What is circuit load?
The total power a series lighting circuit draws from its CCR — lamps plus transformer and cable losses, all within the CCR rating.
Circuit load is the total power a series lighting circuit draws from its CCR. It is the power of every lamp on the loop, plus the losses in the isolation transformers and in the series cable. The CCR holds the current constant, so a bigger load makes it supply a higher voltage, not more current.
The total load must stay within the CCR rating. An overloaded CCR cannot hold rated current on the higher brightness steps, so lamps run dim, or it overheats or trips. Some margin is left for later additions, but a CCR running far below its rating is inefficient. Circuit load is different from transformer load, which is the load on a single isolation transformer.
- List. Every fixture and sign transformer on the circuit.
- Add lamp power. Fixture power multiplied by the quantity of each type.
- Add transformer losses. Use the figures on the transformer data sheet.
- Add cable losses. Estimate from the cable resistance and loop length.
- Compare. Check the total at the highest brightness step against the CCR rating and choose the next standard size up.
- Re-check. Repeat whenever fixtures or cable are added.
Frequently Asked Questions
Q1What is circuit load in a series lighting circuit?
The total power the circuit draws from its CCR. It is the sum of the power of every lamp on the loop plus the losses in the isolation transformers and the cable, and it has to stay within the CCR rating.
Q2What makes up the load on a series circuit?
The lamps or LED fixtures, the losses in each isolation transformer, and the losses in the series cable, which grow with cable length. The total of all three is what the CCR must supply.
Q3How does load affect a constant-current regulator?
The CCR holds the circuit current fixed at each brightness step, so a bigger load makes it supply a higher output voltage rather than more current. Adding fixtures or cable therefore raises the voltage and power the CCR must deliver.
Q4What happens if a circuit is overloaded?
The CCR may be unable to hold the set current on the higher brightness steps, so lamps run dimmer than intended, or it may overheat or trip on its protection. The total load must be checked against the CCR rating before fixtures are added.
Q5Why leave margin, and why not size a CCR far above the load?
Some margin allows for later additions and ageing, so the CCR is chosen at the next standard rating above the calculated load. A CCR running far below its rating is less efficient and may fall below the manufacturer's minimum load, so oversizing is not the answer either.
Q6How is circuit load different from transformer load?
Circuit load is the total of everything on the whole loop, and sets the size of the CCR. Transformer load is the load on the secondary of a single isolation transformer, which sets the size of that transformer.
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