LED power-draw and CCR re-sizing after retrofit
LEDs cut the fixture load, but not the cable loss. Add up the load again with the new figures, then decide whether the CCR still fits or needs re-sizing.
Fitting LEDs cuts the fixture power sharply, but it does not cut the circuit load by the same share. The loop current is unchanged, so the cable loss stays, and each transformer still has its own losses. Re-sizing means adding up the load again with the new figures, then deciding whether the CCR still fits.
1What changes, and what does not
The fixture power falls from tens of watts to about 10 W per light in this example. The transformer loss falls in watts, but a transformer carrying a fraction of its rating is less efficient. The cable loss is the loop current squared times the loop resistance, and the CCR still drives the same current at the top step, so it does not change. The output voltage, which is the load divided by the current, falls with the load. In the example the fixture load falls by about 78% and the circuit load by about 61%.
2Re-add the load, do not scale it
| Item | Halogen (before) | LED (after) |
|---|---|---|
| Fixtures | 60 × 45 W = 2,700 W | 60 × 10 W = 600 W |
| Transformer input | 2,700 ÷ 0.90 = 3,000 W | 600 ÷ 0.78 ≈ 769 W |
| Cable loss | 6.6² × 15.4 ≈ 671 W | 6.6² × 15.4 ≈ 671 W (unchanged) |
| Circuit load | 3,671 W | ≈ 1,440 W |
| Output voltage | ≈ 556 V | ≈ 218 V |
| Share of a 7.5 kW CCR | 49% | 19% |
3When the CCR is now too large
A CCR works between a minimum and a maximum load. At 19% of its rating the circuit may sit below the manufacturer’s minimum loading, with a lower power factor and less accurate regulation. There are four ways to deal with it, and the choice is a design decision made with the CCR and fixture manufacturers.
| Option | What it does | Advantage | Watch |
|---|---|---|---|
| Keep the CCR | Runs the lighter load on the existing regulator | No equipment cost | Only if the manufacturer states it is approved for the load; check power factor, regulation and waveform |
| Add a balancing load | Adds a dummy load so the CCR sits back in its range | Quick; keeps the CCR | Turns part of the saving into heat; needs space, cooling and monitoring |
| Combine circuits | Feeds more fixtures from one CCR | Uses the CCR capacity; fewer regulators | Changes the circuit design; one CCR carries more lights, so check redundancy and cable |
| Fit a smaller CCR | Matches the regulator to the new load, with margin | Best match; keeps the full saving | Cost and an outage. In the example, 1.44 kW plus margin suits a 2.5 kW unit at about 58% (sizes vary by manufacturer) |
What to check when re-sizing
The real fixture draw
Use the manufacturer’s data for the installed fixture, driver included, at the loop current, not the label wattage of the old lamp.
Transformer loading
Compare each transformer rating with the new fixture load, and decide whether to keep it or fit a smaller one.
Cable loss
Recalculate it from the loop resistance at operating temperature and the top-step current. It is now a much larger share of the load.
CCR limits
Compare the load with the rating and the manufacturer’s minimum loading, and see how far it falls as fixtures fail.
Every step
After any change, measure the series current and the light output at every brightness step.
Supply and records
Recheck input kVA and any generator or UPS loading, then update the circuit schedule with the new load and the CCR rating.
Frequently Asked Questions
Q1Why does the circuit load fall less than the fixture load after an LED retrofit?
The fixture power falls a lot, but the cable loss depends on the loop current and the loop resistance, and neither changes. The transformer losses also fall by less than the fixture power. In the worked example the fixture load falls by about 78% and the circuit load by about 61%.
Q2How is the new CCR load calculated?
In the same way as the original: fixture power, plus the transformer input power at the new, lighter loading, plus the cable loss at the top step. Use the manufacturers' data for the installed fixtures and transformers, then compare the total with the CCR rating and its minimum loading.
Q3Does the CCR have to be replaced after an LED retrofit?
Not always. If the CCR manufacturer states that it is approved for the lower load, and its regulation, power factor and output waveform check out at every step, it can stay. If the load falls below the minimum loading or the performance suffers, the options are a balancing load, combining circuits or a smaller CCR.
Q4What is a balancing load, and when is it used?
It is an extra load added to the circuit to bring the total back into the CCR’s working range. It is a quick fix that keeps the existing CCR, but it turns part of the saving into heat, needs space and cooling, and is one more item to maintain. It is a design decision to make with the CCR manufacturer.
Q5How much margin should be allowed when choosing a smaller CCR?
The same approach applies as for any CCR: work out the load at the highest step, add margin for growth, ageing and tolerance, and take the next standard size up. Then check that the result is not too large again. The amount of margin comes from the site design basis and the manufacturer.
Q6What changes on the supply side after the retrofit?
The circuit draws less real power, but a lightly loaded CCR can have a lower power factor, so the supply kVA does not fall as far as the kW. Recheck the loading of any generator or UPS on kVA as well as kW, and do not downsize standby power until the future load is settled.
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