HT vs LT Panel Sizing Errors | Electrical Design Consultant Delhi
Wrong HT-LT panel sizing drains budgets and safety margins on Delhi NCR sites. Here’s what Electrical Design Consultants Delhi teams check first.
HT vs LT Panel Sizing Errors Every Electrical Design Consultant Delhi Project Must Avoid
A panel that looks perfectly finished on handover day can still be wrong. Paint, labelling, and a clean single-line diagram tell you nothing about whether the busbar can survive a fault, or whether the breaker downstream will trip before the one upstream does. Sizing errors between HT and LT panels rarely show up during testing; they show up eighteen months later, during a monsoon peak load or a festival-season demand spike, when the building is full, and the stakes are highest. For developers and facility owners across the NCR, this is where a project’s electrical reliability is quietly decided long before the switchgear ever ships.
Why HT and LT Sizing Isn’t the Same Calculation
Treating high-tension and low-tension panels as scaled versions of the same design is where most sizing errors begin. HT panels, typically fed at 11kV or 33kV directly from the discom network, carry fault currents and insulation requirements that behave nothing like a 415V LT board. The Electrical Design Consultants Delhi teams work with most often sees this gap appear at the transformer interface, where HT switchgear is specified using LT-style safety margins instead of proper short-circuit and insulation coordination studies. A transformer’s impedance, the discom’s available fault MVA, and the CEIG approval requirements all shape HT panel sizing in ways that a standard LT load schedule simply doesn’t capture.
LT panel sizing carries its own separate risk. Diversity factors get applied too generously, future load headroom gets ignored to save panel cost, and busbar ratings get matched to present-day connected load rather than the sanctioned or contracted demand. On a mixed-use development, this means the panel that passed factory testing at 80% load starts nuisance-tripping the moment retail tenants, HVAC plant, and lift banks all draw simultaneously during peak hours. It’s a pattern that shows up repeatedly in the retrofit audits Electrical Design Consultants Noida teams run on ageing commercial complexes, where sanctioned loads were never revisited as tenant mixes shifted over the years.
Where the Numbers Actually Go Wrong
Three calculation gaps account for most of the rework Sanelac gets called in to fix after another firm’s design has already gone to the site.
Fault level mismatch: Busbars and breakers rated below the actual prospective short-circuit current at that point in the network, often because the fault level was assumed rather than calculated from actual transformer and cable impedance data.
Cable derating oversights: Ambient temperature, grouping, and burial depth factors get skipped, so a cable sized correctly on paper runs hot in a real trench or cable tray.
Selectivity failures: Upstream and downstream breakers aren’t coordinated, so a fault on one floor’s LT distribution board trips the main incomer instead of isolating locally.
Any one of these on its own is manageable. Together, they compound, a slightly undersized cable heats a slightly undersized breaker faster, and a coordination gap turns a local fault into a building-wide outage. This is the layered failure pattern that separates a functioning panel from one that merely passed commissioning.
The Real Cost Shows Up After Handover
Sizing errors are expensive precisely because they don’t fail immediately. Cable insulation degrades gradually under sustained overheating, breaker contacts wear faster under repeated nuisance tripping, and discom inspection teams flag CT/PT ratio mismatches only when they audit the installation against sanctioned load, sometimes years into occupancy. By then, correcting the issue means partial shutdowns, re-terminating live busbars, and coordinating outage windows with tenants who were promised uninterrupted power. What would have been a design-stage correction becomes a retrofit involving structural clearances, updated approvals, and disruption costs nobody budgeted for.
This is also where the difference between HT and LT gets expensive in different ways. HT retrofits require fresh CEIG clearance and discom coordination, which can stall a project for weeks, a sequence that Electrical Design Consultants Gurgaon industrial and township clients navigate often, given the DHBVN approval timelines involved. LT retrofits are faster to execute but often need to happen live, floor by floor, while a building is occupied, a far more delicate exercise than getting the sizing right the first time.
Designing It Right the First Time
Correct sizing starts with treating HT and LT as two separate design exercises that meet at the transformer, not one continuous calculation. Load schedules need to reflect sanctioned demand plus a realistic growth margin, not just what’s connected on day one. Short-circuit studies should use actual discom fault-level data rather than generic assumptions, and breaker coordination needs to be verified with time-current curves, not just nameplate ratings.
Firms offering Electrical Design Consultants Gurgaon developers rely on for large township and commercial projects increasingly build this verification into the design stage itself, running discrimination studies before panels are even ordered, rather than discovering coordination gaps during commissioning. The same discipline applies whether the site is a 33kV industrial feed or a compact 415V LT distribution board serving a mid-rise residential tower.
Manufacturer coordination matters just as much as the calculation. A panel builder working from an incomplete single-line diagram will size to the drawing, not to the site’s actual fault conditions, which is why the design consultant’s fault-level study, cable schedule, and coordination curves all need to reach the panel shop before fabrication begins, not after.
What Delhi NCR’s Growth Is Exposing
The pace of commercial and residential development across the region means panel sizing decisions made five years ago are now being tested against load profiles nobody originally planned for. Data centres, EV charging infrastructure, and denser occupancy are pushing demand well past original sanctioned loads on older installations. Teams offering Electrical Design Consultants Noida clients trust for retrofit and expansion work spend a meaningful share of their time re-validating panels that were sized correctly for 2019 loads but not for 2026 ones.
This is less a story about bad design and more about a design that didn’t build in enough margin, verification, or future-load thinking. An experienced design partner brought in early, before panels are finalized, not after complaints start, is the difference between a building that scales quietly and one that needs emergency intervention during its busiest season. Getting HT and LT sizing right isn’t a compliance checkbox; it’s the calculation that decides whether a building’s electrical backbone holds up for the next twenty years or the next two.
FAQs
1. What’s the core difference between HT and LT panel sizing? HT panel sizing centres on fault-level coordination and insulation clearances at 11kV or 33kV, while LT sizing focuses on busbar ratings and cable derating at 415V. The two require separate short-circuit studies because fault behaviour, breaker technology, and approval requirements differ sharply between the two voltage classes.
2. Why do LT panels trip even when the connected load seems within capacity? Nuisance tripping usually points to poor breaker selectivity, not overload. When upstream and downstream breakers aren’t coordinated using time-current curves, a fault anywhere downstream can trip the main incomer instead of isolating locally, cutting power to the whole building unnecessarily.
3. How does discom-sanctioned load affect panel sizing decisions Sanctioned load, not just connected load, should define busbar and cable ratings from the start. Sizing to present-day usage alone leaves no headroom for tenant additions, HVAC upgrades, or EV charging, forcing costly retrofits once actual demand catches up to the sanctioned limit.
4. Can an undersized HT panel be corrected without a full shutdown Rarely, and not safely. HT corrections typically require fresh CEIG approval and coordinated discom outages because busbars and breakers are live at high voltage, unlike LT retrofits, which can sometimes be phased floor by floor during occupied operation.
5. What causes cable sizing errors even when calculations look correct on paper? Skipped derating factors are the usual cause; ambient temperature, cable grouping, and burial depth all reduce a cable’s real current-carrying capacity below its rated value. A cable sized only against nameplate current, without these adjustments, runs hotter than the design intended.
Don’t let guesswork size your panels. Talk to Sanelac Consultants for an HT-LT design that holds up on site, not just on paper.
