Control Transformer Sizing: How to Size a Control Transformer
How to size a control transformer — sealed VA vs inrush VA, calculating the load, why inrush matters for contactor coils, and selecting the right VA rating.
Undersizing a control transformer is one of the most common — and most avoidable — panel wiring mistakes in machine building. The symptom is almost always the same: a contactor coil that pulls in reliably on the bench but chatters or refuses to latch the moment another contactor energizes on the same circuit. The root cause is nearly always voltage sag caused by inrush current, and the fix is a transformer sized for the actual inrush demand, not just the steady-state load.
This guide walks through the complete sizing procedure: what a control transformer does, the difference between sealed VA and inrush VA, how to calculate your total load, a worked example with multiple contactors, and how to select the correct VA rating with the right margin.
What Is a Control Transformer?
A control transformer (sometimes called a machine control transformer or an industrial control transformer) is a step-down transformer that converts the main line voltage — typically 480 V AC or 240 V AC — to a lower control voltage, most commonly 120 V AC, to power the control circuit inside a panel.
Its job is simple: isolate and power the relay coils, contactor coils, solenoid valves, pilot lights, PLC power supply inputs, and other control-voltage devices that make the machine logic work. Because one secondary winding feeds all of those loads at once, the transformer must be sized for the combined demand of every device that could be energized simultaneously — including the brief, very high current surge that inductive loads draw the moment they are first energized.
Control transformers are dual-wound (primary and secondary are electrically separate), which provides the isolation needed to reference one side of the 120 V secondary to ground through a fuse for fault protection. This is the grounded-control-circuit configuration required by NFPA 79 and NEC Article 409 for industrial machinery.
Key specifications on the nameplate:
- Primary voltage (e.g., 480 V, 240 V, 208 V — often multi-tap)
- Secondary voltage (e.g., 120 V)
- VA rating — the continuous output capacity in volt-amps
Why Size Carefully? Inrush vs. Steady-State
Most engineers instinctively add up the coil ratings from the component datasheets and pick the next standard transformer size above that total. That approach works fine for a circuit with only resistive loads like pilot lights and PLC digital inputs. It fails when inductive loads — contactor coils, relay coils, solenoid valves — are on the same secondary.
Every inductive coil draws a short-duration surge of current the instant it is energized. This inrush current can be 6–10× the coil's steady-state (sealed) current. The surge lasts only a few AC cycles (roughly 20–100 ms), but during those milliseconds it creates a voltage dip across the transformer's internal impedance. If the dip is large enough to pull the secondary voltage below roughly 85 % of nominal (about 102 V on a 120 V circuit), any coil that was already sealed in will drop out — because holding voltage for most AC contactor coils is around 85–90 % of rated voltage.
The result is a cascade: energizing one contactor drops the voltage, which de-energizes another contactor, whose coil then tries to re-energize, pulling in more inrush, dropping the voltage further. The panel chatters, trips faults, or fails to start at all.
Sizing for inrush is the core skill in control transformer selection.
Sealed VA vs. Inrush VA
Before calculating a load, you need to understand what these two numbers mean and where to find them.
| Term | What it represents | Typical source |
|---|---|---|
| Sealed VA | The continuous volt-amp draw when the coil is fully energized and the armature has closed | Component datasheet or nameplate |
| Inrush VA | The peak volt-amp demand during the first few cycles of energization | Component datasheet (sometimes labeled "pickup VA" or "inrush VA") |
| Inrush ratio | Inrush VA ÷ Sealed VA — typically 6:1 to 10:1 for AC coils | Calculated from datasheet values |
DC coils (24 V DC contactors, solenoids) have a much lower inrush ratio, often 1.5:1 to 2:1, because inductance limits the current rise differently than with AC. If your control circuit is 24 V DC, inrush is much less of a concern — though the 24 V DC supply itself still needs to be sized correctly. See our guide on 24VDC power supply selection for that calculation.
AC coils (120 V AC contactors and relays) are the high-inrush devices that drive control transformer sizing.
Where to find the numbers:
- Check the manufacturer datasheet for the exact coil model. Eaton, Schneider Electric, Siemens, and ABB all publish sealed VA and inrush VA in the coil specification table.
- If you only have sealed VA, use a conservative inrush multiplier of 10× for general-purpose AC contactors sized up to NEMA Size 4, or 6× for small IEC contactors.
- Pilot lights (LED type): treat as resistive; VA = W rating.
- PLC power supply inputs (120 V AC to internal DC): check the datasheet for inrush. Many switching power supplies have high inrush; some have built-in inrush limiters. If uncertain, use 2× the nameplate VA as a conservative inrush figure.
