height - The center of the grip of the highest breaker handle must not be more than 6 feet 7 inches above the floor (NEC 240.24(A)).

Step-by-Step Sub-Panel Installation

Step 1: Plan the Installation

Calculate the load, select the sub-panel size and feeder breaker rating. Determine the routing for feeder conductors. Verify working space clearances at both the main panel (for the new feeder breaker) and the sub-panel location. Pull the permit.

Step 2: Mount the Sub-Panel

Secure the sub-panel to the wall at the proper height. Use appropriate fasteners for the wall type (wood screws for studs, toggle bolts or masonry anchors for concrete). Ensure the panel is level and plumb. For outdoor installations, use a NEMA 3R rated enclosure.

Step 3: Run the Feeder Conduit or Cable

Install conduit or route cable from the main panel to the sub-panel. Support conduit per NEC Table 344.30(B)(2) for rigid metal conduit or appropriate table for the conduit type used. If using NM cable (where permitted), it must be protected from physical damage and properly supported. SER cable is commonly used for interior sub-panel feeds.

Step 4: Pull Feeder Conductors

Pull all four conductors (L1, L2, neutral, ground) through the conduit. Leave adequate length at both ends for termination (12-18 inches). Use pulling lubricant for long runs to avoid conductor damage. If using cable, secure with proper connectors at each panel.

Step 5: Prepare the Sub-Panel

Critical step: Remove the bonding screw or bonding strap that connects the neutral bus to the panel enclosure. Verify the neutral bus is isolated (floating). Ensure the ground bus is bonded to the enclosure. Install additional ground bars if needed.

Step 6: Terminate Feeder Conductors at Sub-Panel

Connect the two hot conductors to the main lugs (or main breaker terminals). Connect the neutral to the isolated neutral bus. Connect the equipment ground to the grounding bus. Torque all connections to manufacturer specifications using a calibrated torque wrench or torque screwdriver.

Step 7: Install and Connect the Feeder Breaker

Turn off the main breaker at the main panel. Install the 2-pole feeder breaker in the main panel. Connect the feeder hot conductors to the breaker. Connect the feeder neutral to the neutral bus in the main panel. Connect the feeder ground to the ground bus in the main panel. Torque all connections.

Step 8: Install Branch Circuit Breakers

Install branch circuit breakers in the sub-panel. Connect branch circuit conductors. Maintain proper separation: neutrals on the isolated neutral bus, grounds on the grounding bus. Apply AFCI and GFCI protection as required by the current NEC edition.

Step 9: Label and Energize

Complete the panel schedule with circuit descriptions. Label the sub-panel with its feeder source (e.g., "Fed from Panel A, Breaker 7/9"). Verify all connections are tight. Turn on the feeder breaker and test voltage at the sub-panel: 240V between hots, 120V from each hot to neutral.

Detached Building / Garage Sub-Panels

Sub-panels in detached buildings (garages, workshops, barns, accessory dwelling units) have additional requirements beyond a simple in-building sub-panel:

Additional Requirements for Detached Structures

Disconnecting means (NEC 225.31) - A disconnect is required at the detached building, nearest the point of entrance. A main breaker sub-panel satisfies this. Alternatively, install a separate disconnect switch ahead of an MLO panel.
Grounding electrode system (NEC 250.32) - The detached building must have its own grounding electrode system. Install ground rods, use a Ufer ground in the slab, or connect to other available electrodes.
4-wire feeder - Always required for new installations to detached buildings. Two hots, neutral, and separate equipment ground.
Underground feeder routing - If running underground, use UF cable at proper burial depth (24 inches for direct burial, 18 inches in rigid conduit, 12 inches under concrete) per NEC Table 300.5, or individual conductors in PVC conduit.
GFCI protection - All 125V, 15A and 20A receptacles in garages require GFCI protection per NEC 210.8(A)(2).
Lighting - At least one switched lighting outlet is required in garages per NEC 210.70(A)(2)(a).

Underground Feeder Installation

Wiring Method	Minimum Cover (Inches)	Notes
Direct-buried UF cable	24"	Most common for residential
Rigid metal conduit (RMC)	6"	Best protection, minimum cover
IMC (Intermediate metal conduit)	6"	Lighter than RMC, same cover
PVC conduit (Schedule 40/80)	18"	Most common method for feeders
PVC under concrete slab	0" (under 4" slab)	Concrete counts as protection

Important: When trenching between buildings, install a warning ribbon (underground electrical line) 12 inches above the buried conductors or conduit. Transition from underground to above-ground must be protected by conduit to a height of at least 8 feet above finished grade per NEC 300.5(D).

