AGL Maximum Fittings per Circuit Calculator
Find the largest number of fittings a CCR can supply on an airfield lighting series circuit from CCR rating, lamp power and cable loop.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
About this calculator
Use this when the regulator is fixed and you need to know how many fittings it can carry. It is the reverse of the CCR loading calculator: it subtracts the cable loss from the usable CCR output and divides by the load of one fitting, including its isolating transformer loss.
Limits of this calculator
Every fitting is assumed to have the same lamp power and transformer efficiency. Confirm the result against the CCR manufacturer output curves, keep spare capacity for future fittings and check the highest brightness step.
Cross-check: 25 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Circuit-loading estimate; manufacturer limits govern.
What this calculator checks
- The arithmetic of the simplified model for the values you entered.
What remains unverified
- Whether the simplifying assumptions listed on this page hold for your case.
- Any standard, manufacturer or site data. Treat the result as an estimate and confirm it by a further engineering check.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 61820-1; IEC 61820-1-2; IEC 61820-3-2; FAA AC 150/5345-10H, IEC 61820-1:2019; IEC 61820-1-2:2024; IEC 61820-3-2:2023; FAA AC 150/5345-10H |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | AGL series-circuit and CCR requirements are relevant. The calculator estimates a maximum fitting count from user-entered lamp, transformer and cable data; it does not replace CCR load curves or manufacturer limits. |
| Cross-check test cases | 25 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| CCR rating | 10 kW |
| Lamp power per fitting | 45 W |
| Isolating transformer efficiency | 0.85 |
| Circuit current | 6.6 A |
| Series cable loop length | 4,000 m |
| Series cable resistance | 3.08 Ω/km |
| Maximum CCR loading | 0.8 |
- Usable = 0.8 × 10 kW = 8,000 W
- Cable loss = 6.6² × 3.08 Ω/km × 4 km = 536.7 W
- Load per fitting = 45 W / 0.85 = 52.94 W
- Maximum fittings = ⌊(8,000 − 536.7) / 52.94⌋ = 140
Assumptions
- Resistive series circuit, power factor near unity
- Cable resistance given for the full loop length
- Every fitting has the same lamp power and the same transformer efficiency
This is the reverse of the CCR sizing calculator. Confirm against the CCR manufacturer's output curves and the isolating transformer open-circuit behaviour, and keep spare capacity for future fittings. Check at the highest brightness step.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| CCR rating | kW | greater than 0 kW | Rated output of the regulator, from its nameplate |
| Lamp power per fitting | W | greater than 0 W | At the circuit current, from the fitting data sheet |
| Isolating transformer efficiency | (number) | greater than 0.5, up to 1 | From the transformer data sheet at the circuit current |
| Circuit current | A | greater than 0 A | |
| Series cable loop length | m | greater than 0 m | |
| Series cable resistance | Ω/km | greater than 0 Ω/km | |
| Maximum CCR loading | (number) | 0.3 or more, up to 1 | Fraction of rated kW you are willing to use |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Maximum fittings | (number) | Shown to 0 decimal places | |
| Circuit load at that number | kW | Shown to 3 decimal places | |
| Cable loss | W | Shown to 0 decimal places | |
| CCR loading at that number (%) | (number) | Shown to 1 decimal place | |
| CCR output voltage | V | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| CCR rating (kW) | Maximum fittings | Circuit load at that number (kW) | CCR loading at that number | CCR output voltage (V) |
|---|---|---|---|---|
| 4 | 50 | 3.184 | 79.6 | 482 |
| 7.5 | 103 | 5.99 | 79.9 | 908 |
| 10 | 140 | 7.948 | 79.5 | 1,204 |
| 15 | 216 | 11.972 | 79.8 | 1,814 |
| 20 | 292 | 15.995 | 80 | 2,424 |
| 30 | 443 | 23.99 | 80 | 3,635 |
Other inputs are the same as in the worked example above.
Standards and further reading
- ICAO Annex 14, Volume I (aerodrome design and operations)
- ICAO Doc 9157, Part 5 (aerodrome design manual, electrical systems)
Photometric and circuit requirements come from the edition of Annex 14 and the regulator's rules that apply at your aerodrome, and from the fitting and regulator manufacturers' data. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter cCR rating, lamp power per fitting, isolating transformer efficiency, circuit current, series cable loop length, series cable resistance and maximum CCR loading. Change the unit of a value with its selector and the result stays the same.
What is the maximum CCR loading setting?
It is the share of the CCR rating you allow to be used, which leaves headroom. Set it to the margin your specification requires.
Why is the number rounded down?
A fraction of a fitting cannot be connected, so the result is the whole number of fittings that fits within the usable output.
How is this different from the CCR loading calculator?
That page starts from the number of fittings and gives the load and the CCR size. This one starts from the CCR and gives the number of fittings.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 25 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- 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…
- 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
- Cable Loss Formulas: Power and Annual Cost: Cable power loss formulas: I squared R loss for single-phase and three-phase cables, annual energy and cost of the loss
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