- The distribution transformer (DT) loading limit caps how much solar can connect to one transformer.
- It is often cited as ~30% of the DT rating, but this varies by state and DISCOM — verify.
- The cap is shared across every consumer on that DT, so it can be used up by earlier installs.
- The DISCOM confirms headroom at the feasibility study; you cannot read it from the field.
- If the DT is full, you can downsize, go zero-export, or fund a DT upgrade.
Net metering can stop for a reason that has nothing to do with your customer's roof. The distribution transformer that feeds the site can be too loaded with solar already. This distribution transformer capacity limit, often called the 30% loading rule, is one of the most common silent killers of a rooftop project. Knowing it early saves you a wasted design and an angry customer.
What the DT loading rule is
The DT loading rule sets a maximum amount of rooftop solar that a single distribution transformer can host. A distribution transformer steps the medium-voltage line down to the low-voltage supply that reaches homes and shops. Many consumers share one DT. The rule limits total connected solar on that DT so the local grid stays stable.
Because the limit is shared, your customer's allowed size depends on how much solar other consumers on the same transformer have already connected. A roof that could physically take a large system may still be capped because neighbours got there first.
Where the rule comes from
The loading limit is a technical and safety condition. The Central Electricity Authority (CEA) sets the broad connectivity and safety framework. Each State Electricity Regulatory Commission (SERC) then fixes the actual percentage in its net-metering regulation, and the DISCOM applies it. So the rule has a national basis but a state-by-state number.
Why DISCOMs cap solar on a transformer
DISCOMs cap solar on a transformer to protect voltage and equipment. At midday, many rooftops on one DT can push power back into the grid at the same time. This reverse flow can raise the local voltage above safe limits and stress the transformer and lines, which were built to send power one way.
The reverse power flow problem
A distribution network was designed to carry power from the substation down to consumers. Solar reverses that on sunny afternoons. A small amount is fine. Too much, and voltage at the far end of the line can climb, trip inverters, and shorten equipment life. The loading cap keeps reverse flow inside a safe band.
It is about everyone on the DT, not one roof
The rule protects the whole group of consumers on a transformer. That is why a DISCOM looks at total connected solar, not just your project. Your system is judged against what the transformer can safely host after all existing solar is counted.
Is it really 30%?
The often-quoted figure is about 30% of the transformer rating, but this is not a single national number — it varies by state regulation and DISCOM, so verify the current figure. Some states have used different percentages, tied the cap to feeder hosting capacity, or amended the rule in recent years. Treat 30% as a common example, never a guarantee.
Some regulations express the limit against the DT rating, some against the feeder, and some against the consumer's own sanctioned load as well. Because the basis differs, two states can both say "30%" and still allow different system sizes. Always read the live regulation for the state you are working in, or ask the DISCOM directly.
The DT loading math (worked example)
The math is simple once you have the inputs, but the inputs come from the DISCOM. You take the transformer rating, apply the state percentage, subtract solar already connected, and what is left is the headroom for new solar on that DT.
The table below shows how a cap translates to a rough system size. It uses ~30% only as an example, and a simple power-factor and unit-conversion derate. Real numbers depend on your state's rule and the DISCOM's data.
Illustration only. ~30% is a commonly cited example; the real cap varies by state — verify.
The key point: the cap is on total solar on the DT. If neighbours already hold most of the headroom, your customer's allowed size shrinks even if the transformer is large.
How to check DT headroom before you design
You check DT headroom by asking the DISCOM, usually through the feasibility study, because the DISCOM holds the transformer rating and the record of solar already connected. An EPC cannot confirm this from the field. So file the feasibility request early, before you finalise the design or quote a firm price.
What the DISCOM checks
- The transformer rating in kVA that feeds the site.
- Existing connected solar on that DT from earlier consumers.
- The state loading rule and how it is measured.
- The consumer's sanctioned load, which can also cap the size.
The feasibility reply tells you the maximum size the DISCOM will sanction. Read more on what a feasibility approval covers and how long it lasts, and on why a feasibility request is rejected.
Design after feasibility, not before
Quote a range until feasibility comes back. If you promise an exact system size before the DISCOM confirms DT headroom, you risk redesigning the array and renegotiating the price. A short delay at feasibility is cheaper than a rebuild.
What to do when the DT is full
When the DT is full, you still have options — you do not always have to cancel the job. The right choice depends on the customer's goals and what the DISCOM allows in your state.
Downsize the system
The simplest fix is to size the array to the headroom the DISCOM offers. The customer gets a smaller but fully sanctioned net-metered system. This keeps the project legal and the bills clean, even if the savings are lower than first hoped.