Common Sub-Panel Mistakes

1. Bonding Neutral and Ground in the Sub-Panel

The most common and dangerous mistake. Leaving the bonding screw or strap installed creates parallel return paths for neutral current on the equipment ground conductor and metallic raceways. This puts current on parts that should be at zero potential, creating shock hazards and causing GFCI nuisance tripping. Always remove the bonding screw/strap in sub-panels.

2. Using a 3-Wire Feeder Instead of 4-Wire

Running only two hots and a neutral without a separate equipment ground is a code violation for new installations. The neutral cannot serve double duty as both the grounded conductor and the equipment ground in a sub-panel. Always run a separate equipment grounding conductor as the fourth wire.

3. Undersized Feeder Conductors

Feeder conductors must have ampacity not less than the feeder overcurrent device rating. Do not forget to account for voltage drop on long runs. A 60A sub-panel 150 feet from the main panel may need conductors sized for 100A to keep voltage drop under 3%.

4. No Disconnect at Detached Building

NEC 225.31 requires a disconnecting means at each detached building supplied by a feeder. An MLO sub-panel alone does not satisfy this requirement. Install a main breaker panel or a separate disconnect switch.

5. No Grounding Electrode at Detached Building

A sub-panel in a detached building must have its own grounding electrode system per NEC 250.32(A). Even with a 4-wire feeder and proper grounding conductor, the detached building needs local grounding electrodes (ground rods, Ufer, etc.).

6. Inadequate Working Space

Sub-panels installed in tight closets, behind water heaters, or in areas without 36 inches of clear depth, 30 inches of width, and 78 inches of headroom violate NEC 110.26. Plan the location before mounting.

7. Mixing Neutral and Ground on the Same Bus

Even when the bonding strap is removed, some electricians land neutral and ground wires on the same bus bar. Neutrals must go on the isolated neutral bus, and grounds must go on the enclosure-bonded ground bus. Mixing them on the same bar defeats the purpose of separation.

8. Not Torquing Connections

NEC 110.14(D) requires electrical connections to be torqued to manufacturer specifications. Loose connections cause arcing, overheating, and fires. Use a calibrated torque wrench on all lugs and terminal screws.

Sub-Panel Inspection Checklist

Use this checklist to verify your sub-panel installation before calling for inspection:

Feeder & Overcurrent Protection

☐ Feeder breaker rating matches or protects the sub-panel amperage
☐ Feeder conductors properly sized per NEC Table 310.16 and 75°C column
☐ Equipment grounding conductor sized per NEC Table 250.122
☐ Voltage drop is within 3% for feeder length
☐ Feeder is a 4-wire system (two hots, neutral, ground)
☐ Conduit properly sized for number and size of conductors

Grounding & Bonding

☐ Bonding screw/strap REMOVED from sub-panel (neutral floats)
☐ Neutral bus is isolated from the panel enclosure
☐ Ground bus is bonded to the panel enclosure
☐ All neutral wires on the isolated neutral bus only
☐ All ground wires on the ground bus only
☐ No neutral and ground wires sharing the same bus
☐ Grounding electrode system installed (detached buildings)
☐ GEC properly sized and connected (detached buildings)

Installation Quality

☐ All connections torqued to manufacturer specifications
☐ Panel securely mounted, level, and plumb
☐ Working space clearances met (36" deep x 30" wide x 78" high)
☐ No storage or obstructions in the working space
☐ Panel schedule completed with circuit descriptions
☐ Sub-panel labeled with feeder source identification
☐ Proper cable/conduit connectors used at all entries
☐ Conductors properly supported and secured

Detached Building Requirements

☐ Disconnecting means at the building (main breaker or separate disconnect)
☐ Grounding electrode system installed
☐ Underground wiring at proper burial depth per NEC Table 300.5
☐ Warning ribbon installed 12 inches above buried conductors
☐ Conduit protection to 8 feet above grade at transitions
☐ GFCI protection on all garage/outbuilding receptacles

Testing & Verification

☐ 240V measured between L1 and L2
☐ 120V measured from each hot to neutral
☐ 120V measured from each hot to ground
☐ Less than 1V measured between neutral bus and ground bus
☐ GFCI and AFCI breakers tested and functional
☐ No neutral current on equipment grounding conductor

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