Existing installations and added loads

Much of Brightwater's work is not new construction. It is adding something to a building that already has a service: a vehicle charger in an older house, a heat pump replacing a gas furnace, a new tenant in a strip mall. Each of those jobs asks the same question. Can the existing service carry the new load? Article 220 gives two ways to answer it without pretending the building is brand new. Section 220.87 lets you use the measured maximum demand of an existing service, and 220.83 offers an optional method for an existing dwelling. This lesson uses both, along with the energy management provisions that can let a new load fit where it otherwise would not.

Using measured demand: 220.87

Section 220.87 lets you determine the existing load of a feeder or service from real data instead of a calculation. If maximum demand data for a one-year period is available, you take the highest demand recorded in that year at 125% and add the new load calculated under Article 220. If a year of data is not available, you may use the maximum demand measured over a 30-day period by a recording meter that averages the load over 15-minute intervals, again at 125%, adjusted to reflect any seasonal loads such as heating or cooling that were not running during the recording. The method applies only where the feeder or service has overcurrent protection in accordance with the code, and the section has conditions for buildings with on-site generation or peak load shaving that you should check in your edition.

The 125% factor is a safety margin for the fact that one year's peak may not be the building's worst. Utilities usually keep demand data for commercial accounts, so the landlord or owner can request it. For a house, a utility may have interval data from a smart meter, or the electrician can install a recording meter for 30 days.

Without the demand data, Dana would have had to calculate the load of the four occupied units from scratch, with all the conservative assumptions that entails, and the total would almost certainly have been much higher. Measured demand reflects how the building really behaves. That is also its weakness: if a tenant's business changes, the old data no longer describes the building, which is why the 125% margin is required.

How to adjust for seasonal loads is a judgment the section leaves to the designer and the inspector; the conservative approach in this example adds the full heating load to the recorded peak. On an exam, follow the adjustment the question describes.

The optional method for existing dwellings: 220.83

Section 220.83 gives a simplified calculation for adding loads to an existing dwelling unit. It has two versions. Section 220.83(A) applies where no additional air conditioning or electric space heating is being installed, and 220.83(B) applies where it is. In both, the load includes the general lighting at the floor area value, 1,500 VA for each small-appliance and laundry circuit, and the nameplate ratings of the fixed appliances, ranges, ovens, cooktops, dryers and water heaters. The first 8 kVA of that load is taken at 100% and the remainder at 40%. In version (B), the air conditioning or heating is added at 100% on top.

Compare that with the new-dwelling optional method of Lesson 5, which takes the first 10 kVA at 100%. The existing-dwelling method starts the 40% factor sooner, because it is meant for adding loads to a house whose real use is already known in a general way. Exam questions sometimes test whether you used 8 kVA or 10 kVA.

Energy management: making a load fit

The Morales family does not want to pay for a service upgrade if they can avoid it. The 2023 edition gives them an option. Section 625.42 lets the load of electric vehicle supply equipment be set by an energy management system that limits its current, and 220.70 recognizes energy management systems in load calculations, under conditions in 750.30. In plain terms, if a listed system monitors the service and automatically reduces the charger's current so that the service is never overloaded, the charger may be calculated at the maximum current the system allows rather than at its full rating.

A charger limited to 12 amperes adds about 9 or 10 miles of range per hour, which is enough for many households that charge overnight. A smarter system that watches the whole service can let the charger run faster when the range and dryer are idle and slow it when they are on. That is how the code expects such a system to work, and it is why the section requires a listed system rather than a hand-set limit.

Other approaches to existing loads

Not every addition needs a full calculation. Adding a single small load to a service with obvious spare capacity is usually handled with judgment and a quick check. When the margin is thin, the measured-demand method of 220.87 or the existing-dwelling method of 220.83 gives a defensible number for the permit. For a house, a load calculation by the standard or new-dwelling optional method may also be used, treating the house as if it were new, which is often the most conservative answer.

One more rule matters when adding loads to an existing building: the new load's branch circuits and the feeders that serve them must meet today's code, even if the rest of the building does not. A charger added to a 1970s house still needs a branch circuit sized at 125% under Article 625, ground-fault protection where required, and a disconnecting means where required. The load calculation decides whether the service can carry it; the rest of the code decides how it is wired.

def existing_dwelling(other_loads, hvac=0):
    return min(other_loads, 8000) + 0.40 * max(other_loads - 8000, 0) + hvac

def measured_demand(peak_va, new_va):
    return 1.25 * peak_va + new_va

total = existing_dwelling(4800 + 4500 + 20700 + 7680, 6000)
print("Morales as planned:", total, "VA =", round(total / 240, 1), "A")
total = existing_dwelling(4800 + 4500 + 20700 + 2880, 6000)
print("With a 12 A managed charger:", total, "VA =", round(total / 240, 1), "A")
va = measured_demand(74000, 80770 + 34525)
print("Harlow Commons:", va, "VA =", round(va / (208 * 3 ** 0.5), 1), "A")

Morales as planned: 25872.0 VA = 107.8 A
With a 12 A managed charger: 23952.0 VA = 99.8 A
Harlow Commons: 207795.0 VA = 576.8 A

How these questions are worded

Questions on existing installations often hide the method choice in the story. A question that mentions a utility bill, a demand meter or a twelve-month history is pointing at 220.87. A question about adding a hot tub or a charger to an existing house, with no mention of measured data, is pointing at 220.83. A question that says a heat pump replaces a gas furnace is adding heating and cooling, so it needs version (B) of 220.83. Each of these is a single sentence in a long stem, which is why Dana reads every stem twice before choosing.

The arithmetic differences between the methods are worth memorizing as a set. The new-dwelling optional method splits at 10 kVA; the existing-dwelling method splits at 8 kVA. The measured-demand method multiplies the peak by 1.25 and adds the new load at its calculated value. The energy management rules let a managed load be counted at the limit the system enforces. If you can state those four facts from memory, you can work nearly any existing-load question the exam offers.