Designing a distribution system is mostly arithmetic done in the right order. Get the order wrong and you end up with a transformer that is too small, protection that will not coordinate, or a cable that is legal on paper and unusable in practice. This is the sequence that works.
Step 1: Establish the Load
Start with a load list, not a guess. Every motor, panel, and fixed load, with its rating and whether it runs continuously. Apply diversity – not everything runs at once – and record what diversity factor you used, because the next engineer will need it. Motor starting current belongs here too: a motor that draws six times its rating for four seconds is a voltage dip problem waiting to happen.
Step 2: Choose the Voltage Levels
Most industrial and commercial systems use one medium-voltage level for distribution and 400 V or 480 V at the point of use. The medium-voltage level is usually dictated by what the utility offers or by what the site already has. Where you are establishing a new site, the comparison is between fewer transformers and longer low-voltage runs, or more transformers and shorter ones. Long low-voltage runs lose voltage and cost cable.
Step 3: Size the Transformers
Rating comes from the diversified load with margin, checked against the starting and peak conditions rather than the average. Then check the environment: altitude and ambient temperature derate a transformer, and a unit in a sealed enclosure runs hotter than the outdoor figure suggests. Load growth is worth a thought here, but resist padding it – an oversized transformer costs money up front and loses money in no-load losses every hour it is energised.
Step 4: Calculate the Fault Levels
Every point in the system has a prospective short-circuit current, and every piece of equipment has to be rated above it. This is what sets switchgear breaking capacity, busbar withstand, and cable thermal withstand. It is also what makes protection coordination possible, because a relay can only discriminate if it knows what current to expect.
Step 5: Select Protection and Check Coordination
Protection is chosen so that the device nearest the fault clears it, and nothing upstream operates first. That means working through the time-current curves of every device in series, at both maximum and minimum fault levels. It is tedious and it is the step most often skipped, and it is why nuisance tripping happens on sites that look fine on the single-line diagram.
Step 6: Size the Cables
Three checks, and the governing one wins:
- Current carrying capacity, derated for installation method, grouping and ambient temperature.
- Voltage drop, across the full run, under load and under motor starting.
- Short-circuit thermal withstand, for the time it takes the protection to clear the fault.
A cable that passes the first and fails the third is a fire risk. A cable that passes the first two and fails the second is a commissioning problem.
Step 7: Decide Earthing and Bonding
The earthing arrangement affects fault current magnitude, protection sensitivity, and touch and step voltages. It is a system-level decision made early, not a detail added at installation. Where the utility or the local code dictates an arrangement, that settles it.
Step 8: Write Down the Site Conditions
Altitude, ambient temperature range, humidity, salt or industrial pollution, indoor or outdoor, and ingress protection. These decide enclosure specification, creepage distances, whether you need anti-condensation heating, and how much corrosion protection the finish needs. They are cheap to handle at design stage and expensive to fix later.
Step 9: Turn It Into Equipment
By this point the single-line diagram, load list, fault levels and site conditions are enough to specify everything. Where a compact substation or prefabricated substation suits the layout, the switchgear, transformer and LV board are supplied as one tested assembly. Where the site calls for separate equipment, each item is specified against the same diagram. Our engineering team works from drawings like these.
The Mistakes Worth Avoiding
- Sizing the transformer from the connected load instead of the diversified load, or padding the diversity factor to be safe.
- Choosing switchgear on rated voltage without checking the fault level at that point.
- Skipping coordination studies and discovering at commissioning that two devices trip together.
- Specifying equipment before the site conditions are known.
- Leaving the standard to be decided after the order is placed.
If you are still fixing the ratings, our MV switchgear selection guide works through the five numbers that decide the arrangement before you look at a catalogue.
Frequently Asked Questions
Should I oversize the transformer for future growth?
Some margin is sensible, but a large margin costs twice: once in capital and once in no-load losses every hour the transformer is energised. Size to the diversified load with a stated margin, and record what margin you used.
How do I know what fault level to design for?
Ask the utility for the fault level at the point of connection, and ask what it will be after planned network changes. Equipment sized to today’s fault level can be under-rated within a few years.
What causes nuisance tripping in a new installation?
Usually protection that was selected without a coordination study. Two devices in series with overlapping curves means the upstream one trips for a fault the downstream one should have cleared.
Do I need to specify the standard at the design stage?
Yes. The standard and clause set determine test requirements, clearances and sometimes the construction. Deciding it after the order is placed is how projects end up with equipment that cannot be certified to the specification.
Get a Drawing and a Price
Send the single-line diagram, the load list and the site conditions – altitude, ambient range, indoor or outdoor, enclosure rating. We come back with a proposed arrangement, the standard set we would work to, and a quotation. Email info@xsdfftransformer.com or call +8615867897761.