How to Install and Wire a Sub-Panel

By Dave Rongey
Summary: Electrical Sub-Panel wiring considerations for the home complete with pictures. Careful planning for your Sub-Panel with immediate and future load considerations will help you understand how to size your Sub-Panel. This information will help as you consider a Sub-Panel and its size.

How to Install and Wire a Sub-Panel the Right Way

Electrical Wiring Video

How to Install and Wire a Sub Panel

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Make a List of all the things you plan to use

120 volt devices
Lighting
Electrical Convenience Outlets
GFCI Protected Outlets

240 volt equipment
Motor Loads:
Hot Tubs [see the Hot Tub Page]
Well Pumps
Air Conditioners
Sub-Panel Considerations-
Correct amperage for all existing loads and future requirements.
Considerations to help determine Sub-Panel Circuit Requirements:

Important Factor: Find out if the Main Service or the Panel that will supply the Sub-Panel Circuit have adequate Load Capacity and space for the needed circuit breaker.

How big should your Sub-Panel be ? (Watts and Amperage)
This will help you consider the circuits to be served by the Sub-Panel including general purpose electrical outlets and any special equipment which will be needed.

Special Considerations:
Large 220 Volt Loads such as Welders, Air Conditioners, Motors, Well Pumps and the like may require larger size wire and conduit to provide the necessary current this equipment requires. Be sure to list all the label requirements and manufacturers specifications.
Long Distances from the power source will produce Voltage Drop, especially over 175 feet. A 2% Voltage drop factor and correction will most likely require an increased size of the wire and conduit.

Wire Types and Capacities
Cables and common wire types

Devices and Junction Boxes
Select the right box for your application

120/240 Volt Sub-Panel Circuit Requirements
4-Wire System consisting of:
2-Insulated Power Conductors (Black & Red)
1-Insulated Neutral Conductor (White)
1-Ground Conductor (Bare or Green)

Don't Make This Mistake!
[click pictures to enlarge]

More about Wiring a Sub Panel

Grounds and Neutrals in Electrical Panel
How to Wire Grounds and Neutrals in Sub Panels – all the neutrals and ground wire and terminal bars must be separated from each other, and your sub feed should be a 4-wire cable that has a separated insulated neutral wire and a separate ground wire.
Sub Panel Grounding and Neutrals
Understanding Grounding for Sub Panels: When you add a second electrical panel with separate neutral and common bars, do you ground the common to the box along with a ground rod connection?
How to Install a Sub Panel for an Attached Garage
Planning and Installing a Garage Sub Panel: Do This Before Installing a Garage Electrical Panel, Check the Main Panel, Select the Wire Type and Wiring Method, Check the Wiring Path, Outlets and GFCI Requirements, Electrical Code Compliance.
Considerations When Planning An Electrical Sub Panel
How to Plan for a Sub Panel Installation: Planning the right amperage and cabling for a sub-panel used in a basement, garage and other locations.
How to Plan for a Sub Panel
Planning for an Electric Sub Panel: Identifying Panel Circuits and Connected Loads. Sub Panels are installed and used for several reasons including expanding the circuit breaker capacity of the main electrical panel.

Learn more about wiring a sub panel

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Complete Guide to Home Electrical Wiring

Learn more about How to Wire Circuit Breakers and Panels
Perfect for Homeowners, Students and Electricians
Includes:
Home Electrical Circuit Breakers
120 Volt Circuits
240 Volt Circuits
Multi-Wired Circuits
Wiring Methods for Installing Electrical Circuit Wiring
Electrical Codes for Home Electrical Wiring
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Questions about Sub-Panels

Questions about Wiring Panels

Let Us Hear Your Question!

Roger in Virginia asks:
Is the Wire Size to my Sub Panel Too Small?

I recently installed a sub panel in a storage building located about 150′ from my home. I fed this sub panel with #10 UF cable. From this sub panel, I am feeding 2 – 20 amp circuits with #14 wire. With nothing but the fluorescent lighting on, I pull about 2.7 amps of current with a measured voltage of 125 volts on this feed. When I turn on my shop vac however, I am pulling about 11 amps of current, and my measured voltage on this circuit drops to 118 volts. This causes my fluorescent light bulbs to cycle on and off. Is the voltage drop enough to cause this issue, or am I missing something?

Dave’s Reply:
Wire Length and Voltage Drop

When planning the wire size for a sub panel, or any panel for that matter, it is important to factor in the length of the distance to the panel from the source, and the amperage of the sub panel. The circuit loads placed upon the sub panel will determine the size of the sub panel as well. So basically, everything starts with planning the load that will be placed on the sub panel, and then we calculate the wire size feeding the sub panel based upon the distance. It is also important to factor in any electric motors that may be used due to the starting load which will effect the amperage and the voltage.

