Control Transformer Sizing: VA Calculation for Control Panels

Why a Separate Control Transformer?

Industrial control panels typically receive 480 V (or 240 V) three-phase power for motor loads. The control circuit — contactors, relays, PLC I/O, indicators — operates at 120 V AC (or 24 V DC). A control transformer provides this voltage conversion and delivers two additional benefits:

1. Isolation — the secondary is isolated from the primary, protecting operators from phase-to-ground faults. A fault on the 120 V secondary does not automatically energise the enclosure.
2. Clean power — separates the sensitive control circuit from line disturbances on the motor power supply.

NFPA 79 Section 9.4 (Industrial Machinery) and NEC Article 430 require proper protection of control circuit transformers. UL 508A requires that control transformers in listed industrial panels comply with these requirements.

Load Types and VA Values

Every load connected to the control transformer secondary must be quantified in VA (volt-amperes). Sources for these values:

• Contactor and relay coils — manufacturer datasheet lists sealed (holding) VA and inrush (pickup) VA. If not listed, sealed VA is approximately: VA = A × V (coil rated current × coil voltage). Inrush is typically 6–10× sealed for AC coils; 1–2× for DC coils through a rectifier.
• Indicator lamps — LED: 1–3 W each; incandescent: 5–15 W each. (1 W ≈ 1 VA for resistive loads.)
• PLC power supply — datasheet lists AC input power (VA or W); use VA rating.
• 24 V DC power supply — if powered from 120 V AC, use the VA input rating from the datasheet.
• Timers, auxiliaries, other coils — from datasheets.

Common approximate sealed VA values for AC contactors (for budgeting only — always use actual datasheet values):

Inrush and Sealed VA

AC contactor coils draw a high momentary current when first energised — called inrush or pickup current. This inrush is caused by the low impedance of the coil before the armature closes the air gap. Once the armature closes, magnetic reluctance drops, inductance increases, and current falls to the much lower sealed (holding) value.

The transformer must supply this inrush current without its secondary voltage dropping below approximately 85% of nominal. Excessive voltage drop prevents the contactor from fully picking up, causing chattering or failure to latch.

The critical sizing rule:

Required transformer VA ≥
  (Sum of all sealed VA loads)
  + (Inrush VA of the largest simultaneously closing load
     − its sealed VA)

Or more conservatively:
  Required VA ≥ (Sum of sealed VA) × 1.25  [no inrush analysis needed]

The conservative 25% adder covers modest inrush but may be insufficient for large contactors. For panels with multiple large contactors, do the full inrush calculation.

Step-by-Step Sizing Procedure

1. List all 120 V AC loads — include every coil, lamp, PLC supply, timer, solenoid, and alarm horn connected to the control transformer secondary.
2. Find sealed VA and inrush VA for each load — from manufacturer datasheets. Record both values.
3. Total the sealed VA — this is the steady-state load.
4. Identify the worst-case simultaneous inrush scenario — which set of loads are energised at the same time? Typically, the first moment of machine start where the most contactors pick up simultaneously is the worst case.
5. Calculate the inrush contribution:
   Inrush adder = (inrush VA of simultaneously starting loads) − (sealed VA of those same loads)
6. Required transformer VA = Steady-state sealed VA total + Inrush adder
7. Add a contingency factor — typically 20–25% for future expansion and estimation uncertainty.
8. Select the next standard size above the calculated required VA.

Worked Example

A panel contains:

Inrush analysis — worst case: all three motor contactors and all four auxiliary relays energise simultaneously on machine start:

Simultaneous inrush VA  = (3 × 160) + (4 × 80) = 480 + 320 = 800 VA
Sealed VA of same loads = (3 × 20) + (4 × 10)  = 60 + 40   = 100 VA
Inrush adder            = 800 − 100 = 700 VA

Required transformer VA = 216 (total sealed) + 700 (inrush adder)
                        = 916 VA

With 25% contingency:   = 916 × 1.25 = 1145 VA

Result: Select a 1500 VA control transformer (next standard size above 1145 VA).

Note: If you used the simple rule (sealed VA × 1.25 = 216 × 1.25 = 270 VA) you would have under-sized significantly for this application due to large inrush from multiple simultaneously operating contactors.

Standard VA Sizes

Control transformers are available in standard VA ratings. Common sizes from major manufacturers (Hammond, Acme, Square D/Schneider, Eaton):

Always select the standard size at or above your calculated requirement. Slightly oversizing is preferred — control transformers run at lower temperature when lightly loaded and last longer.

