AGL Series Circuit Load and CCR Sizing Formulas

AGL series circuit formulas: the load of a 6.6 A constant current circuit, the cable loss, the regulator output voltage and the smallest standard CCR size.

The formulas

In a series airfield lighting circuit one constant current regulator (CCR) drives the same current through the primary of every isolating transformer, and it raises or lowers its output voltage to hold that current. n is the number of fittings, Plamp the lamp power of one fitting at the circuit current, ηT the efficiency of the isolating transformer, I the circuit current in amperes (6.6 A at the highest step), R the cable resistance per unit length and L the full length of the series cable loop, with R and L in matching units. The load is treated as resistive with a power factor near 1. The maximum loading is the share of the rated kW that you are willing to use, and the standard sizes here are 4, 7.5, 10, 15, 20, 25 and 30 kW. All values are examples. Take the lamp power and the transformer efficiency from the data sheets, check the result against the manufacturer's output curves and your project specification, and size the circuit at the highest brightness step.

Worked example: 60 fittings of 45 W on 4 km of cable at 6.6 A

The transformer efficiency is 0.85, the cable resistance is 3.08 Ω/km and the maximum loading is 0.8.

1. Plamps = 60 × 45 / 0.85 = 3,176 W
2. Pcable = 6.6² × 3.08 × 4 = 536.7 W
3. Pt = 3,176 + 536.7 = 3,713 W
4. Vout = 3,713 / 6.6 = 562.6 V
5. Rating needed = 3.713 / 0.8 = 4.641 kW, so the smallest standard size is 7.5 kW
6. The 7.5 kW regulator is loaded to 3,713 / 7,500 = 49.51%

The example circuit on each standard size

Frequently asked questions

Why does the regulator voltage change with the load?

A constant current regulator holds the current and lets the voltage follow the load. More fittings or a longer cable need a higher output voltage at the same 6.6 A.

Why is the cable loss I² × R × L?

The same current flows through the whole series loop, so every metre of conductor loses I² × R. L must be the full conductor length of the loop, not the distance to the last fitting.

Why size the circuit at the highest step?

The load is largest at the highest brightness step, which is 6.6 A, so that step decides the regulator size. The lower steps carry less current and need less power.

What loading limit should I use?

The calculator uses 0.8 as an example. Some of the rating is kept for lamp ageing, cable changes and future fittings, so use the limit in your specification or the manufacturer's advice.

What if the load is above the largest size?

Above 30 kW the circuit is split into two or more circuits, each with its own regulator.

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