How to Calculate the Control Transformer Load
Follow this four-step procedure:
Step 1: List every load on the secondary
Create a table with every device the transformer will power:
- All AC coils (contactors, control relays, solenoid valves)
- Pilot lights and indicating lamps
- PLC power supply (if powered from the control transformer secondary)
- Hour meters, timers with integral coils
- Audible alarms or buzzers
Step 2: Record sealed VA for each device
Pull the sealed VA from each component's datasheet. Sum all sealed VA values to get the total sealed VA for the circuit.
Step 3: Find the largest single inrush load
Identify the single device with the highest inrush VA. This is typically the largest contactor coil.
Inrush VA (largest device) = Sealed VA of that device × inrush ratio
You do not add the inrush of every coil simultaneously. In practice, inrush events are staggered — a PLC output card typically energizes one output per scan cycle. The worst-case assumption that governs transformer sizing is that one coil energizes while all others are already sealed in. This "one inrush at a time" model is the standard industry approach used by transformer manufacturers including Hammond, Acme, and Square D.
Step 4: Calculate the required transformer VA
Required VA = Total sealed VA of all other loads + Inrush VA of the largest single load
Then apply a safety margin of 20 % to account for load additions, cable losses, and transformer aging:
Design VA = Required VA × 1.20
Select the next standard VA rating at or above your Design VA.
Worked Example: Sizing a Transformer for a Multi-Motor Panel
This is the kind of electrical control panel design scenario that comes up constantly in machine building: a conveyor system with three motors, starter contactors, a PLC, and indicator lights.
Panel load list
| Device | Qty | Sealed VA each | Total sealed VA | Inrush VA each | Notes |
|---|---|---|---|---|---|
| NEMA Size 1 starter contactor (120 V coil) | 3 | 12 VA | 36 VA | 144 VA | 12× inrush ratio |
| Auxiliary control relay (120 V coil) | 4 | 5 VA | 20 VA | 30 VA | 6× inrush ratio |
| 120 V AC PLC power supply input | 1 | 30 VA | 30 VA | 60 VA | 2× conservative inrush |
| LED pilot lights (120 V) | 6 | 2 VA | 12 VA | 2 VA | Resistive — no significant inrush |
| TOTAL | — | — | 98 VA | — | — |
Identify the largest single inrush device
The NEMA Size 1 contactor has the highest inrush at 144 VA per coil.
Calculate required VA
All other loads run continuously at their sealed VA while one contactor pulls in:
- Sealed VA of all loads = 98 VA
- Subtract the sealed VA of the largest inrush device (it is transitioning to sealed during inrush): 98 − 12 = 86 VA
- Add its inrush VA: 86 + 144 = 230 VA
Apply 20 % margin
230 × 1.20 = 276 VA
Select standard VA rating
Standard control transformer ratings commonly available: 50, 75, 100, 150, 200, 250, 300, 350, 500 VA.
The next standard size at or above 276 VA is 300 VA.
Specify: 300 VA, 480 V primary / 120 V secondary control transformer.
A 250 VA unit would be technically undersized at 276 VA required. A 300 VA unit gives a working margin and room for modest future load additions.
Voltage Regulation, Voltage Dip, and Why Inrush Trips Coils
The math above tells you what VA to buy. Understanding why the voltage drops during inrush helps you troubleshoot when something still chatters.
Every transformer has internal impedance — typically expressed as a percentage (%Z), usually 1–5 % for small industrial control transformers. When a large inrush current flows, it creates a voltage drop across this impedance:
Voltage drop (%) ≈ (%Z) × (Inrush VA / Transformer VA)
For a 300 VA transformer with 3 % impedance during a 144 VA inrush event:
Voltage drop ≈ 3 % × (144 / 300) = 1.44 %
That's negligible — good, that's why the margin calculation above works. Now see what happens if someone installs a 100 VA transformer on that same panel (common mistake when spares are grabbed from the shelf):
Voltage drop ≈ 3 % × (144 / 100) = 4.32 %
A 4.3 % dip takes 120 V down to 114.9 V. Doesn't sound catastrophic, but add in the fact that the actual supply voltage may already be at the low end of tolerance (108 V on a nominal 120 V circuit = −10 %), and sealed coils that were holding at 108 V now see 103.3 V — right at the edge of the 85 % dropout threshold. The coils let go.
Transformer impedance also means the transformer cannot deliver its full rated VA indefinitely without regulation droop. Always size so that the continuous (sealed) load is no more than 80–85 % of rated VA to keep voltage regulation within spec. The 300 VA transformer in the example above has a sealed load of 98 VA — about 33 % of rating — with plenty of headroom.
Selecting the Right VA Rating: The Decision Rules
| Situation | Rule |
|---|---|
| All loads are resistive (LEDs, PLC inputs only) | Size to 120–125 % of total sealed VA |
| Mixed loads with AC coils | Use the inrush calculation above, then add 20 % margin |
| Large contactors (NEMA Size 3 or larger) | Consider inrush ratios up to 10–12×; verify datasheet |
| Future expansion planned | Add 25–50 % margin or go to the next two standard sizes up |
| High ambient temperature (>40 °C enclosure) | Derate transformer by 10–15 % or select a ventilated/open-style unit rated for higher ambient |
| Long secondary wiring runs (>15 m) | Account for conductor voltage drop; may need slightly higher secondary tap |
Never round down to save cost on a control transformer. The cost difference between a 250 VA and a 300 VA unit is typically under $20. The cost of a field service call to troubleshoot chatter-related faults is orders of magnitude higher.
