Adding a 48 A EVSE to a Dwelling With a Fully Loaded 100 A Service
A 2,000 sq ft single-family dwelling has a 100 A, 120/240 V single-phase service with a 24-space panel and six open spaces. Existing loads: 12 kW electric range, 5,500 W dryer, 4,500 W water heater, 1,200 VA dishwasher, 900 VA disposal. Heat is a heat pump with a 4,800 VA compressor and 8,000 VA of supplemental strip heat; calculated cooling is 6,500 VA. The owner signed for a hardwired 48 A EVSE on a 60 A circuit and wants it fed from the existing panel. The utility will not change the overhead drop without a load letter, and the owner does not want to wait three weeks for a service change.
1. Using the optional method of NEC 220.82, calculate the existing dwelling load in amperes and determine whether the 48 A EVSE can be added to the 100 A service.
Read the worked answer
General loads, 220.82(B)(1)
- Lighting: 2,000 ft x 3 VA = 6,000 VA
- Small appliance: 2 x 1,500 = 3,000 VA
- Laundry: 1,500 VA
- Subtotal = 10,500 VA
Appliances at nameplate, 220.82(B)(3)
12,000 + 5,500 + 4,500 + 1,200 + 900 = 24,100 VA
Largest of heating or cooling, 220.82(C)
- Heat pump: 4,800 + (0.65 x 8,000) = 10,000 VA, using the 65% supplemental-heat allowance in 220.82(C)(4)
- Cooling: 6,500 VA
- Use 10,000 VA
Demand
Total connected = 10,500 + 24,100 + 10,000 = 44,600 VA Demand = 10,000 + 0.40 x 34,600 = 23,840 VA 23,840 / 240 = 99.3 A
Available capacity on a 100 A service = 24,000 - 23,840 = 160 VA, about 0.7 A.
Adding the EVSE
48 A x 240 V = 11,520 VA New total = 44,600 + 11,520 = 56,120 VA Demand = 10,000 + 0.40 x 46,120 = 28,448 VA = 118.5 A
118.5 A exceeds the 100 A service rating. The EVSE cannot be added to this service as it stands, and the existing calculated load already uses essentially all of the available capacity. That answer has to be given to the owner in writing before any work starts, because the customer will hear 'the panel has six open spaces' and assume it fits.
2. If the service is changed out, what minimum size is required, and how does the answer change if the AHJ requires the EV charging load at 125%?
Read the worked answer
At 100% of EVSE rating (28,448 VA = 118.5 A): a 125 A service (30,000 VA) arithmetically works.
At 125% of EVSE rating (11,520 x 1.25 = 14,400 VA):
Total = 44,600 + 14,400 = 59,000 VA Demand = 10,000 + 0.40 x 49,000 = 29,600 VA = 123.3 A
Still under 125 A, so the arithmetic minimum is 125 A either way. Do not sell 125 A.
- Margin at 125 A is 400 VA, under 2 A. The first added window unit or hot tub puts the service back over.
- The EVSE is a continuous load: 48 A x 125% = 60 A circuit per 625.41. That single branch circuit is nearly half the 125 A service rating.
- A 200 A service leaves 48,000 - 28,448 = about 19.5 kVA of headroom and is the defensible recommendation.
If 200 A is installed and this is a dwelling service, 310.12 permits the service conductors at 83%: 200 x 0.83 = 166 A, so 2/0 Cu (175 A at 75 C) is acceptable with a 200 A disconnect. Note that allowance in the load letter so the inspector and the utility see the same basis.
3. The owner insists on keeping the 100 A service. What code-permitted options actually exist, and which one would you put in the proposal?
Read the worked answer
Option 1 - smaller EVSE setting. Available capacity is 160 VA, so no fixed setting works. Even a 16 A / 240 V unit (3,840 VA) gives 44,600 + 3,840 = 48,440 VA, demand 25,376 VA = 105.7 A. Rule it out with numbers, not opinion.
Option 2 - 220.87 measured demand. If a 30-day recording showed a maximum demand of 14.2 kW, existing load = 59.2 A x 1.25 = 74.0 A; adding 48 A gives 122 A, still over 100 A. Metering does not rescue this service unless the recorded demand is very low.
Option 3 - listed energy management. A listed EMS that limits the total service load or curtails the EVSE so the service stays within 100 A is permitted for reducing the calculated load (see 625.42 and the load management provisions of Article 750). Requirements: the equipment is listed for the purpose, the controlled loads are wired so they actually shed, the setpoints are documented, and the AHJ accepts the design. It also has to be maintained, which means a service agreement.
Recommendation. Propose the 200 A service as the base bid, and offer the load-managed 100 A configuration as a clearly priced alternate with the EMS, the controlled-load list, and the maintenance requirement spelled out. Never sign a contract that says 'install 48 A charger on existing 100 A panel' without one of those two paths attached.