100 A Service, 10 kW Strip, and a Thermostat With No C Wire
An 1,800 ft² ranch has a 100 A, 20-space panel with four open spaces. The existing 2.5-ton air conditioner is being replaced with a 3-ton heat pump whose nameplate lists a minimum circuit ampacity of 21 A and a maximum overcurrent protection of 35 A. The matching air handler contains a 10 kW electric resistance strip with an MCA of 54 A and MOP of 60 A. Existing major loads include a 40 A electric range, a 30 A dryer, a 30 A water heater, and a 20 A garage circuit. The existing thermostat cable is 4-wire (R, W, Y, G) and the customer wants a Wi-Fi thermostat that requires a C wire. The outdoor unit will be about 35 ft from the panel and the air handler about 15 ft. The homeowner does not want a service upgrade.
1. Evaluate the 100 A service capacity and recommend practical options for the auxiliary heat.
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Use a load calculation, not breaker spaces. A 100 A service at 240 V is 24,000 VA. The proposed heat pump circuit is 21 A × 240 V = 5,040 VA; the 10 kW strip is 10,000 VA, plus the air-handler blower (included in its MCA). Those two alone are about 15,040 VA, leaving little for the range, dryer, water heater, lighting, and small appliances. Under the standard method the service is likely overloaded.
NEC 220.60 can omit noncoincident loads only where controls prevent simultaneous operation. A heat pump and strip can run together during defrost, so do not assume they are noncoincident unless the control sequence positively interlocks them.
Practical options:
- Reduce the strip to 5 kW, about 17,100 Btu/h, which usually suffices in an 1,800 ft² house and may avoid the upgrade.
- Keep the existing furnace as dual fuel.
- Upgrade the service.
Have the electrician run the formal calculation and confirm the panel bus rating, not just the available spaces.
2. How should the missing C wire be resolved? Explain why the obvious shortcuts are risky.
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The existing 4-wire cable has R, W, Y, and G. A smart thermostat needs a C conductor to power its display and Wi-Fi without stealing current through the fan or compressor circuit.
Do not reconnect G as C: that gives up independent fan control and can damage the equipment or cause nuisance trips. An add-a-wire device can work on a simple 24 V system, but it may not be listed for a communicating heat pump or dual-fuel control, and it can cause erratic operation if the transformer is already loaded.
Best practice is to pull a new 18/7 or 18/8 cable from the air handler to the thermostat, even if it means fishing a new route. If that is impossible, use a properly rated 24 V transformer and isolation relay only where the thermostat has separate Rc/Rh terminals. Check the air-handler transformer VA: thermostat, contactor, and any accessories must be within rating.
3. Specify the conductor, breaker, disconnect, and labeling requirements for the two circuits.
Read the worked answer
Size conductors from the nameplate MCA, and protect them at or below the nameplate MOP. For the outdoor unit, 21 A MCA / 35 A MOP: a 30 A or 35 A breaker is acceptable, but not a 40 A; use conductors rated at least 21 A, typically 10 AWG copper at 75°C. For a 10 kW strip with 54 A MCA / 60 A MOP, use 6 AWG copper and a 60 A breaker; if you drop to 5 kW, recalculate from the new nameplate.
Keep the heat pump and auxiliary heat on separate circuits and label them. Install a disconnect within sight of the outdoor unit, and follow local rules for GFCI protection of outdoor receptacles. Do not upsize the breaker to stop nuisance trips; that usually indicates a loose connection, a failing capacitor, or a wiring error. If the strip is replaced with a smaller kW, update the nameplate and panel directory.