GrannyFlatFinderGrannyFlatFinder
Utilities

The Comprehensive Guide to PG&E ADU Metering (2026): Rule 31, Load Calcs & Submetering

Most contractors will tell you to upgrade your main panel for $25k. They are wrong. This technical guide explains how to use Rule 31 adapters, pass load calculations, and legally submeter your ADU.

The Comprehensive Guide to PG&E ADU Metering (2026): Rule 31, Load Calcs & Submetering

Editor’s Note: This guide was comprehensively updated in February 2026 to reflect PG&E Electric Rule No. 31, NEC 2023/2026 updates, and the latest California Civil Code regarding submetering.

If you are planning a detached ADU in the Bay Area or anywhere in PG&E territory, you have likely encountered the most feared phrase in construction: "You need a Service Upgrade."

It starts innocently enough. You submit your plans to the city. The plan checker asks for a "Load Calculation." Your electrician runs the numbers on your standard 100-Amp or 125-Amp main panel, and the math fails.

Suddenly, your $200,000 ADU project has a $25,000 line item added for a "Main Panel Heavy Up," and your timeline pushes out 6-9 months for PG&E engineering review.

For years, this was the only way. But in 2026, the landscape has changed.

New hardware, new utility rules (specifically Rule 31), and smart load management strategies now allow 90% of homeowners to bypass the service upgrade entirely.

This guide is not a summary. It is a deep technical dive for homeowners, architects, and owner-builders who want to understand the exact mechanics of avoiding this cost.

Part 1: The Physics of the Problem

To defeat the service upgrade requirement, you must first understand why it is being imposed. It is rarely a "legal" requirement from the city; it is a physics requirement from the National Electrical Code (NEC).

The "All-Electric" Trap

California’s Title 24 Energy Code has effectively banned gas in new ADUs (or made it prohibitively expensive to comply). This means your 500-800 sq. ft. ADU is a dense block of electrical load.

Here is the typical "Nameplate Rating" vs. "Demand Load" for a modern ADU:

  1. Induction Cooktop/Range: 40-50 Amps
  2. Heat Pump HVAC (Mini-split): 20-30 Amps
  3. Heat Pump Water Heater: 30 Amps
  4. EV Charger (Required "EV Ready" circuit): 40-60 Amps
  5. General Lighting/Plugs: ~15 Amps

When you add this to a main house that already has an electric dryer, AC, and oven, a standard 100-Amp service (common in homes built before 1990) is mathematically overwhelmed.

The NEC 220.87 Loophole

Most electricians run the "Standard Calculation" (NEC 220.82), which uses the nameplate rating of every appliance. This is conservative and often results in a theoretical load of 180+ Amps, forcing an upgrade.

The Pro Move: Ask for an NEC 220.87 (Determining Existing Loads) calculation.

This code section allows you to use actual historical peak demand instead of theoretical nameplate ratings.

  • Method A: Obtain 1 year of 15-minute interval data from PG&E (available via your online account or "Green Button" data).
  • Method B: Install a recording ammeter for 30 days.

Why this works: Your 100-Amp panel likely peaks at only 35-40 Amps in reality. If you can prove your existing peak is 40A, and your ADU adds 40A (after demand factors), you are at 80A total. You pass. No upgrade needed.

  • Action Item: Before accepting a quote for a panel upgrade, demand that your electrician perform an NEC 220.87 study using your smart meter data.

Part 2: The "Rule 31" Solution (Meter Socket Adapters)

If you cannot pass the load calculation even with the historical data method, you still don't need a heavy up. Enter PG&E Electric Rule No. 31, effective January 2025.

What is a Meter Socket Adapter (MSA)?

An MSA (often referred to by the brand name ConnectDER) is a collar that plugs in between your existing glass utility meter and the metal meter socket.

It physically taps the power ahead of your main breaker panel.

Why It Changes Everything

  1. Bypasses the Busbar: Your main panel's "busbar rating" (e.g., 100A or 125A) becomes irrelevant for the ADU load. The MSA draws directly from the service entrance conductors, which are often rated for higher capacity than the panel itself.
  2. No Trenching: You utilize the existing service drop (overhead) or lateral (underground).
  3. Speed: Because you aren't changing the main service wire, PG&E treats this as a "Greenbook" administrative review rather than a "New Business" engineering project.

