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
Make a List of all the things you plan to use
120 volt devices
Electrical Convenience Outlets
GFCI Protected Outlets
240 volt equipment
Hot Tubs [see the Hot Tub Page]
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.
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)
(Bare or Green)
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[click pictures to enlarge]
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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.
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?
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.
Join the Discussion!
Ask Dave Your Electrical Question
200Amp Main Panel Circuits
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.
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.