Can 12 months of utility data help me avoid an ADU service upgrade?

Sometimes. Section 220.87 of the electrical code lets your designer use your home's actual measured maximum demand for the existing load, instead of a calculated estimate, when a year of demand data is available; that peak is multiplied by 125 percent and the new ADU load is added. If the result fits within your existing service rating, the measured approach can show capacity that a standard calculation would not.

How the method works

1. Get maximum demand data for the existing service covering a full year.
2. Take the highest demand and multiply it by 125 percent.
3. Add the calculated load of the new ADU, figured under Article 220.
4. Confirm the total does not exceed the service rating and that the existing equipment is properly protected.

If a full year of data isn't available, the code allows an alternative: a recording of the service's demand over at least 30 days, using the highest average load held over a 15-minute interval, taken while the home is occupied and accounting for the larger of the heating or cooling load and other seasonal loads.

Where the data comes from

• Your utility account. Smart meter interval data can often be downloaded from your utility's website.
• A load study. An electrician can install a recording meter for the 30-day option.

Building departments differ on the format they accept, so ask early how yours wants the data presented.

When it doesn't help

• Homes with solar. Panels offset what the meter sees, so net readings can understate the home's real load. Your designer may need production data or a different method.
• Planned changes in the main house. If you are also replacing a gas furnace, water heater or range with electric equipment, those are new loads and must be added on top.
• An unusual year. If the house sat empty or a major appliance was out of service, the data won't reflect normal use.

Other ways to make room

If the measured method still comes up short, an energy management system can cap the load, since Section 220.70 lets the calculation use its setpoint, or a separate utility service for the ADU may be the better path.

What this means for your electrical plan

The load calculation sheet should state the method, the data source and dates, the measured peak, the 125 percent factor, the new ADU load and the result compared with the service rating, with the utility data or load study attached. See whether your ADU needs a panel upgrade, why load calculations are required and when a separate service is required.

Sources: dranetz.com, ecmag.com, mikeholt.com, dgs.ca.gov.