Electrical Power Load Allocation, Generators & Busbar Trunking

A garment factory buys a generator the way it buys a machine: by looking at what is available and what the budget allows. That habit causes the two classic power failures. The first is an oversized generator bought from connected load with no demand factor, which wastes capital and runs inefficiently at low load for its whole life. The second is an undersized generator bought from a guess, which trips on the first peak day and stops every sewing line at once. Electrical power load allocation exists to prevent both failures. It is a chain of four conversions, and each conversion must be done in order: connected load to demand load, demand kilowatts to kVA, demand kVA to generator size, and generator kVA to busbar current.

The method in this lesson uses one worked example all the way through, so every number can be followed and reused. The loads are illustrative for a medium sewing floor, but the method is standard. Where a rating, factor, or clearance depends on a code or a utility rule, the lesson says so explicitly: verify against the current electrical code and utility requirement before building anything.

Connected load is the sum of the rated power of everything installed. Build it section by section, walking the floor, because a load list built from memory always misses something. The usual missing items are the compressor, the finishing irons, and the cutting room: sewing-only lists are the most common source of undersized generators.

The worked example floor has: 120 sewing machines at 0.55 kW each, which is 66 kW; 20 overlock machines at 0.75 kW each, which is 15 kW; cutting room equipment at 8 kW; finishing irons at 25 kW; lighting at 12 kW; the air compressor at 30 kW; and miscellaneous loads at 10 kW. Add them: 66 plus 15 plus 8 plus 25 plus 12 plus 30 plus 10 is 166 kW of connected load.

Shares help management see where the power goes. Sewing machines are 66 divided by 166, which is 39.8 percent. Overlock machines are 15 divided by 166, which is 9.0 percent. Cutting is 8 divided by 166, which is 4.8 percent. Finishing irons are 25 divided by 166, which is 15.1 percent. Lighting is 12 divided by 166, which is 7.2 percent. The compressor is 30 divided by 166, which is 18.1 percent. Miscellaneous loads are 10 divided by 166, which is 6.0 percent. The shares sum to 100.0 percent, which is a useful check that nothing was dropped from the table. Two facts jump out: sewing plus overlock together are under half the load, and the compressor plus irons together are a third. Any plan that sizes power from sewing machines alone is missing a third of the factory.

Not every load runs at the same time. Some lines are down for style changeover, some irons cycle on thermostats, and lighting zones switch independently. The demand factor is the fraction of connected load that actually runs at peak, and demand load is connected load multiplied by that factor.

For this floor, a demand factor of 0.80 is used: 166 kW multiplied by 0.80 is 132.8 kW of demand load. The factor itself is a judgement that should be confirmed, not copied. The right way to confirm it is a clamp meter survey during peak production: measure the real current at the main panel over a full peak day, convert to kilowatts, and divide by connected load. If the measured factor comes out at 0.72, use 0.72. A demand factor taken from another factory is a guess wearing a decimal point, and it will mis-size everything downstream.

Generators are rated in kVA, apparent power, while loads are usually described in kW, real power. The two are linked by power factor, the ratio of real power to apparent power. Motor-heavy loads like sewing machines and compressors pull current that does not all do useful work, so power factor sits below 1.0, typically around 0.80 to 0.90 unless correction equipment is installed.

Demand kVA equals demand kW divided by power factor. Here, 132.8 kW divided by 0.85 is 156.2 kVA. This step is where many sizing errors hide: a buyer who orders a 132 kW generator for a 132.8 kW demand has actually ordered about 132 kVA of capacity against 156.2 kVA of demand, a shortfall of roughly 15 percent, before any growth is considered. Always convert before comparing with a generator rating plate.

Power factor correction deserves a mention. Capacitor banks can raise power factor toward 1.0, which reduces kVA for the same kW and can reduce the generator size or free capacity on an existing set. Correction equipment must be sized and maintained by a qualified electrician, and the corrected power factor should be re-measured after installation, because an unmaintained capacitor bank quietly stops correcting and the sizing assumption silently fails.