The Application Process (Step-by-Step)

This is not a DIY install. Here is the workflow:

  1. Compatibility Check: Your electrician must verify you have a "Form 2S" meter (standard residential) and that there are no physical obstructions (like a flush-mount stucco wall preventing the collar from snapping on).
  2. Submit Form 79-1226: This is the "Application for Meter Socket Adapter Appointment." You submit this to PG&E.
  3. Purchase the Device: You (or your contractor) buy the specific MSA approved by PG&E.
  4. PG&E Appointment: A PG&E technician meets your electrician on-site. The PG&E tech unlocks the ring and pulls the meter. Your electrician snaps in the MSA. The PG&E tech replaces the meter and re-seals it.

Total Cost: ~$1,500 - $3,000 (Device + Labor). Total Savings: ~$20,000+ compared to a panel upgrade.

Part 3: The "Greenbook" Minefield

Whether you use an MSA or upgrade your panel, you must comply with the PG&E Greenbook (Electric & Gas Service Requirements). This is where 50% of ADU projects fail their first inspection.

The "36-Inch" Gas Rule

PG&E requires a strict 3-foot (36-inch) radial clearance between the gas regulator vent and any "source of ignition" or electrical equipment.

  • The Trap: You want to mount your new ADU subpanel (or disconnect switch) right next to the main panel. But your gas meter is also there.
  • The Fail: If your new subpanel is 24 inches from the gas regulator, the inspector will fail you. You will have to relocate the panel, potentially re-wiring the entire feed.

Working Space (NEC 110.26)

You need a clear space 30 inches wide, 36 inches deep, and 78 inches high in front of any electrical equipment.

  • The Trap: Building the ADU too close to the existing meter location, or placing a heat pump unit on the ground in front of the meter.
  • The Fix: Mark the "working space" on your concrete/ground with spray paint before you pour any pads or mount any equipment.

Part 4: Submetering & Tenant Billing

If you use an MSA or share the main panel, you have one master bill for the property. How do you handle a tenant?

Do not use flat-rate billing. Charging "$150 for utilities" is risky. If the tenant mines Bitcoin or charges two Teslas, you lose money. If they use nothing, they feel cheated.

The Legal Strategy (Civil Code 1940.9)

California law explicitly allows you to submeter and bill tenants, provided:

  1. Disclosure: You state in the lease that utilities are submetered.
  2. No Markup: You cannot charge more than the utility rate. You cannot add "administrative fees" for reading the meter.
  3. Transparency: You must provide the opening and closing readings on the invoice.

Hardware Recommendations

You need a Revenue-Grade meter (ANSI C12.20 accuracy). Do not buy cheap $20 meters from Amazon; they are not legal for billing.

Top 3 Options for 2026:

  1. Leviton VerifEye: The industry standard. "Dumb" but bulletproof reliability. Hard to tamper with.
  2. E-Mon D-Mon: The commercial standard. Expensive, but widely recognized by inspectors.
  3. Emporia Vue (with caveats): Very popular smart monitor. Warning: Ensure you use the "Utility Connect" or specific revenue-grade CTs if you intend to bill. The standard consumer version is for "monitoring," not strictly certified for "billing" in all jurisdictions. Check with your local weights & measures if unsure.

Part 5: When to Pay for a Separate Meter

Despite the high cost ($20k+), there are two strategic reasons to choose a completely separate PG&E service.

1. AB 1033 (Selling the ADU)

If your long-term goal is to sell the ADU as a separate condo under Assembly Bill 1033, you must have fully independent utilities. A shared meter (even with a submeter) is generally not allowed for separate real estate parcels.

2. The "EV Rate" Arbitrage

PG&E offers specific rate plans (like EV2-A) that are great for car charging but expensive for daytime AC use.

  • Scenario: Your tenant has an EV and wants the EV rate. You work from home and want a standard rate.
  • Solution: Separate meters allow you to be on different rate plans.

Summary Checklist for Homeowners

Before you sign a contract with a builder or architect, verify these 5 items:

  1. - Demand NEC 220.87: Instruct them to use historical data for the load calc, not just nameplate ratings.
  2. - Check Rule 31 Eligibility: Have them verify your existing meter socket is compatible with an adapter (Form 2S).
  3. - Greenbook Scan: Measure the distance from your gas meter to where the new disconnect will go. Is it >36 inches?
  4. - Conduit Path: If trenching, are you crossing any water/sewer lines? (Requires 12-inch vertical separation).
  5. - Lease Language: Ensure your standard lease agreement includes the specific Civil Code 1940.9 disclosure for submetering.

Need an Architect who knows these rules?

Disclaimer: This guide is for informational purposes only. Electrical work involves high voltage and significant danger. Always hire a licensed C-10 Electrician. Codes and PG&E Rules (Rule 18, Rule 31) are subject to change without notice.