Go zero-export or limited-export
A zero-export or export-limit controller stops the system pushing surplus to the grid. Some DISCOMs allow a larger system this way, because it does not add reverse flow on the DT. The trade-off is that the customer earns nothing for surplus units, so size it to self-consumption.
Fund a transformer upgrade
For a larger commercial site, upgrading or adding a transformer can unlock the capacity. This has a real cost, and who pays it varies by DISCOM. Our DT upgrade cost guide covers who bears the charge and how to budget for it. Confirm the route and the cost with your DISCOM before you commit.
Design around the cap from day one
The best EPCs treat the DT cap as a design input, not a surprise. They run feasibility early, size to sanctioned load, and tell the customer that the final size depends on the DISCOM. This honest framing protects your margin and your reputation.
Tie the system to the customer's sanctioned load. A system far above sanctioned load can be capped on that basis too, separate from the DT rule. Designing to sanctioned load keeps both checks happy. And if the customer wants more than the grid will host, talk about battery storage versus net metering so they keep more of their own generation.
Set expectations early with the customer
The DT cap is the easiest feasibility risk to explain badly. Tell the customer at the quote stage that the final system size depends on the DISCOM's feasibility reply, and that a shared transformer can limit it. Frame it as a normal part of grid-connected solar, not a problem with their roof. A customer who understands this up front accepts a downsized system far more easily than one who was promised a fixed size that the DISCOM then cut.
How SuryaHub helps you manage DT and feasibility
The DT cap is a feasibility-stage risk, and SuryaHub is built to manage that stage cleanly. SuryaHub runs each net-metering job through DISCOM and net-metering workflows, stores the transformer rating, sanctioned load and feasibility reply against the project, and keeps the quote linked to project execution so a changed system size updates everywhere. SuryaHub is pre-revenue; the real pilots are Suryantra Energy and RGESPL, and every figure here is a scheme or technical fact, not a guarantee.
Catch feasibility risk before you quote
See how SuryaHub tracks DT, sanctioned load and DISCOM replies on every job.
Related guides
Frequently asked questions
What is the 30% DT loading rule for net metering?+
The DT loading rule caps how much rooftop solar can connect to one distribution transformer, often cited as about 30% of the transformer rating. The exact figure varies by state regulation and DISCOM, so verify the current percentage with your DISCOM or the SERC regulation before you design.
Why do DISCOMs limit solar on a distribution transformer?+
DISCOMs limit solar on a distribution transformer to keep voltage stable and protect the transformer. A transformer feeds many consumers, and too much reverse power flow at midday can push voltage up and stress equipment. The cap shares safe hosting capacity across everyone on that transformer.
How do I check the DT loading before applying for net metering?+
You check DT loading by asking the DISCOM during the feasibility study, which reads the transformer rating, existing connected solar and the loading rule for your state. The DISCOM holds this data; an EPC cannot confirm headroom from the field alone, so file the feasibility request early.
What happens if the distribution transformer is already full?+
If the distribution transformer is already full, the DISCOM may reject or reduce your sanctioned solar capacity, ask for a smaller system, or require a transformer upgrade. Options include downsizing, a zero-export design, or funding a DT upgrade. Confirm the route and any cost with your DISCOM.
Is the 30% DT loading limit the same across all of India?+
No. The DT loading limit is not the same across all of India. About 30% is commonly cited, but each State Electricity Regulatory Commission and DISCOM sets its own figure, and several states have amended it. Treat any percentage as an example and verify the current rule for your state.
How does SuryaHub help with DT capacity checks?+
SuryaHub tracks each net-metering job through the DISCOM feasibility stage, stores the transformer and sanctioned-load details, and flags when a feasibility reply changes the workable system size. SuryaHub is pre-revenue; the real pilots are Suryantra Energy and RGESPL.
Sources & references
Technical and safety standards for grid connection come from the CEA, with the loading percentage set by each state regulator. Always confirm the current figure with your DISCOM and the SERC regulation before you design.
- Central Electricity Authority (CEA) ↗
Technical and safety standards for grid connection of distributed generation.
- Ministry of Power ↗
Electricity (Rights of Consumers) Rules and rooftop solar connectivity policy.
- Ministry of New & Renewable Energy (MNRE) ↗
Rooftop solar programme guidelines that DISCOMs implement locally.
Written by the SuryaHub team · reviewed against CEA, MoP & SERC sources · updated 19 June 2026.
Method: Technical rules are taken from CEA and SERC sources above and re-checked every 30 days. The 30% figure is a commonly cited example only; loading caps vary by state and must be verified. SuryaHub is pre-revenue; only Suryantra Energy and RGESPL are real pilots.
Change log: 19 Jun 2026 — first published.