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
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.
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 ....and much more. Be Sure to Get Your Copy!
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.
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?
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.
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.
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.
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.
Electrical Project Skill Level:
Intermediate to Advanced - Best performed by a Licensed Electrical Contractor.
Electrical Tools Required:
Basic Electricians Pouch Hand Tools and a Voltage Tester.
Estimated Time:
Depends on personal level experience, ability to work with tools, install electrical circuit wiring, and the available access to the project area. Electrical Safety:
Identify the panel circuit, turn it OFF and Tag it with a Note before working with the wiring. Working in an electric panel is dangerous due to arc flash hazards and the possibility of electric shock.
Electrical Wiring Parts and Materials:
Electrical parts and materials for circuit wiring projects should be approved for the specific project and compliant with local and national electrical codes.
Electrical Codes and Inspections:
Installing additional electrical circuit wiring should be done according to local and national electrical codes with a permit and be inspected.
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