ADU Electrical Service Upgrades: When Your Panel Can't Handle a Second Unit and What the Utility Process Involves

Your existing electrical panel can support an ADU only if it has enough spare capacity to handle the additional load, which most GTA homes do not. A typical ADU requires 60 to 100 amps of dedicated capacity for heating, cooling, appliances, and outlets. If your home already runs on a 100-amp service with a full panel, you need either a panel upgrade, a service upgrade from the utility, or both. The service upgrade is the longer process because it involves Toronto Hydro or Alectra running new conductors from the street to your property, which requires their engineering review, scheduling, and potentially transformer work.

How to Tell If Your Current Panel Is Enough

Open your electrical panel and look at the main breaker. It shows your service amperage, typically 100, 125, 150, or 200 amps. Then count the breaker slots and note how many are occupied versus available. But slot availability alone does not determine capacity. What matters is the total calculated load versus your service size.

An electrical load calculation, performed by a licensed electrician or electrical engineer, adds up your existing home's demand plus the projected ADU demand. The Ontario Electrical Safety Code requires this calculation for any permit involving service changes. If the total exceeds roughly 80 percent of your service amperage, you need more capacity.

The Red Flags We See on Site Visits

• 100-amp service on homes built before 1985, which is nearly universal in older Toronto neighbourhoods
• Panel already full with no spare breaker slots, even if service amperage seems adequate
• Electric heating in the main house, which consumes significant capacity before the ADU adds anything
• Planned electric heating or cooking in the ADU rather than gas, which dramatically increases load requirements
• Existing sub-panels already fed from the main panel, reducing available capacity for new circuits

If your home has 200-amp service with a modern panel that has spare slots and your ADU will use gas heating, you might avoid a service upgrade entirely. A licensed electrician can add a sub-panel for the ADU fed from your existing service. But this scenario represents maybe 20 percent of the ADU projects we see. Most require utility involvement.

Panel Upgrade vs Service Upgrade: Two Different Projects

These terms get confused constantly, but they describe completely different scopes. A panel upgrade replaces the electrical panel box inside your home with a larger one that has more breaker slots. This work stays entirely on your property and requires only an electrical permit from the city. Your electrician handles it, ESA inspects it, and the utility is not directly involved.

A service upgrade increases the amperage coming from the utility to your property. This involves replacing the conductors from the transformer to your meter, potentially upgrading the transformer itself if it cannot handle the increased load, and installing a new meter base. The utility owns the infrastructure up to your meter, so they must approve and schedule this work.

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The panel is the box on your wall. The service is the pipe feeding it from the street. You can upgrade one without the other, but ADUs usually need both.

When You Need Both

If you have 100-amp service and need 200-amp service, you need a service upgrade. Your existing 100-amp panel cannot accept 200-amp service, so you also need a new panel rated for 200 amps. These projects happen together. The electrician installs the new panel and meter base, ESA inspects the work, and then the utility connects the upgraded service.

If you already have 200-amp service but your panel is full, you might only need a panel upgrade or a sub-panel addition. No utility involvement required. This is the faster path when it applies.

The Toronto Hydro Process for Service Upgrades

Toronto Hydro handles service upgrades for properties within the City of Toronto. The process involves several stages, and understanding the sequence prevents scheduling conflicts with your construction timeline.

Step One: Application and Load Assessment

Your electrician or electrical contractor submits a service upgrade application to Toronto Hydro. This includes the electrical load calculation showing why the upgrade is needed and the proposed new service size. Toronto Hydro reviews the application to determine if their existing infrastructure can handle your request or if transformer work is required.

Step Two: Engineering Review and Quote

Toronto Hydro's engineering team assesses the work required on their side. If your street's transformer has capacity, they quote the connection work. If the transformer needs upgrading or the overhead lines need replacement, the scope expands. They provide a quote for the work, which you must accept and pay before they schedule the installation.

This engineering review stage is where timelines vary dramatically. Simple connections where infrastructure exists might clear in a few weeks. Projects requiring transformer upgrades or underground conversions can take months for engineering alone.

Step Three: Your Electrician Prepares the Property

While waiting for the utility, your electrician installs the new meter base, panel, and all internal wiring. ESA inspects this work and issues a connection authorization. Toronto Hydro will not connect service until ESA confirms the installation meets code.

Step Four: Utility Connection

Toronto Hydro schedules the actual connection work. A crew comes to your property, disconnects the old service, connects the new conductors, and energizes the upgraded system. For overhead service, this typically happens in a single day. Underground service conversions take longer.

