Rewiring an old shed can seem like a daunting task, but with some planning and basic electrical skills, it is quite manageable. Here is a comprehensive guide on how to fully rewire a 50 year old shed from start to finish.

Assessing the Existing Wiring

The first step is to carefully evaluate the existing electrical wiring in the shed. This will help you understand what needs to be replaced and what can potentially be reused.

Inspect the Main Power Supply

• Check where the main power supply enters the shed. It may come underground from the main house, or overhead from a power pole.
• Evaluate the condition of the wires, insulation, conduits, and connections. Look for cracks, exposed wires, corrosion etc.
• If the supply line is degraded, it will likely need replacing.

Check the Main Distribution Panel

• Open up the main circuit breaker panel.
• Assess if it is up to modern safety codes. Older fuse boxes will need upgrading to a modern circuit breaker panel.
• Inspect all the circuit breakers and fuses. Make sure they are appropriately rated for the wire gauge they serve.

Evaluate Existing Outlets and Lighting

• Plug a tester into all the outlets and check if they are properly wired and grounded.
• Verify if there are any loose, damaged, or overloaded outlets. These will need replacement.
• Check that all light fixtures are properly mounted and wired through the junction boxes.

Look for Visible Wiring Hazards

• Is the wiring modern copper or outdated aluminum? Aluminum will need replacement.
• Is the insulation cracked or peeling away? This is a fire hazard.
• Are there loose hanging wires, improper splices etc? These can cause shorts.

Planning the New Wiring Layout

Once the extent of the rewiring is determined from the inspection, it is important to plan the layout of the new wiring.

Consider How the Shed Will Be Used

• Plan wiring to support current and future power needs based on usage. More outlets near workbenches etc.
• Factor in specialized loads like large tools, heaters, dust collectors etc. These require dedicated heavy duty circuits.

Choose a Main Service Panel Location

• Pick a spot that is dry, accessible, and away from foot traffic. Near the shed entryway is ideal.
• Plan for the main circuit breaker panel and associated components like meters and disconnects.

Map Locations for Outlets and Lighting

• Mark where all outlets need to be conveniently placed. Follow all code requirements for spacing.
• Plan adequate lighting placement for safety and visibility. Use junction boxes to distribute wires.

Allow Space for Cable Runs and Junction Boxes

• Map paths for wiring between the panel, outlets, switches, and lighting. Plan junction boxes spacing.
• Ensure access to concealed wiring for maintenance via junction boxes and removable panels / drywall.

Modernizing the Main Circuit Breaker Panel

Upgrading the outdated fuse box or panel to a modern circuit breaker panel is a critical part of the project.

Choose a Large Enough Panel

• Get a panel with sufficient circuit breaker spaces for existing and planned circuits. Allow room for future expansion.
• For a 50 year old shed, a good minimum is a 16 space, 100 amp main breaker panel.

Pick a Location Convenient for the Main Service Entry

• Frequently near the front corner or back wall is ideal. Group it with the meter and disconnect.
• Ensure 3 feet clearance space in front of the panel as per codes.

Install a Sturdy Mounting Backboard

• Mount the new panel on plywood or OSB. This provides a secure base.
• Properly ground the backboard and panelcase.

Use Appropriate Conduit for Wiring Entry

• Connect insulated conduits for the mains and all circuit wiring to the panel.
• Use weatherproof conduit for outdoor incoming service wires.

Carefully Label All Circuit Breakers

• Creating a detailed circuit directory is useful for maintenance.
• Clearly label each breaker with tape and a marker.

Running New Circuit Wiring

Laying out the new wiring between the panel and outlets is the most labor intensive phase. Careful planning ensures this goes smoothly.

Choose Appropriate Wire Gauges

• For 15-20 amp branch circuits, use modern copper building wire with #14 AWG conductors.
• Go heavier (#12 AWG) for longer runs over 50 feet from the panel.

Plan Junction Boxes for Access

• Junction boxes should be conveniently accessible, but not hazardous.
• Avoid putting boxes right next to plumbing, gas lines, or directly above counters.

Use the Right Conduit Type

• Outdoor and wet area wiring requires moisture resistant conduit like PVC or metal.
• Make sure to get conduit sizes matched to the wire gauge being used per code.

Distribute Across Multiple Smaller Circuits

• Plan multiple 15-20 amp circuits instead of overloading a few circuits.
• Utilize AFCI / GFCI breakers were appropriate to add safety.

Label Everything Clearly

• At both ends, use numbered wire labels to identify all wire runs.
• This eases troubleshooting down the road.

Installing New Outlets, Switches and Lighting

The final phase is installing updated outlets, switches, and lighting fixtures. Use the opportunity to add ample quality lights.

Use Spec Grade Weatherproof Outlets for Wet Areas

• For outdoor receptacles and those in damp areas, install special covers.
• GFCI protection is required for any outdoor outlets or those near plumbing.

Follow Codes for Outlet Spacing

• General use outlets must be spaced no more than 12 feet apart on walls.
• Islands and peninsulas require an outlet if wider than 12 inches.

Upgrade Old Two-Wire Outlets to Grounded Three-Wire

• When possible, replace outdated ungrounded outlets with modern 3-wire grounded outlets.
• Use pigtails to connect the ground wires if grounding at the box is unavailable.

Choose Switches Rated for Motor Loads if Required

• For heavy duty loads like large compressors or dust collectors, use motor rated switches.
• Similarly, very long runs may need higher horsepower rated switches.

Install Junction Boxes for All Lighting

• Ensure lighting is wired through accessible electrical junction boxes.
• Use LED lights as much as possible for brightness and efficiency.

Safety Checks, Inspections and Documentation

After the installation is complete, perform safety checks and get the necessary final inspections done by the authorities.

Thoroughly Check All Electrical Connections

• Verify secure neutral and ground connections, and tight terminal screws at every outlet and switch.
• Check for loose wire nuts, splices etc. These can cause shorts and fires.

Test GFCI and AFCI Protection

• Use a GFCI tester to verify protection at receptacles connected to these breakers.
• Push the test buttons on AFCI breakers to ensure proper functioning.

Schedule a Final Inspection by the Authority

• Most jurisdictions require an inspection by the electrical authority before energization.
• Make any corrections needed to get a green light on the inspection.

Create a Detailed Circuit Map

• Prepare a circuit directory showing all outlets and lighting, mapped to their corresponding breakers.
• Laminate it and affix it prominently next to the panel.

With meticulous planning and strong attention to detail throughout, an old shed can be fully updated with modern, safe electrical wiring that will serve it well for decades to come.