Fusing and Protection for the Control Transformer
Proper fusing is required by NFPA 79 and NEC for industrial machinery. The UL 508A panel standard also specifies overcurrent protection requirements for control circuit transformers.
Primary-side fusing
Size the primary fuse at 125 % of the primary full-load current (FLC), per NEC 450.3(B) for transformers 1 kVA and under, or per the appropriate table row for larger units. Never exceed the maximum fuse size allowed by the table; an oversized primary fuse will not protect the transformer winding.
Primary FLC = Transformer VA / Primary voltage
For a 300 VA / 480 V unit: 300 / 480 = 0.625 A. Size the primary fuse at 0.625 × 1.25 = 0.78 A → use a 1 A fuse (next standard size up).
Secondary-side fusing
The secondary fuse protects the control wiring and devices. NFPA 79 requires the secondary circuit to be grounded on one conductor and fused on the ungrounded conductor. Size the secondary fuse at:
Secondary fuse = Transformer VA / Secondary voltage × 1.25 (or per NEC 450.3 table)
For 300 VA / 120 V: 300 / 120 = 2.5 A × 1.25 = 3.125 A → use a 3 A fuse.
In practice, many panel builders use a 3 A or 4 A secondary fuse for a 300 VA transformer and 120 V control circuit. Use the smallest fuse that does not nuisance-trip during normal inrush.
Fuse type
Use time-delay (slow-blow) fuses on both primary and secondary. Inrush will nuisance-trip fast-blow fuses even when the transformer is correctly sized. Class CC or Class J time-delay fuses are common choices for control circuit protection.
Common Sizing Mistakes to Avoid
- Using only nameplate VA without checking inrush — the most common error; always verify the datasheet.
- Forgetting the PLC power supply — switching power supplies have their own inrush and can be significant loads.
- Adding every device's inrush simultaneously — correct practice adds only the worst single inrush event on top of all sealed loads.
- Reusing undersized spares — "we have a 150 VA on the shelf" is not a sizing calculation.
- Ignoring temperature derating — a transformer in a warm enclosure without ventilation will run hotter and may not deliver its rated VA without accelerated insulation degradation.
- Omitting primary-side fusing — always fuse both primary and secondary; fusing only the secondary leaves the transformer winding unprotected.
For the full panel wiring context — including enclosure sizing, wire numbering, and component layout — see our electrical control panel design guide. For the motor control logic that runs on these circuits, see the motor start/stop ladder logic tutorial.
Frequently Asked Questions
How do you size a control transformer?
List all loads on the secondary and find their sealed VA from datasheets. Identify the single device with the highest inrush VA. Add the sealed VA of all loads, subtract the sealed VA of the largest inrush device, then add its inrush VA. Multiply the result by 1.20 for a 20 % margin and select the next standard VA rating above that figure.
What is the difference between sealed VA and inrush VA?
Sealed VA is the continuous volt-amp draw of a coil once it has fully energized and the armature has closed. Inrush VA is the brief surge — typically 6–10 times the sealed value — that an AC coil demands during the first few AC cycles when it is first energized. Sizing a transformer only to the sealed load ignores inrush and can lead to voltage dips that drop out other coils on the circuit.
Why does inrush matter for a control transformer?
When one coil draws high inrush current, it causes a voltage drop across the transformer's internal impedance. If the voltage drops below roughly 85 % of nominal, any coil already sealed in will drop out because its holding voltage threshold is exceeded. This causes chatter, false faults, and failed starts — even though each individual component is within its rated specifications.
What size control transformer do I need?
It depends on your specific load list. Run the four-step calculation: list loads, sum sealed VA, find highest single inrush VA, then add (total sealed VA − largest device sealed VA) + largest device inrush VA, multiply by 1.20, and select the next standard size up. Common panel sizes range from 100 VA for small PLC panels to 500 VA or larger for multi-motor systems.
Can I use a larger transformer than calculated?
Yes — going one standard size larger than required is generally fine and provides extra margin for future additions. Going too large (e.g., a 1 kVA transformer on a 100 VA load) is wasteful and may complicate fuse sizing, but it will not harm the control circuit.
Do DC coils require the same inrush analysis?
DC coils have a much lower inrush ratio (typically 1.5:1 to 2:1), so inrush is rarely the dominant factor when sizing a 24 V DC power supply. The governing factor for DC supplies is usually total continuous current draw plus startup sequencing. See the 24VDC power supply selection guide for that sizing approach.