Derating and Altitude

Transformers are rated at sea level (0–1000 m) and at a standard ambient temperature (usually 40°C continuous). Derating is required when these conditions are exceeded:

• Altitude above 1000 m — air is less dense, reducing cooling convection. Derate approximately 0.5% per 100 m above 1000 m. At 2000 m: multiply rated VA by 0.95.
• Ambient temperature above 40°C — for every 5°C above 40°C, derate by approximately 5–7%. A transformer in a panel where internal ambient reaches 55°C should be derated by ~15%.
• Harmonic-rich loads — switching power supplies (PLCs, VFD front-end, 24 V DC supplies) draw non-sinusoidal current with significant harmonics. K-factor rated transformers or derating of 15–20% is recommended when >50% of the load is switching supplies.

Primary and Secondary Protection

Primary Protection (NEC 450.3 / NFPA 79 Section 9.4)

The transformer primary must be protected against overcurrent. Per NEC 450.3(B) for transformers ≤1000 V:

• Primary fuse rated no more than 125% of primary full-load current (FLA) if there is no secondary overcurrent protection
• Primary fuse rated no more than 250% of primary FLA if secondary is also protected

Primary FLA calculation: FLA_primary = VA_rated / V_primary. For a 1500 VA, 480 V primary: FLA = 1500/480 = 3.1 A. At 250% = 7.75 A → select a 7 A or 8 A fuse.

Secondary Protection (NEC 430.72 / NFPA 79 Section 9.4)

NFPA 79 and NEC 430.72(C) require secondary protection for control circuit transformers. The secondary fuse must be sized to protect the secondary wiring:

• For transformers ≤2 kVA: fuse rated no more than 200% of secondary FLA
• For transformers 2–10 kVA: fuse rated no more than 167% of secondary FLA

Secondary FLA: FLA_secondary = VA_rated / V_secondary. For 1500 VA, 120 V secondary: FLA = 1500/120 = 12.5 A. At 200% = 25 A. Select a 15 A or 20 A secondary fuse to protect the 14 AWG or 12 AWG secondary wiring.

In practice, panel builders typically mount a dedicated fuse block on the secondary with dual fuses — one on each leg of the secondary (Line and Neutral from ungrounded systems, or L1 and L2 on 240 V centre-tapped systems). On a grounded neutral system, only the ungrounded conductor is fused.

Short-Circuit Current Rating (SCCR)

The control transformer and its protective devices must be included in the panel's Short Circuit Current Rating (SCCR) calculation per UL 508A and NFPA 79. The transformer's SCCR is typically found on its nameplate or datasheet. The panel's overall SCCR is the lowest SCCR of any component in the fault current path.

Secondary Grounding

NFPA 79 Section 9.4.3 and NEC 250.30 require the secondary of a control transformer to be grounded unless the system is ungrounded with ground fault detection. For the standard 120 V grounded system:

• One secondary terminal (X2, neutral) is connected to the equipment grounding conductor (PE/ground bar)
• Only the ungrounded conductor (X1, Line, 120 V) is switched by control devices and fused
• The grounded conductor (X2, Neutral) is not interrupted by switches or fuses (it goes directly to the ground bar)

This grounding creates a reference that limits the voltage-to-ground on the secondary circuit. A ground fault on the 120 V side then produces a measurable fault current that trips the overcurrent device — providing shock protection.

Frequently Asked Questions

How do I size a control transformer?

Add up the steady-state VA of all loads: coil VA of each relay/contactor (from datasheets), indicator lamp wattage, PLC power supply VA, and other 120 V AC loads. Identify the largest inrush load (typically the largest contactor coil, inrush = 6–10× sealed VA). Apply the rule: transformer VA ≥ (total steady-state VA) + (largest inrush VA × inrush factor − sealed VA of that load). Select the next standard VA size and verify the transformer can supply inrush without excessive voltage drop.

What is the difference between sealed VA and inrush VA for a contactor coil?

Sealed VA (also called holding VA) is the power drawn by the coil when the contactor is fully picked up and held closed — typically 5–15 VA for a small AC contactor. Inrush VA (also called pickup VA) is the momentary power drawn as the contactor first closes — typically 6–10× the sealed VA. Inrush is brief (20–100 ms) but the transformer must supply it without collapsing the secondary voltage below 85% of nominal.

What fuse size does a control transformer secondary need?

Per NEC 430.72(C) and NFPA 79 Section 9.4, the secondary of a control transformer must be protected by an overcurrent device (fuse or breaker) rated at no more than 2× the secondary full-load ampere rating (for transformers ≤2 kVA). For larger transformers, secondary protection is limited to 125–167% of secondary FLA. Many panel builders use a dedicated fuse block on the secondary with a 2 A or 3 A fuse for 150–500 VA transformers.

Can I use a 120 V control transformer on a 208 V supply?

Only if the transformer has a 208 V primary tap. Most industrial control transformers have multiple primary taps: 240 V, 480 V, and sometimes 208 V. Always match the primary tap to the supply voltage. If the transformer has only 240 V and 480 V taps, it cannot be used directly on 208 V without exceeding the primary tap voltage tolerance (typically ±5–10%). Using a 240 V tap on 208 V will reduce secondary output voltage proportionally (~104 V instead of 120 V).