Match a series isolating transformer rating to the fixture load it serves, with loading percentage, secondary voltage and losses.
Each fixture on a series circuit is fed through an isolating transformer that isolates the lamp circuit from the high-voltage series circuit and keeps the series circuit closed if the lamp fails. A transformer that is too small overheats, and one that is too large runs lightly loaded, with lower efficiency and a poorer power factor that add to the circuit load. This tool finds the smallest standard size that carries the load and shows how every size compares.
Most fixtures have their own transformer. Some, such as certain stop bar arrangements, share one.
Power at the operating current, from the manufacturer's data sheet. At a lower brightness step the lamp draws less than at full brightness, so enter the power at that current.
Anything else on the same secondary, such as a heater or a monitoring device. Leave 0 if none.
Normally 6.6 A. For a 20/6.6 A transformer enter 6.6 A (the secondary), not the 20 A primary current, because the secondary voltage is worked out from this value.
Current at which the nominal VA rating applies, normally 6.6 A. Some 300 W and 500 W types have a 20 A secondary. At a lower brightness-step current the usable capacity is the rating × (operating ÷ rated current).
1.00 for plain lamps. For LED fixtures, enter the driver's power factor.
Planning assumption for the primary-side load. Efficiency falls at light loading, so use manufacturer data, or see Calculator #18.
Your project's own loading window, not a standard value.
Nominal sizes, treated as VA. Confirm against your transformer data sheet.
Used for the circuit totals. Set 0 to skip.
| Size (W / VA) | Loading (%) | Spare (VA) | Nominal secondary (V) | Status |
|---|
Nominal sizes treated as VA at the rated secondary current. Nominal volts are the rating divided by the rated current; the test-load resistances in AC 150/5345-47C Table 2 do not always equal the nominal rating exactly, so use the data sheet for the actual full-load voltage. Calculated sizing only, not a final order list.
Every step is shown so the result can be checked by hand.
Add the fixtures and any extra load on the secondary, then convert to apparent power with the fixture power factor.
Each standard size is compared as a percentage of its nominal rating, treated as VA at the rated secondary current (6.6 A). At a lower operating current the usable capacity scales down in proportion.
The recommended size is the smallest one at or below your maximum loading. If that size is below your preferred minimum, the tool says so, because a lightly loaded transformer wastes capacity and can run at a poorer power factor.
The secondary carries the lamp current. It equals the circuit current for a 6.6/6.6 A transformer, while a 20/6.6 A transformer has 20 A in the primary and 6.6 A in the secondary. The voltage across the load follows from the apparent power and the secondary current.
Dividing by efficiency gives the real power the primary circuit must supply. This is the figure that feeds the circuit load calculation. It is real power only: for CCR kVA the transformer's magnetising current also counts, and FAA AC 150/5345-47C allows a power factor as low as 0.95 at rated load (see Calculators #13 and #16).
| Item | Value |
|---|---|
| 1 fixture × 45 W, PF 1.00 | S = 45 VA |
| Circuit current | 6.6 A |
| Secondary voltage | 6.82 V |
| 30/45 W transformer (rated 45 VA) | 100 %, at the limit |
| 65 W transformer | 69 % |
| Recommended size | 30/45 W |
| Primary load at 90 % | 50 W, 5 W loss |
| 100 transformers | 5.00 kW primary |
It does not model transformer efficiency curves, magnetising current, secondary open-circuit voltage or lamp-out behaviour, and the sizes offered are nominal. A fixture with a switching driver can behave differently from a plain lamp, so use the manufacturer's guidance for the pairing. This tool covers sizing and primary load; for efficiency across loading use Calculator #18, and for a multi-fixture order list use Calculator #49.
Quick answers on matching a series isolating transformer rating to the fixture load it serves.
It matches a series isolating transformer rating to the fixture load it serves. It finds the smallest standard size that carries the load and shows how every size compares, with loading percentage, secondary voltage and losses. A transformer that is too small overheats, and one that is too large runs lightly loaded, with lower efficiency and a poorer power factor that add to the circuit load.
Add the fixtures and any extra load on the secondary, then convert to apparent power with the fixture power factor. The real load is the number of fixtures times the watts per fixture plus the extra load, and the apparent load is that real load divided by the power factor. Use 1.00 for plain lamps, and the driver's power factor for LED fixtures.
Each standard size is compared as a percentage of its nominal rating, treated as VA at the rated secondary current of 6.6 A. At a lower operating current the usable capacity scales down in proportion. The recommended size is the smallest one at or below your maximum loading. If that size is below your preferred minimum, the tool says so, because a lightly loaded transformer wastes capacity. The loading window is your project's own, not a standard value.
The sizes are 10/15 W, 20/25 W and 30/45 W (dual-rated, treated as 15, 25 and 45 VA), then 65 W, 100 W, 150 W, 200 W, 300 W and 500 W, the ratings listed in FAA AC 150/5345-47C. They are nominal sizes treated as VA, so confirm them against your transformer data sheet.
Enter 6.6 A, which is the secondary, and not the 20 A primary current. The secondary voltage is worked out from this value. The rated secondary current is the current at which the nominal VA rating applies, normally 6.6 A. Some 300 W and 500 W types have a 20 A secondary, and for those you enter 20 A.
The secondary voltage is the apparent power divided by the secondary (lamp) current, which is 6.6 A for most types. The primary real load is the secondary real load divided by the efficiency, and the transformer loss is the difference. The primary load is real power and feeds the circuit load calculation. For CCR kVA the transformer's magnetising current also counts, and FAA AC 150/5345-47C allows a power factor as low as 0.95 at rated load, so use Calculators #13 and #16 for kVA. Efficiency is a planning assumption, so use manufacturer data; the AC sets minimums of 70 to 90 percent depending on size.
One 45 W fixture with a power factor of 1.00 gives 45 VA, and at 6.6 A the secondary voltage is 6.82 V. A 30/45 W transformer is loaded 100 percent, at the limit, and a 65 W transformer is loaded 69 percent, so the recommended size is 30/45 W. At 90 percent efficiency the primary load is 50 W with 5 W of loss, and 100 transformers give 5.00 kW of primary load.
It does not model transformer efficiency curves, magnetising current, secondary open-circuit voltage or lamp-out behaviour, and the sizes offered are nominal. A fixture with a switching driver can behave differently from a plain lamp, so use the manufacturer's guidance for the pairing. For efficiency across loading use Calculator #18, and for a multi-fixture order list use Calculator #49. Final selection shall be verified against the isolating transformer and fixture manufacturers' data.
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