Okay, let's break this down. "Existing insulation to remain" with an existing joist does present a common challenge when running MC cable or conduit. Here's how to approach it, and some things to consider

Understanding the Problem
Insulation

Leaving the insulation in place is common practice (especially in existing structures) for energy efficiency. However, it complicates running cable or conduit because you can't just drill straight through it, and packing it around the cable can cause overheating issues.

Joists

Joists are structural members. You cannot just cut or notch them haphazardly to make way for your wiring. Weakening a joist can be a serious safety hazard.

Code Considerations

The NEC (National Electrical Code) has rules about supporting cables, not damaging insulation, and how you can (and can't) run wires through or near structural members.

Solutions and Best Practices1. Plan Your Route Carefully



Before you start, map out the entire run. Look for the path of least resistance that minimizes interference with insulation and joists. Consider running parallel to the joists whenever possible. This avoids the need for boring through them entirely. Use existing pathways

Look for any existing holes, chases, or other openings that might be usable. Someone else might have already faced this issue.2.

Drilling Through Joists (When Necessary and Permitted)



Location, Location, Location

The NEC and local building codes dictate where you're allowed to drill.

Generally, the middle third of the joist's span is the safe zone. Avoid drilling near the top or bottom edges, as these are the areas under the most stress. Hole Size



Keep the hole small. Typically, the NEC limits the size of holes to no more than 1/3 of the joist's depth. Always check your local codes! Proper Drilling Technique

Use a drill bit specifically designed for wood (auger bit, spade bit, or hole saw). Drill straight and true. Avoid splintering the wood by backing up the hole with a scrap piece.

Number of Holes

Minimize the number of holes you drill in any single joist.3.

Running Parallel to Joists



Securing the Cable/Conduit

You'll need to properly support the MC cable or conduit.

Cable Staples/Clamps

Use appropriately sized staples or clamps that are approved for MC cable or conduit. These should be secured to the side of the joist. Do NOT compress the insulation.

Support Spacing

The NEC specifies how often you need to support the cable (typically every 4.5 feet for MC cable, but check your local codes).

Running Boards/Runners

If you need to protect the cable from physical damage, consider installing a "running board" (a piece of lumber attached to the joists) and securing the cable to that.4.

Dealing with Insulation



Leave a Gap

Do not compress the insulation around the cable or conduit. This can trap heat and create a fire hazard. You need airflow.

Careful Placement

Try to position the cable so it naturally sits on top of the insulation, if possible, rather than burrowing within it.

Insulation Shields (Optional)

In some situations, you might consider using a small metal shield or standoff to create an air gap between the cable and the insulation. However, this isn't usually necessary for typical MC cable runs.

Consider using a box extension to allow for more room.5. Conduit Specifics Bending

Bending conduit around obstructions (like existing pipes or HVAC ductwork) can be challenging, especially in tight spaces. Use appropriate conduit benders and plan your bends carefully.

Fittings

Use the correct fittings (couplings, connectors, elbows) to ensure a secure and watertight connection.

Support

Rigid conduit needs more robust support than MC cable. Use straps or hangers designed for the conduit size and type.

Important Considerations and Cautions Local Codes



Always consult your local electrical codes and building codes. These are the final authority on what is permitted. What's acceptable in one jurisdiction may not be in another. Permits

You might need a permit for electrical work. Check with your local building department.

Professional Help

If you're uncomfortable with any aspect of this,

hire a qualified electrician. Working with electricity can be dangerous, and improper wiring can create a fire hazard. Safety First



Turn off the power to the circuit you're working on before starting any electrical work. Use insulated tools. Wear safety glasses. If you're working in an attic or crawl space, wear appropriate protective gear (respirator, gloves, etc.).* Documentation

Take pictures of your wiring before covering it up with insulation. This can be helpful for future reference.

In Summary:

Running MC cable or conduit with existing insulation and joists requires careful planning, adherence to code, and proper techniques. Prioritize running parallel to the joists whenever possible, minimize drilling, and always leave an air gap around the cable to prevent overheating. When in doubt, consult a qualified electrician. Flag for review