« Electric Panel Circuit Listing

Electrical Code Articles »

Electrical Tips to Help You Wire it Right

The Safest Way to Test Electrical Devices and Identify Electric Wires!

The Non-Contact Electrical Tester
This is a testing tool that I have had in my personal electrical tool pouch for years, and is the first test tool I grab to help identify electrical wiring. It is a Non-contact tester that I use to easily Detect Voltage in Cables, Cords, Circuit Breakers, Lighting Fixtures, Switches, Outlets and Wires. Simply insert the end of the tester into an outlet, lamp socket, or hold the end of the tester against the wire you wish to test. Very handy and easy to use.

The Quickest Way to Check for Faulty Electrical Wiring!

The Plug-In Outlet Tester
This is the first tool I grab to troubleshoot a problem with outlet circuit wiring. This popular tester is also used by most inspectors to test for power and check the polarity of circuit wiring.
It detects probable improper wiring conditions in standard 110-125 VAC outlets Provides 6 probable wiring conditions that are quick and easy to read for ultimate efficiency Lights indicate if wiring is correct and indicator light chart is included Tests standard 3-wire outlets UL Listed Light indicates if wiring is incorrect Very handy and easy to use.

Strip Off Wire Insulation without Nicking and Damaging the Electric Wire!

The Wire Stripper and Wire Cutter
My absolute favorite wire stripping tool that I have had in my personal electrical tool pouch for years, and this is the tool I use to safely strip electrical wires.
This handy tool has multiple uses:
The wire gauges are shown on the side of the tool so you know which slot to use for stripping insulation.
The end of the tool can be used to grip and bend wire which is handy for attaching wire onto the screw terminals of switches and outlets..

The wire stripper will work on both solid and stranded wire. This tool is Very Handy and Easy to Use.

Circuit Breakers, Panels, Parts and Accessories

Circuit Breaker Siemens ITE

Circuit Breaker ITE GFCI

Circuit Breaker ITE AFCI

Circuit Breaker Square D

Hot Tub Panel

Sub Panel 120 240volt 100amp

Sub Panel 120 240volt 125amp

Panel 120 240volt 200amp

200Amp Main Panel Circuits

Michael asks:
We bought a house with a detached shop.
The shop has a 200 amp service, it has it’s own pole and meter. With every breaker shut off, the lights and outlets still work. They still have power. Only when I shut the 200 amp breaker off does the lights and outlets quit working.
I can understand a possible bad breaker, but all defective. I highly doubt that. What do you think?

Additional Comments: The feedback I have read is very informative and professionally stated.

Dave’s Reply:

From what you have described, I think there is another panel somewhere. One way to know for sure is to open the panel and see if there is a spare set of lugs that are used to feed another panel.

More about Sub panel electrical

Why Is My Bathroom Light Fixture Not Working?
How to Fix a Bathroom Light Fixture and a Bath Exhaust Fan that has Stopped Working. The Most Common Problems with Bathroom Light Fixtures and How You Can Fix Them.W-1383 C-40

220 240 Volt Air Compressor Wiring
How to Wire a 220 Volt Air Compressor and Control Switch: Wiring a standard 240 volt motor for equipment such as an air compressor there are only two insulated wires for the 220/240 volts and one separate wire for the ground.W-734 C-15

How to Install an Electrical Sub Panel
Thinking about installing a Sub Panel? Here is what must be considered first, and you may find that you really do not need a sub panel at all.W-777 C-6

Electric Well Pump Circuits
Can I power my shed with 120volt from the 240volt well circuit? Installing a 240volt 3wire Circuit for a Well Pump Panel, Dedicated Circuit for Well Pumps.W-608 C-6

Generator Transfer Switch Using Circuit Breakers
Can You Make a Generator Transfer Switch Using Circuit Breakers? How to Use a Circuit Breaker Interlock Kit for a Generator Transfer Switch.W-801 C-6

Hot Tub Cable and Electrical Wiring
How to Install Hot Tub Electrical Circuit Wiring: Can I run a regular 6AWG 3 conductor wire plus ground through a one inch conduit or do I need to run outdoor 6AWG wire?W-700 C-4

Fix an Electrical Circuit for a Sump Pump or Septic Pump
My pump is not working and I am trying to troubleshoot the problem: How to Identify Typical Septic Pump Problems.W-1046 C-4

What Size Sub-Panel Should I Install for My Garage?
How to Install a Sub Panel for a Garage: Where to Begin when Installing a Sub Panel for a Garage, Planning for a Sub Panel, Identify the Sub Panel Circuits, Sizing a Garage Sub Panel, 120 Volt and 240 Volt Circuits, Planning for Future Circuits.W-663 C-2

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How to Install and Wire a Sub-Panel