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Control Transformer sizing for contactor actuation

Updated: Sep 02, 2026 | 6 min read

Technical Articles

Published: Jul 14, 2026 by Sandeep Pawar

Technical Articles

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Introduction

A contactor is an electromagnetic device consisting of a coil and magnet system along with fixed and moving contacts. When the coil is energized, it produces a magnetic field thereby attracting the moving magnet. This causes the fixed and moving contacts to connect and the contactor is said to be actuated. The energization of contactor coil is usually done through a control transformer.

This is mainly done because voltage requirements vary with control systems and with an intermediary control transformer the desired voltage can be obtained.

When a contactor coil is energized, it draws in a high inrush current momentarily. Apart from contactor coils, Relays and solenoids are some other devices which draw inrush current when energized. The control transformer selected must be able to accommodate this momentary high inrush current for a satisfactory operation.

Selection of a control transformer

For a proper selection of control transformer, three parameters of the load circuit must be determined in addition to the minimum voltage required to operate the circuit. These are Hold on VA, Pick-Up VA, and Inrush load power factor.

Hold-On VA: Hold-On VA is the product of load voltage (V) multiplied by the current that is required to operate the circuit after initial start up or under normal operating conditions. It is calculated by adding the hold-on VA requirements of all the electrical devices of the circuit that will be energized at any given time. Hold-On VA is also sometimes referred as steady state VA.
Pick-Up VA: Pick-Up VA is the product of load voltage (V) multiplied by the current (A) that is required during start up. It is calculated by adding the pick-up VA requirements of all devices (contactors, timers, relays, solenoids, etc) which will be energized together. Energization of electromagnetic devices takes 20-50 milliseconds.
Inrush Load power factor: Inrush load power factor is difficult to determine without a detailed vector analysis of all the load components. Generally such analysis is not feasible; hence a safe assumption would be 40% power factor. Until recently 20% power factor was commonly used for transformer calculations; however tests conducted on major brands of control devices indicate that 40% power factor is a same assumption.

It is recommended that a control transformer be sized at 40% power factor. Some electromagnetic devices typically operate at that level due to their inherently low power factor. Selecting a control transformer at 40% power factor will be more than the adequate size for all the various loads in the circuit.

Besides the above parameters there are two parameters of primary and secondary voltage. Primary voltage is the voltage available from electrical distribution system which is connected to the transformer supply terminals. Secondary voltage is the voltage required for load operation which is connected to the transformer load voltage terminals.

Steps for selection of control transformer

Determine the supply and load voltages as per requirement. The supply voltage is the voltage available to control transformer and load voltage is the operating voltage of all the devices connected to the Transformer output.
Determine the hold-on and pick-up VA of each coil in the control circuit. This data is provided by the product manufacturer in the datasheet.
Calculate the hold-on VA by adding the VA requirements of all the equipment that will be energized together(timers, contactors, relays, solenoids, pilot lamps etc).
Calculate the Pick-Up VA of all the coils that will be energized together. Be sure to include the hold-on VA of components that don't have inrush (lamps, timers) as they present load to the transformer during maximum inrush.
Calculate the application Inrush VA by using the following industry accepted formula.
Based on the value of application Inrush VA obtained, use regulation chart for selecting the control transformer rating.

Continuous VA Transformer Name Plate Rating (A)	Inrush VA @ 40% Power Factor
Continuous VA Transformer Name Plate Rating (A)	85% Secondary Voltage	85% Secondary Voltage	85% Secondary Voltage
25	160	130	95
50	270	210	160
75	435	365	255
100	635	520	370
150	1300	1010	700
200	1975	1500	1020
250	2680	2030	1340
350	3665	2820	1895
500	6300	5035	3305
750	10555	7920	5050
1000	15225	11160	6000

Continuous VA Transformer Name Plate Rating (A)	Inrush VA @ 40% Power Factor
Continuous VA Transformer Name Plate Rating (A)	85% Secondary Voltage	85% Secondary Voltage	85% Secondary Voltage
25	160	130	95
50	270	210	160
75	435	365	255
100	635	520	370
150	1300	1010	700
200	1975	1500	1020
250	2680	2030	1340
350	3665	2820	1895
500	6300	5035	3305
750	10555	7920	5050
1000	15225	11160	6000

Using regulation chart to select the transformer rating

The above regulation chart gives the continuous rating of the control transformer and the corresponding Inrush VA at different secondary voltage levels. This secondary voltage value depends on internal losses in The transformer.

After calculating the application inrush VA as discussed above, determine the secondary voltage level of the transformer. Column B indicates that during inrush, 90% of the rated voltage would be available at the transformer secondary, which is an acceptable drop in rated voltage. Once this is determined, read down the column until you arrive at a value which is more than the application inrush VA calculated. Corresponding to this value, the value in column A would be the nameplate rating of the control transformer.

