Mike Dolt Enterprises
Mike Dolt Enterprises
User: Franz, Location: SC, USA
Asked on: Wednesday, June 4, 2025 at 1:40 am EDT
Answered on: Wednesday, June 4, 2025 at 4:11 am EDT
Question: I realize many of you are professionals and I am likely to get numerous “Call an electrician…” responses, but I want to understand what I’ve got and best approach to get what I want after modifying existing circuitry. Here’s the existing condition… and yes I have contacted an electrician who is going to come to my location….At my newly purchased house which we are remodeling (built circa 1968) I have the following set of panel/subpanels:1 Main/primary/first Square D panel with a 200A breaker - two black 120V lines, one black w white stripe neutral conductor and one black ground wire from the meter directly adjacent. This panel and meter are mounted to an exterior brick wall outside the house, directly opposite a utility room with 3 secondary or subpanels. This panel supplies power to the three subpanels, all of which are Square D QO Load Center catalog number QO 12-24. They each have one black 120V black line, one red 120V line, and one large twisted aluminum wire connected to one (or both?) of the ground and neutral bars which are directly adjacent to one another. The bare aluminum wire runs back to the main panel.One subpanel is fed by a 125V breaker and only has 120V “mini/narrow” Square D breakers. This subpanel has what appears to be separate but adjacent neutral and ground bars, but all the white neutrals and bare copper ground wires from the circuits are connected to just one of these…it’s packed.The other two subpanels are each fed by a 100A breaker in the main. They have same black and red 120V lines inbound which have lugs at top and the twisted aluminum wire attached to one or both of the neutral and ground bars which are near the bottom of the panel box. Both of these other two subpanels only have 240V breakers. Each breaker serves an old ceiling mounted radiant heat system above each room. The circuits in these two panels consist of two wire Romex without a ground in the NM cable jacket. Each black and white wire carries 120V to a box and were connected at a controller (rheostat?) mounted in each room. From there the “heat wires” run up to the ceiling where they disappear into the Sheetrock. I presume they attach to a grid/mat embedded in the plaster layer over the Sheetrock. The heat wires and controllers will be removed. These two panels appear to have one green insulated ground wire for each breaker connected to one of the bars. These green wires enter the panel separately from the two wire Romex which connect to each breaker.Finally, my objectives/questions:1) I want to convert a number of the existing 240V radiant heating circuits to 120V circuits and use them to minimize the number of outlets/lights/appliances on the existing 120V circuits.2) Can I use the two existing subpanels which only have 240V breakers by swapping out for 120V breakers? Connect white (now neutral) wire to the unoccupied bar?3) I’m assuming I (and electrician) must ensure the neutral and ground bars are not bonded in the old 240V panels(?)4) what about the existing subpanel with only 120V breakers - should we split up the neutrals and bare copper ground wires onto separate bars and ensure they are not bonded?Appreciate your patience with the lengthy description and any suggestions. Tried to attach photos but couldn’t get it to work.
Okay, let's break this down. You've got a good grasp of the situation, and your questions are relevant. Here's an analysis and some pointers to discuss with your electrician
Understanding Your Current Setup
Main Panel (200A)
This is the service entrance, grounding point, and where the feeder circuits to the subpanels originate. The description sounds fairly typical for older installations.
Subpanels (QO 12-24)
These are distribution points for your various circuits. They are "sub" panels because they derive power from the main panel. The catalog number confirms they are standard, quality panels.
Aluminum Wiring
The large twisted aluminum wire is a concern. Aluminum wiring has a history of problems, especially when connected to copper devices due to dissimilar metals and expansion/contraction. You must use proper connectors (like those with an anti-oxidation compound) rated for aluminum-to-copper connections. Your electrician will need to inspect these connections closely.
125A Subpanel (120V breakers)
You described this panel having what appear to be separate bars for grounds and neutrals but both are connected to one bar. This is a concern you can address with your electrician when they visit.