A generator sized exactly to current demand is overloaded the day a new line is added. Add a growth margin of 15 to 20 percent, then select the next standard size above the result. Here, 156.2 kVA multiplied by 1.20 is 187.5 kVA, and the next standard size above that is a 200 kVA set. The margin is not generosity: it covers a new line, a second compressor, or a finishing expansion without a second capital purchase, and it lets the generator run at a healthier load fraction instead of at its limit.

Standard sizes vary by manufacturer, so the selection step is to take the calculated figure to the supplier and choose the next catalogue rating above it. If the calculated figure lands just above a standard size, that is not a reason to round down. Rounding down removes the margin the calculation just added, and the factory is back to an exactly-sized set with no future.

Power must travel from the generator and the main panel to the floor, and busbar trunking is the usual distribution method on a sewing floor because tap-off points can be added as lines move. Trunking is chosen by current, not by power, so the kVA must be converted once more.

For a three phase supply, current in amperes equals kVA multiplied by 1,000, divided by the square root of 3 multiplied by the line voltage. For the selected 200 kVA set at 400 volts, that is 200,000 divided by 1.732 multiplied by 400. The denominator is 692.8, and the current is 288.7 amperes. The next standard trunking rating above that, with margin for heat and future load, is 400 amperes. Choosing trunking at exactly 288.7 amperes would leave no margin for a hot day, a future tap-off, or voltage drop along a long run, so the standard practice is to step up to the next rating. Cable sizes, earthing, and protection settings must all be verified against the current electrical code and utility requirement by a qualified electrician; this lesson gives the current calculation, not the installation design.

Every factory loses grid power or generator capacity at some point, and the shedding order decided during the outage is always worse than the order decided during planning. Write it down now. Protect sewing lines first, because a stopped line loses earned minutes immediately and breaks the day target. Protect finishing next, because finished goods must keep moving to packing. Shed cutting and warehouse loads early, because cut panels already issued will keep sewing running for hours. Shed office and non-essential lighting first of all. Post the list at the main panel and in the maintenance office, and rehearse it once so the electrician on duty does not have to invent it in the dark.

A woven factory in Narayanganj bought a 150 kVA generator because that was the size the supplier had in stock and the price fit the budget. Connected load was 220 kW. On peak days the generator tripped repeatedly, sewing lines stopped mid-bundle, and the factory paid overtime to recover lost hours. Nobody had run the chain.

Rebuilt properly, the numbers are clear. Demand load at a factor of 0.80 is 220 multiplied by 0.80, which is 176 kW. At a power factor of 0.85, demand kVA is 176 divided by 0.85, which is 207.1 kVA, before any growth margin. The 150 kVA set had been undersized by more than 25 percent from the day it was installed. The factory replaced it with a 250 kVA set, giving headroom for demand plus growth, and tripping stopped. The capital approval SOP was changed the same week: no generator purchase without a load list, a demand factor, and a power factor calculation on file. The cost of the second purchase dwarfed the cost of the calculation that would have prevented it.

The most expensive failure is sizing from connected load with no demand factor, which buys a generator far larger than needed and runs it inefficiently for years. The most disruptive failure is sizing from a guess, which trips on peak day. The quietest failure is forgetting power factor, which leaves a 15 percent shortfall hidden inside a correct-looking kilowatt number. The most overlooked failure is the load list itself: irons and compressors left off a sewing-only list, or a list never updated after a new line is added, so the margins the factory thinks it has no longer exist.

Walk the floor and build the connected load inventory section by section, including compressors, irons, and cutting. Confirm the demand factor with a peak-day clamp meter survey. Convert demand kilowatts to kVA with the real power factor. Add 15 to 20 percent growth margin and select the next standard generator size above the result. Calculate busbar current from the selected kVA and choose trunking above it, with installation verified against the current electrical code and utility requirement. Write the load shedding priority list and post it. Review the load list quarterly and after every machine addition, because a power plan is only as current as its last update.