AGL Maximum Fittings on a CCR Formulas

AGL maximum fittings formulas: how many fittings a constant current regulator of a given kW rating can carry on a series circuit after the cable loss.

The formulas

This is the reverse of CCR sizing: the regulator is fixed and the question is how many fittings it can carry. The symbols are those of the series circuit guide. The brackets ⌊ ⌋ mean rounding down to a whole fitting, because a part of a fitting cannot be connected. Every fitting is taken to have the same lamp power and the same transformer efficiency, and the load is resistive. If the cable loss alone is larger than the usable power, no fitting fits, and the loop must be shorter, the conductor larger or the regulator bigger. Keep spare capacity for future fittings and check the circuit at the highest brightness step. The values are examples, so confirm them with the manufacturer's output curves.

Worked example: a 10 kW CCR at 6.6 A with 45 W fittings on 4 km of cable

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

1. Pusable = 0.8 × 10,000 = 8,000 W
2. Pcable = 6.6² × 3.08 × 4 = 536.7 W
3. p = 45 / 0.85 = 52.94 W
4. nmax = ⌊(8,000 − 536.7) / 52.94⌋ = ⌊140.97⌋ = 140
5. Pt = 140 × 52.94 + 536.7 = 7,948 W, which is 79.48% of the rating, and Vout = 7,948 / 6.6 = 1,204 V

How the cable length changes the answer

Frequently asked questions

Why round down?

The last fitting must fit completely. Rounding up would load the regulator above the limit you chose.

Why does a longer cable carry fewer fittings?

The cable loss comes out of the same usable power, and it grows with the length. Every watt lost in the cable is a watt that a fitting cannot use.

Should I connect the maximum number?

Not normally. Keep some capacity for future fittings, lamp ageing and cable repairs, and take the limit from your specification.

Does this work for mixed fittings?

Not directly. The method takes one lamp power for every fitting. For a mix, add the load of each type: Σ(ni × pi) + Pcable must stay below the usable power.

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