As a final check, make sure that the transformer VA rating is equal to or greater than the total circuit Hold-On Requirements.

Let us further understand this with the help of an example,

Pick Up VA = 550 VA

Hold On VA = 36 VA

Now from the above table we consider the secondary voltage delivered by the transformer as 90% of the nameplate secondary voltage under maximum inrush conditions at rated input voltage.

In column B, under 90% secondary voltage, we have to select a value more than 552 VA. The nearest value greater than 552 VA is 1010 VA. Corresponding to the value the control transformer nameplate rating is 1 50 VA in column A.

Hence rating of the control transformer for energizing MNX 110 is 150 VA

Let us now consider MO 110 contactor,

Pick-up VA = 240 VA

Hold-on VA = 25 VA

Now from the above table we consider the secondary voltage delivered by the transformer as 90% of the nameplate secondary voltage under maximum inrush conditions at rated input voltage.

In column B, under 90% secondary voltage, we have to select a value more than 242 VA. The nearest value greater than 242 VA is 365 VA. Corresponding to this value the control transformer nameplate rating is 75 VA in column A.

Hence rating of the control transformer for energizing MO 110 is 75 VA

Summarizing for an 110A AC3 contactor the comparison is as below,

Contactor	MNX 110	MO 110
Pick-Up VA	550	240
Hold-On VA	36	25
Application Inrush	550	241.2
Size of Control Transformer	150 VA	75 VA

Contactor	MNX 110	MO 110
Pick-Up VA	550	240
Hold-On VA	36	25
Application Inrush	550	241.2
Size of Control Transformer	150 VA	75 VA

It can be seen that with MO contactors, there is a significant reduction in control transformer size.

This will in turn result in cost savings for the user.

The above method assumes that all contactors are picked up at the same time.

There is also an alternative, more accurate way for sizing the control transformer. This method is more application specific and depends on the exact number of components that are actuated at a given point of time.

Let us consider a general example of a system having five Star-Delta motor feeders each of 50 HP motors. A typical Star-Delta feeder would consist of a Star contactor, Main contractor and a delta contactor. It would also have two auxiliary contactors, one for start interlocking and one for emergency stop.

We will now compute the pick-up VA requirements at different instants of time and would select the control transformer corresponding to the highest Pick-up VA requirement at any given time instant.

Case 1: Considering MNX contactors

For a 120 HP Star delta feeder as per Fuse based type 2 charts, selection would be,

Star Contactor: MNX 80
Main/Delta Contactor: MNX 95
Auxiliary contactor: MX0 (One for start interlocking and one for emergency stop)

Since there are 5 feeders total contactors are,

MNX 80: 5 Nos

MNX 95: 10 Nos

MXO: 10 Nos (2 in each feeder)

The Pick-up and Hold-on VA of individual contactors is as given below,

Contactor	Pick-Up VA	Hold-On VA
MNX 80	190	21
MNX 95	550	36
MNX 0	26	4.5

Contactor	Pick-Up VA	Hold-On VA
MNX 80	190	21
MNX 95	550	36
MNX 0	26	4.5

At t = 0 (At Panel Power On)

The emergency contactors in all five feeders will pick up.

Total Pick-up VA = 26 x 5 VA

= 130 VA

At t = 1 (When Start command is given)

The star contactor, main contactor and start interlocking auxiliary contactor will pick-up

Total Pick-up VA = (190 x 5) + (550 x 5) + (26 x 5)

= 3830 VA

At t = 2 (At Start to Delta Changeover)

The Star contactor will drop off and the delta contactor will pick-up

Total Pick-up VA = 550 x 5

= 2750 VA

At t = 0 except star contactor all contactors will be picked up.

Hence Hold on VA would be maximum at t=2

Total Hold-on VA = (36 x 10) + (4.5 x 10)

= 405 VA

Maximum Pick-up VA requirement = 3830 VA

Maximum Hold-on VA requirement = 405 VA

Assuming secondary voltage to be 90% of the rated value,

The control transformer rating to be selected is 500 VA

Had we assumed that all the contactors pick-up at the same time the control transformer selection would have been computed as given alongside.

