How is the load on a circuit calculated?
Adding up every watt a series circuit draws, from the fixtures, the transformers and the cable, so the regulator that feeds it is the right size.
A regulator can only drive so much load. Load calculation finds how much a circuit really draws, so the regulator is neither too small to hold the current nor so large that it runs poorly at light load.
1Add up what the circuit draws
The load is more than the lamps. Start with the fixture power of every light, then add the isolating transformer losses, because each transformer draws more than it delivers. Signs, other connected devices and any planned future lights go in too.
2Do not forget the cable
The same current flows through the whole series loop, so the cable wastes I²R as heat. On a long circuit that can be a large share of the load, and because it grows with the square of the current, it is largest at the top step. That is why the calculation is done at the highest step.
3Add margin, then size up
Total the load, add margin for growth, ageing and measurement error, then choose the next standard CCR size up. A regulator at its limit cannot take another fixture, and if the load outgrows it the current falls and the lights dim.
| Item | How | Example |
|---|---|---|
| Fixtures | Nameplate lamp or LED power × number of fixtures | 60 × 45 W = 2,700 W |
| Transformer input | Fixture power ÷ transformer efficiency at that load | 2,700 ÷ 0.90 = 3,000 W |
| Cable loss | I² × total series loop resistance | 6.6² × 15.4 = 671 W |
| Circuit load | Transformer input + cable loss | 3,000 + 671 = 3,671 W |
| Output voltage | Load ÷ rated current (approximate, assumes power factor near 1; otherwise use VA) | 3,671 ÷ 6.6 ≈ 556 V |
| CCR choice | Load plus margin, then the next standard size up | 3.67 kW + 25% ≈ 4.6 kW, above 4 kW → 7.5 kW |
- List the circuit. Count fixtures by type and note any signs or other devices.
- Get the data. Fixture power, transformer ratings and efficiency, cable resistance and length.
- Add transformer losses. Divide fixture power by transformer efficiency at that load.
- Add cable loss. Use the total loop resistance and the top-step current squared.
- Total and add margin. Allow for future lights, ageing and tolerance.
- Choose the CCR. Take the next standard size up and check it at light load.
| Term | Meaning |
|---|---|
| Circuit load | Total power the circuit draws from the regulator at a given step |
| Transformer loss | Power used in the isolating transformer, not delivered to the fixture |
| Cable loss | I²R heating in the series cable, rising with the square of current |
| Loop resistance | Total resistance of the series cable out and back |
| Margin | Spare capacity kept for growth, ageing and error |
| CCR rating | Output the regulator is built for, in kW or kVA, at its rated current |
Frequently Asked Questions
Q1What is load calculation in airfield lighting?
Adding up all the power a series circuit draws, from the lamps or LED fixtures, the isolating transformers and the cable, so that a constant current regulator of the right size can be chosen.
Q2What goes into the load?
The power of every fixture on the circuit, the losses in each isolating transformer, and the I squared R loss in the series cable. Sign transformers, any other connected devices and spare capacity for future lights are added too.
Q3Why calculate at the highest step?
The highest step has the largest current, and cable loss rises with the square of current, so that is when the circuit draws the most power. If the CCR can carry that, it can carry the lower steps.
Q4Why not pick a CCR that matches the load exactly?
A regulator run at its limit has no room for added fixtures, ageing, a changed lamp type or measurement error, and loses the ability to hold current if the load grows. Designers add margin and then choose the next standard size up.
Q5Do LED fixtures change the calculation?
Yes. LED fixtures draw far less power than halogen lamps, so the load is smaller. The method is the same, but isolating-transformer efficiency and power factor are usually lower at these light loads, so check the circuit in VA as well as watts, and check the regulator's minimum-load behaviour and harmonics, using the manufacturer's data.
Q6What happens if the load is too large?
The CCR cannot reach the voltage needed to push the set current through the loop, so the current falls, the lights dim and the regulator may alarm. The cure is to reduce the load, split the circuit or fit a larger regulator.
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