The Alectra Process for 905 Municipalities

Alectra serves Mississauga, Vaughan, Markham, Brampton, and several other 905 municipalities. Their service upgrade process parallels Toronto Hydro's but with different forms, portals, and timelines.

You submit a new service or service upgrade request through Alectra's online portal. The application requires your electrical contractor's information, the proposed service size, and the load calculation. Alectra reviews the request and determines if their distribution system can accommodate the upgrade.

• Standard upgrades where infrastructure exists typically process faster than Toronto Hydro
• Subdivision areas with newer infrastructure often have transformer capacity already available
• Older areas of Mississauga and established Vaughan neighbourhoods may require more extensive utility work
• Alectra provides a connection timeline estimate after engineering review

One difference we see: Alectra tends to be more responsive on initial applications but can have longer wait times for actual crew scheduling during peak construction season. Plan accordingly if your project targets a spring or summer completion.

Timeline Reality: Why Utility Work Drives Your Schedule

Here is what catches ADU builders off guard: the utility process often takes longer than the building permit process. You can have architectural drawings approved, structural reviews completed, and your permit in hand, but if the utility has not connected your upgraded service, you cannot close the electrical permit or get final occupancy.

At PermitsHub, we coordinate electrical drawings with the overall permit package specifically because utility timing affects everything downstream. Starting the utility application while permit review is underway keeps these parallel tracks aligned.

Typical Timeline Ranges

For straightforward service upgrades where transformer capacity exists and no underground conversion is needed, expect four to eight weeks from application to connection. This assumes you submit a complete application with accurate load calculations and your electrician has the property ready when the utility is ready to connect.

For projects requiring transformer upgrades or infrastructure modifications, timelines extend to three to six months. We have seen downtown Toronto projects wait even longer when utility work requires coordination with road permits or other municipal infrastructure.

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The clients who finish on schedule are the ones who started their utility application the same week they submitted their building permit. The ones who wait until framing is done end up with a finished ADU they cannot legally occupy.

What Your Electrical Drawings Must Show

Your building permit application needs electrical drawings that demonstrate the ADU's load requirements and how they integrate with your existing or upgraded service. These drawings serve double duty: the city reviews them for code compliance, and they form the basis for your utility application.

• Single-line diagram showing the service entrance, main panel, and ADU sub-panel relationship
• Load calculation for the ADU including heating, cooling, appliances, and general circuits
• Combined load calculation showing total property demand after the ADU is added
• Panel schedules identifying circuit assignments and breaker sizes
• Notation of service size required based on the load calculation results

If your load calculation shows you need 200-amp service and you currently have 100-amp service, this documentation triggers the service upgrade requirement. Building inspectors will not sign off on final electrical without confirming the service matches what the drawings specified.

Cost Factors That Affect Service Upgrade Scope

Service upgrade costs vary enormously based on factors outside your control. Understanding what drives scope helps you budget realistically and avoid surprises.

Overhead vs Underground Service

If your current service comes from overhead lines and you are staying overhead, the utility work is relatively straightforward. If you want to convert to underground service, or if the utility requires underground for new connections in your area, costs increase substantially. Underground work involves trenching, conduit installation, and more complex utility infrastructure.

Transformer Capacity

The transformer serving your street has a capacity limit. If your upgrade pushes total neighbourhood demand beyond that limit, the utility must upgrade the transformer before connecting your service. You may bear some or all of that cost depending on the utility's policies and how much capacity your project consumes.

Distance from the Transformer

Properties farther from the transformer require longer conductor runs. For underground service, this means more trenching and conduit. For overhead, it may require additional poles. Distance directly affects utility charges.

Your Property's Electrical Infrastructure Age

Homes with very old electrical systems sometimes need more than just a panel upgrade. If your meter base, service mast, or grounding system does not meet current code, these must be replaced as part of the upgrade. ESA will not authorize connection to upgraded service through non-compliant equipment.

Coordinating Electrical With Your Overall ADU Permit

The building permit process and the utility process run on separate tracks with different agencies. Your job, or your permit consultant's job, is keeping them synchronized so neither holds up the other.

Submit your utility application as soon as you have electrical drawings complete, even if your building permit is still under review. The utility does not need your building permit to process a service upgrade application. They need load calculations and contractor information.

During construction, schedule your electrician's rough-in work so it is inspection-ready before the utility connection date. ESA must inspect and authorize before the utility connects. If your rough-in fails inspection and needs corrections, you may miss your utility appointment and face rescheduling delays.

For final occupancy, the electrical permit must be closed, which requires both ESA final inspection and utility connection. This is often the last permit to close on ADU projects, so build buffer time into your completion estimates.