100A Subpanels (240V circuits)
These are the panels serving the radiant heat. The lack of ground wires in the original Romex for the heating elements is typical for older installations. The separately run green ground wires for each breaker are puzzling.
Addressing Your Objectives1. Converting 240V Circuits to 120V
Feasibility
Yes, this is generally possible, but it involves careful consideration and code compliance.
Neutral Wire
You'll be repurposing one of the existing "hot" wires as a neutral.
Important
The white wire was being used as a hot wire previously. The wire needs to be re-identified (e.g. with white electrical tape) so that it is used as the neutral conductor, in accordance with the current electrical code.
Grounding
The biggest hurdle. You need to ensure each new 120V circuit has a proper ground. This means either running a new 3-wire (hot, neutral, ground) cable from the subpanel to each new outlet/fixture or utilizing an exception in the National Electrical Code (NEC) which may allow you to install a GFCI outlet if the wiring can be grounded to a suitable grounding source.2.
Using Existing 240V Subpanels for 120V Circuits
Swapping Breakers
Yes, you can replace 240V breakers with 120V breakers. The panel itself doesn't care what voltage the breakers are serving.
Neutral Connection
Yes, you will connect the neutral (white) wire of the new 120V circuits to the unoccupied neutral bar (after properly re-identifying the previously-used white wire as a neutral conductor).
Circuit Capacity
You'll effectively double the number of available circuits in these panels, as each 240V breaker slot can now accommodate two 120V circuits.
Load Balancing
Be careful to balance the load across the two "legs" (A and B phases) of the subpanel. This means alternating the breakers for the new 120V circuits on each side of the panel. This prevents one phase from being overloaded. Your electrician can help with this.3.
Neutral and Ground Bar Bonding in 240V Subpanels
Crucial
In a subpanel,
the neutral and ground bars MUST be kept SEPARATE. The neutral and ground are only bonded together at the main panel. This is vital for safety and to prevent neutral current from flowing on the ground wires, which can cause electrical noise and potentially create shock hazards. Inspection
Your electrician needs to inspect the current bonding configuration in those two 240V subpanels to ensure they are NOT bonded. If they are, the bonding jumper (usually a green screw or strap) must be removed.4.
120V Subpanel Neutral and Ground Separation
Yes, the 120V subpanel should also have separate neutral and ground bars, and they should not be bonded. Separation
The neutral wires should be landed on the neutral bar, and the bare copper ground wires should be landed on the ground bar.
Clean Up
This is an opportunity to tidy up the wiring in the panel. Neatly arrange the wires and ensure all connections are tight.
Important Considerations and Questions for Your Electrician
Aluminum Wiring Mitigation
Discuss strategies for dealing with the aluminum wiring, including proper connectors and possibly pigtailing with copper wire.
Grounding
In particular, ask them about the feasibility of running new ground wires from the electrical boxes to a suitable ground source, such as the subpanel's grounding busbar.
Arc Fault Circuit Interrupters (AFCIs)
Are AFCI breakers required for the new 120V circuits in your area? AFCI breakers detect arc faults (a common cause of electrical fires) and are often required in bedrooms, living rooms, and other areas.
Ground Fault Circuit Interrupters (GFCIs)
GFCIs are required for outlets in bathrooms, kitchens (near sinks), outdoor locations, and other damp or wet locations.
Load Calculation
Have your electrician perform a load calculation to ensure the existing service (200A) and subpanel feeders (100A, 125A) are adequate for the increased load from the new 120V circuits.
Permits and Inspections
Discuss whether permits and inspections are required for the electrical work you're planning.
Safety First NEVER work on electrical circuits without turning off the power at the breaker. Double-check that the power is off with a non-contact voltage tester before touching any wires. If you are unsure about anything, stop and consult with a qualified electrician.This information should give you a solid foundation for discussing your plans with your electrician and understanding the work that needs to be done. Good luck with your remodeling project! Flag for review