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How to Install Wire for an Outbuilding

Installing Electric Wiring for an Outbuilding: How to Size and Calculate an Electric Panel for an Outbuilding, Electrical Load Calculator, Voltage Drop Calculator.


Electric Wiring for an Outbuilding

Electrical Question: I want to run a wire from my cottage out to a 9 x 12 outbuilding with a bedroom.

  • I intend on having probably 4 plugs, one outdoor light, and a ceiling fan and light.
  • I would like to run a 1500 watt  space heater (approximate size) from one of the plugs periodically.
  • The main electrical line needs to run about 100 yards out from the cottage and the main electric service panel.
  • How many amps would I need on that circuit and do I need to run thicker wire to run that far?

This home electrical wiring question came from John, in , Canada.

Dave’s Reply:
Thanks for your electric wiring question John.

John, the electrical circuits for an outbuilding must be carefully planned out so all of the wiring will be correctly identified and installed to make your outbuilding project is safe and according to code. This information will help you prepare for your outbuilding project.

Preparations for Wiring an Outbuilding

  • Application: Outbuilding Electric Wiring.Skill Level: Intermediate to Advanced – Best installed by a Licensed Electrical
  • Contractor or a Certified Electrician. This is Not Recommended for Homeowners.
  • Tools Required: Electricians Tools including pouch of hand tools and the various power tools necessary for installing the sub-panel.
  • Estimated Time: Depends on the type and size of the panel and available access to the project area, and trenching requirements for underground electrical wiring.
  • Precaution: Any existing electrical wiring within the immediate area that may interfere with the installation should be identified and turned OFF and Tagged if necessary. Working in an electric panel is dangerous due to arc flash hazards and the possibility of electric shock.
  • Notice: Installing additional electrical wiring should be done through the local building authority, installed according to applicable local and national electrical codes, with a permit, and have the required inspections.

How to Size an Electric Panel for an Outbuilding

  • List all of the known electrical devices that will be used in the outbuilding.
  • Add up all of the know electrical load information and add at least 25%.
    • If high wattage electrical devices will be used such as motors or electric heating devices then additional factoring will be required based upon the area and specific circuit size.
    • If electrical equipment will be used that requires a dedicated circuit for equipment such as laundry equipment or a well pump then more factoring will be required based on the length of wire that will be required.
  • Add up how many circuits will be required for all of the electrical loads that have been identified.
    • Depending on the size of larger outbuildings where multiple circuits will be installed then an electrical sub panel may be required.
  • The outbuilding may require a sub panel with a main breaker and a dedicated ground system which will depend on the feeder cables that will be installed for the specific project.
  • The outbuilding circuit size can only be determined when the total connected load has been identified and adjusted as required.
  • Select the type of wire that will be installed for the outbuilding circuit, such as direct burial cable, or wiring that will be in conduit.
  • Determine if the main feeder wire for the outbuilding will be copper or aluminum.
    • NOTE: Aluminum wire may not be used for smaller gauges and is restricted in some areas.

IMPORTANT: If the electrical circuits for the outbuilding have not been well planned and installed correctly using the right methods and materials then the circuit will be vulnerable to breakdown and damage.

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Be Careful and Be Safe - Never Work on Energized Circuits!
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