Fixture Schedule
| Fixture type | Watts each | Power factor | Fixtures per transformer | Transformer type | Transformers (qty) | Remove |
|---|
Notes on power factor, LED and transformer type
Power factor is 1.00 for plain (halogen) lamps. For LED fixtures, enter the watts and the driver's power factor, or the VA with a power factor of 1.00, and follow the fixture manufacturer's recommended transformer size where one is given. LED loads are often below 22.5 VA, so a "lightly loaded" result on the smallest size is expected. The type must match the current of the series circuit the transformer sits on (6.6 A or 20 A primary). FAA lists no 65 W transformer with a 20 A primary, so the 20/6.6 A type skips 65 VA. Smaller 10/15 W and 20/25 W types also exist for very light loads, but are not on this list. Availability of each size and type depends on the manufacturer.
Selection Rules
Rule note
Extra allowance added to the load before choosing a size, for example for a future lamp change. Your own rule.
Rule note
Used for the secondary voltage of each transformer (S ÷ I). The secondary of a standard airfield series isolating transformer is rated 6.6 A, even when the primary circuit is 20 A. This is the secondary voltage at full rated load.
Rule note
Your project's own loading window, not a standard value. Some specifications set a maximum below 100 % to allow for lamp tolerance and ageing, so set yours to match the project specification. Standard sizes: 30/45, 65, 100, 150, 200, 300 and 500, treated as nominal VA. The 30/45 size is one dual-rated transformer, treated here as 45 VA.
Report details
Selection by fixture type
| Fixture type | Load (VA) | Design load (VA) | Selected size | Type | Loading (%) | Nearest size | Secondary (V) | Qty | Status |
|---|
Build-up
How the size is selected
Every step is shown so the result can be checked by hand.
1. Load per transformer
Multiply the fixtures per transformer by the watts each, then divide by the power factor to get apparent power. The design margin is added on top.
2. Selected size
The selected size is the smallest standard size where the design load is at or below your maximum loading. Loading is then reported for the actual load, without the margin.
If the loading falls below your preferred minimum, the row is flagged as lightly loaded.
3. Nearest size
The nearest standard size to the actual load is shown beside the selected one. When it is smaller than the load, the row says so, because the nearest size alone would be overloaded.
4. Order count
Transformers are counted per selected size and type across all rows. The secondary voltage is S ÷ the secondary current (6.6 A).
Worked example
| Item | Value |
|---|---|
| 1 × 45 W, PF 1.00 | S = 45 VA |
| Selected size, no margin | 30/45 VA at 100 % |
| Same fixture, 10 % margin | 49.5 VA → 65 VA at 69 % |
| 2 × 20 W, PF 0.90 | S = 44.4 VA |
| Selected size | 30/45 VA at 99 % |
| 1 × 150 W, PF 1.00 | 150 VA at 100 % |
| Secondary at 6.6 A (44.4 VA) | 6.73 V |
What this tool does not do
The standard sizes follow the series isolating transformer ratings in FAA AC 150/5345-47 (L-830 and L-831). Confirm them against your project specification and the manufacturer's range. It does not model efficiency curves, magnetising current, secondary open-circuit voltage or lamp-out behaviour. The sizes are nominal, and the margin and loading window are your own rules. For primary-side load and losses, use Calculator #17.