How is power calculated for a loop?
Add up what the fixtures use and what the transformers and cable waste — that total sizes the CCR.
Power calculation finds the real power a series loop draws from the CCR. Power is voltage times current (on AC, real power also includes the power factor), and because the CCR holds the current fixed, the load is the sum of every fixture’s power plus the losses in the isolating transformers and the cable.
Cable loss follows I² × R, so it rises with the square of the current and in direct proportion to cable length. Calculate at the highest brightness step, add a margin for ageing and future expansion, then round up to the next standard CCR rating. An undersized CCR cannot hold the set current at full brightness. The transformers also draw magnetising current, so the circuit power factor is below one: check the kVA as well as the kW, and keep within the CCR manufacturer’s minimum-load limit.
- List the loads. Count fixtures and note each one’s power at the design current.
- Add the losses. Include transformer losses and I²R loss for the total cable resistance.
- Sum and add margin. Total the power, add a margin, and pick the next standard CCR size.
Frequently Asked Questions
Q1What is the basic power formula?
Power equals voltage multiplied by current (P = V x I) for DC or resistive loads; on AC, real power also includes the power factor. Using Ohm's law it can also be written as I squared times R, which is the form used for losses in cable.
Q2How do I find the total load on a CCR?
Add up the lamp or LED power of every fixture, the losses in each isolating transformer, and the I squared R loss in the cable. The total is the real power the CCR must deliver at full brightness.
Q3Why is the CCR rated in kVA or kW?
It supplies a fixed current at whatever voltage the loop needs, so its load is a voltage-times-current product. kW is real power; kVA is apparent power and is larger when the power factor is below one.
Q4Why add a margin when selecting a CCR?
To allow for ageing, extra cable or fixtures added later, rising losses and tolerances. The calculated load is then rounded up to the next standard CCR size.
Q5Which brightness step is used for the calculation?
The highest step, because it has the highest current and therefore the highest power. Lower steps draw less power.
Q6How does cable length affect power?
Longer cable has more resistance, so more power is lost as heat at the same current. That loss adds to the load the CCR must supply but produces no light.
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