Pick-Up VA = 190 x 5 + 550 x 10 + 26 x 10

= 6710 VA

Hold-on VA = 21 x 5 + 36 x 10 + 4.5 x 10

= 510 VA

Hence the control transformer rating would be 750 VA, which is much more than the earlier calculated rating. So the earlier method which takes into account the application gives a more accurate control transformer sizing

Case 2: Considering MO contactors

For a 120 HP Star delta feeder as per Fuse based type 2 charts, selection would be,

Star Contactor: MO 80
Main/Delta Contactor: MO 95
Auxiliary contactor: MX0 (One for start interlocking and one for emergency stop)

Since there are 5 feeders total contactors are,

MO 80: 5 Nos

MO 95: 10 Nos

Mx0: 10 Nos (2 in each feeder)

The Pick-up and Hold-on VA of individual contactors is as given below,

Contactor	Pick-Up VA	Hold-On VA
MNX 80	240	25
MNX 95	240	25
MNX0	26	4.5

Contactor	Pick-Up VA	Hold-On VA
MNX 80	240	25
MNX 95	240	25
MNX0	26	4.5

At t = 0 (At Panel Power On)

The emergency contactors in all five feeders will pick up.

Total Pick-up VA = 26 x 5 VA

= 130 VA

At t = 1 (When Start command is given)

The star contactor, main contactor and start interlocking auxiliary contactor will pick-up

Total Pick-up VA = (240 x 5) + (240 x 5) + (26 x 5)

= 2530 VA

At t = 2 (At Start to Delta Changeover)

The Star contactor will drop off and the main contactor will pick-up

Total Pick-up VA = 240 x 5

= 1200 VA

At t = 0 except star contactor all contactors will be picked up.

Hence Hold on VA would be maximum at t=2

Total Hold-on VA = (25 x 10) + (4.5 x 10)

= 295 VA

Maximum Pick-up VA requirement = 2530 VA

Maximum Hold-on VA requirement = 295 VA

Assuming secondary voltage to be 90% of the rated value,

The control transformer rating to be selected is 350 VA

Had we assumed that all the contactors pick up at the same time the control transformer selection would have been computed as below,

Pick-Up VA = 240 x 5 + 26 x 10 + 26 x 10

= 3860 VA

Hold-on VA = 25 x 15 + 4.5 x 10

= 420 VA

Hence the control transformer rating would be 500 VA, which is much more than the earlier calculated rating. So the earlier method which takes into account the application gives a more accurate control transformer sizing

From the above computation it is clear that control transformer size with MO contactors is much lower than that obtained by using MNX contactors. This greatly reduces the cost of the control transformer providing direct benefit to the user. Thus MO turns out to be an economical solution over MNX with regards to control transformer sizing for the end user.

Annexure

Below is the table for control transformer ratings for MNX & MO range of contactors (single contactor) at 90% secondary voltage.

MNX Power Contactors

MNX Frame Wise	Pick-up VA (Single Coil)	Hold-on VA (Single Coil)	Application Inrush VA (Single Coil)	Transformer VA rating (For single Coil)
MNX 9-22	68	11	69	25
MNX 25-40	68	11	69	25
MNX 50-80	190	21	191	50
MNX 95-140	550	36	551	150
MNX 185-225	960	56	962	150
MNX 300-400	2100	95	2102	350
MNX 550-650	1000	25	1000	150

MNX Frame Wise	Pick-up VA (Single Coil)	Hold-on VA (Single Coil)	Application Inrush VA (Single Coil)	Transformer VA rating (For single Coil)
MNX 9-22	68	11	69	25
MNX 25-40	68	11	69	25
MNX 50-80	190	21	191	50
MNX 95-140	550	36	551	150
MNX 185-225	960	56	962	150
MNX 300-400	2100	95	2102	350
MNX 550-650	1000	25	1000	150

MO Power Contactors

MO Frame Wise	Pick-up VA (Single Coil)	Hold-on VA (Single Coil)	Application Inrush VA (Single Coil)	Transformer VA rating (For single Coil)
MO 9-45	77	9	77.5	25
MO 50-70	144	15	144.8	50
MO 80-110	240	25	241.3	75
MNX 140-225	1000	50	1001.25	150
MNX 250-300	1400	65	1401.5	200

MO Frame Wise	Pick-up VA (Single Coil)	Hold-on VA (Single Coil)	Application Inrush VA (Single Coil)	Transformer VA rating (For single Coil)
MO 9-45	77	9	77.5	25
MO 50-70	144	15	144.8	50
MO 80-110	240	25	241.3	75
MNX 140-225	1000	50	1001.25	150
MNX 250-300	1400	65	1401.5	200

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About the Author

Sandeep Pawar,

General Manager - Product Management

Electrical Engineer with over 20 years of experience in Product Management and Application Engineering for Protection Relays, Power Quality Solutions, and Intelligent Motor Control Centres (MCCs). Possesses strong expertise in LV and MV power system protection schemes, motor protection, monitoring, and control applications. Well-versed in both conventional and intelligent motor management systems, with extensive knowledge of protection, control, and condition monitoring solutions.