Mike Dolt Enterprises
Mike Dolt Enterprises
User: gp1200x, Location: NY
Asked on: Sunday, February 2, 2025 at 9:52 pm EST
Answered on: Thursday, March 6, 2025 at 8:10 am EST
Question: I just signed a solar contract for a 13.8kW system so now I am looking at replacing my 2 2ton AC units from 1992 (InterCity Products - Arco Air using R-22)House is in NY and 2550 sq ft two story with half of the top floor being a vaulted ceiling. The 2 2ton units were installed by the second builder in 1992 when he finished the upstairs room and installed 2 skylights. I may have a manual j done to confirm if the units are large enough. House built in 1983, R13 walls -siding and R38 ceiling with a walkout basement in back and underground in front..2 car garage under house.I use oil for winter heating so I will keep that for winter. In fall and spring I was thinking that a 2 stage heat pump (2 of them - one per floor) with a heat strip may be better since I will have 50 percent more energy than I currently use from the solar.I installed an EV charger two years ago using MC6/2 with ground to privide 50 amps of continous current to my charger (70 amp breaker since 65 is not available). Wire in the MC6.2 is THHN and rated for 65 amps so 52 was the NEC limit on this line so I was under. Charger was a 12kW unit. (Never bought an EV..yet).Applying this to the heat pump...if I have them install 12KW strips in both...can I wire them up using this same anology to get 50 amps of heating out of the wiring? Runs would be 30 and 60 feet. Next question... Basement run is easy since MC6/2 can be attached to the bottom of joints in the basement according to code. Getting it up from the basement to the attic where the 2nd floor unit is may be a chore. However, I have an open metal duct top and bottom that is from the basement ceiling to the attic floor. I installed it when the house was built and only ran a few thermostat type wires down it. It is 8x16 inches. Can I run the MC6/2 straight up this (probably 18 feet of duct length) if I secured attached the outside metal conduit to the top and bottom witih multiple clamps? There is no way of clamping it inside the metal duct itself.Thanks.
Okay, let's break down your AC/Heat Pump replacement project, focusing on the electrical considerations for those heat strips.1. Heat Pump Sizing & Justification
Manual J
Excellent idea to get a Manual J load calculation done. This is crucial to determine the actual heating and cooling needs of each zone. Your gut feeling and the existing AC size may not be accurate for modern, more efficient heat pumps. Factors like window efficiency, air leakage, and your preferred comfort levels all play a role.
Two-Stage Heat Pumps
A great choice for your climate and situation. Two-stage units offer improved comfort (fewer temperature swings) and efficiency compared to single-stage models. They also dehumidify better on milder days.
Heat Strips (Electric Resistance Heat)
Understand that heat strips are
expensive to operate. They are fine for emergency heat or supplemental heat during the coldest days but will negate some of the savings you get from solar. It's usually better to size the heat pump so it can handle most of the heating load and not rely heavily on the strips. However, you should consider how often you are going to use this and how much it would cost to heat via Oil.2. Electrical Considerations for Heat Strips
Electrical Load Calculation is Key
Before you even think about wiring, you must have a licensed electrician perform a complete load calculation for your entire home. Adding 24kW of heat strips could potentially overload your service panel. This is non-negotiable for safety and code compliance.
12kW Heat Strip Amp Draw
A 12kW heat strip at 240V (typical residential voltage) will draw approximately 50 amps. `Amps = Watts / Volts` `Amps = 12000 / 240 = 50`
Wiring Requirements (for ONE 12kW Heat Strip)
Wire Gauge
You need a wire gauge capable of safely handling 50 amps continuously (NEC requirement is usually 125% of the continuous load). For 50 amps continuous, you'd need at least #6 AWG copper wire, possibly larger depending on the run length and local codes. The #6 AWG needs to be rated for 75C or 90C to avoid de-rating.
Breaker Size
You will need a 60 amp breaker for EACH 12kW heat strip.
Wiring Configuration
You'll need a 2-pole breaker (240V) and a 3-wire circuit (two hots, one neutral, and a ground).
MC6/2 Cable Suitability
Your MC6/2 cable, as you've described it, is likely
NOT suitable for 50 amps continuous. You stated that the wire is THHN and rated for 65 amps, NEC requires you to derate this wire to 80% of its rating for continuous operation. This would limit the capacity of the wire to 52 amps. Even if it was rated for 75 amps, the NEC de-rates this to 60 amps. I believe you are referring to the outside aluminum sheath but the most important piece is the wires that are inside this sheath. This cable is usually stranded. The breaker must be 125% of the continuous load so for 50 amps, you need a 62.5 amp breaker. Breakers only come in increments of 5 and you must round up so you would need to use a 70 amp breaker. Even if the wire was able to handle 50 amps, it would not be suitable to put this on a 70 amp breaker as it cannot take the amps. Important
Don't rely on internet advice for this. Consult a qualified electrician to verify the exact amperage rating of your existing cable and its suitability for the heat strip load. They need to inspect the cable and its markings. They'll also know your local electrical codes.
3. Running Wire in the Metal Duct
Code Concerns
Running electrical cable inside a duct specifically designed for HVAC is generally
not allowed by electrical codes. HVAC ducts are considered part of the environmental air handling system. Reasoning
Electrical cables can generate heat, and if the duct is used for cooling, that heat could reduce efficiency. Also, there's a potential for damage to the cable from the HVAC system.
Alternative Routing
Explore other options for running the cable to the attic. Consider: Exterior conduit run (weatherproof). Interior wall chase (if available).* Running cable alongside existing plumbing or gas lines (check code requirements, and maintain proper separation).
Recommendations
Hire an Electrician
Engage a qualified electrician to perform a load calculation, inspect your existing EV charger wiring, and advise on the best wiring solution for the heat strips. This is paramount.2.
Get a Manual J
Have a qualified HVAC contractor perform a Manual J load calculation for each zone of your home.3.
HVAC Contractor
Work with a reputable HVAC contractor who can properly size and install the heat pumps, ensuring they meet your heating and cooling needs. Discuss heat strip sizing with them based on your load calculation and heating preferences.4.
Verify Solar Production
Make sure you have a realistic estimate of your solar energy production, considering factors like shading, panel orientation, and weather patterns.
In summary: Don't cut corners or try to DIY the electrical work for the heat strips. It's a potentially dangerous and code-violating situation. Focus on getting professional advice and ensuring a safe and compliant installation. Good luck with your project! Flag for review