A relay with a 24 VDC coil can separate the control circuit from its contacts without isolating the switched actuator from mains. If those contacts switch incoming 120 VAC, the actuator still uses that supply. An isolation transformer is needed only when the equipment instructions, circuit design, or applicable requirements call for a separately isolated supply. Decide that first. Then allocate enclosure space for the transformer, its losses, circuit protection, bonding connections, and field terminals.
What the relay actually isolates
There are two different isolation boundaries. One lies between the relay coil and its contacts. The other would lie between the incoming supply and the actuator power circuit.
A suitably rated electromechanical relay can provide the first boundary. A transformer with separate primary and secondary windings can provide the second, when designed for the required isolation.
| Circuit boundary | What provides separation | What it does not establish |
|---|---|---|
| 24 VDC control circuit to relay contacts | The relay's documented insulation system | Isolation between the actuator supply and incoming mains |
| Open relay contact | A switching gap in the controlled conductor | A suitable servicing disconnect or a safe actuator circuit |
| Incoming mains to transformer secondary | A suitably specified isolation transformer | Touch-safe secondary voltage or removal of machinery hazards |
| Electrical circuit to enclosure metal | Insulation and protective bonding serve different functions | Permission to leave exposed conductive parts unbonded |
“Dry contact” describes a contact that does not supply its own voltage. Once connected to 120 VAC, its terminals and connected wiring must be treated accordingly.
For a grounded 120 VAC supply, normal control switching generally interrupts the ungrounded conductor. Opening that contact does not disconnect every actuator connection. Do not treat the operating relay as the means of isolation for maintenance.
The separate question of physically fitting these components is covered in putting a 120 VAC power supply and relay inside a small enclosure. Adding a transformer changes the circuit architecture, not just the component count.
When a transformer is justified
Start with the actuator manufacturer's installation instructions. Establish whether it accepts the available mains supply directly or requires a particular control-power arrangement.
A transformer may be justified when:
- The equipment instructions require an isolated supply with specified characteristics.
- The machine electrical design requires a separate control-power circuit.
- The available supply voltage does not match the actuator voltage.
- A documented system requirement calls for galvanic separation between supply circuits.
- A specific electrical-noise problem requires a transformer with documented shielding or attenuation performance.
Do not add a transformer merely because an actuator is outside the enclosure. External wiring needs suitable cable, protection, terminals, environmental sealing, and protective bonding where required. Physical distance alone does not establish an isolation requirement.
Likewise, an unknown actuator is not a reason to assume a floating supply is safer. It is a reason to obtain the actuator's electrical requirements before completing the design.
A 120-to-120 VAC isolation transformer still supplies hazardous voltage. It does not make exposed terminals safe to touch. Its isolation also does not protect against contact with both secondary conductors.
For a timed ejector, address unexpected motion separately. A relay contact can weld closed, and a control command can fail. An isolation transformer does not correct either fault. Where motion creates a hazard, the risk assessment must establish the required safety functions and stopping arrangement.
What component ratings prove and leave unproven
A relay dielectric test is not permission to apply any voltage across any terminals. Check the documented working voltage, insulation classification, terminal arrangement, and installation conditions.
The coil-to-contact rating may differ from the rating between adjacent contacts or separate poles. Accessories and sockets can introduce their own spacing and voltage limits. Use the complete relay-and-socket combination within its documented ratings.
The 24 VDC power supply also needs suitable isolation for the intended control circuit. Relay isolation does not compensate for an unsuitable power supply or wiring that bridges the separation boundary.
For the transformer, verify that its documentation establishes the required primary-to-secondary separation. A 1:1 voltage ratio alone does not prove this. An autotransformer shares a winding and does not provide galvanic isolation.
Neither component establishes compliance for the completed assembly. The final design still needs suitable wiring, fault protection, clearances, enclosure construction, and equipment approval where required. Standards such as NFPA 79 and UL 508A may apply to the completed equipment, depending on its scope and installation.
An enclosure rating addresses another boundary: protection against the surrounding environment. Our wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed. That rating does not establish the safety or approval status of the installed electrical assembly.
Size the transformer and protection around the actuator
Obtain the actuator's voltage, frequency, running current, inrush current, duty cycle, and permitted voltage range. Also record how often it operates and whether other loads can energize simultaneously.
For a single-phase load, apparent power is voltage multiplied by RMS current. A hypothetical actuator drawing 0.5 A at 120 VAC presents a running load of 60 VA. If its documented pickup current is 2 A, the corresponding pickup demand is 240 VA.
That does not automatically require a 240 VA transformer. The selection depends on pickup duration, transformer regulation, permitted voltage drop, and repetitive duty. Use the transformer's documented load-selection method rather than sizing from running current alone.
Check the relay contacts against the actual load category. A resistive current rating is not automatically suitable for an AC solenoid. Inductive interruption and repeated pickup can shorten contact life or cause welding.
Use actuator-approved suppression where needed. Any suppression network must suit the AC circuit and the required release time. Leakage through suppression components can also affect the apparent off state.
Coordinate protection for the complete circuit:
- Protect the transformer primary according to its instructions and applicable installation requirements.
- Establish the required secondary protection rather than assuming primary protection covers every secondary fault.
- Protect field conductors and connected equipment within their ratings.
- Select protective-device characteristics that accommodate legitimate inrush without compromising fault protection.
- Provide the required disconnecting arrangement and identify every supply entering the enclosure.
Do not select protective-device ratings without the transformer, actuator, conductor, and installation data. These are coordinated decisions, not enclosure accessories chosen afterward.
Keep protective bonding separate from supply isolation
An isolated secondary is not an instruction to disconnect protective earth. Maintain the required bonding of the enclosure, mounting panel, exposed conductive parts, and actuator metalwork.
Whether a secondary conductor must be grounded depends on the circuit and applicable requirements. A grounded secondary and an intentionally ungrounded secondary need different protection and identification arrangements. Do not improvise this choice by adding or removing a neutral-to-earth connection.
The enclosure grounding and bonding guide explains the protective fault-current path. A grounding electrode is not a substitute for the equipment grounding conductor.
Bring the external actuator wiring to identified field terminals. Specify terminal voltage and current ratings, conductor sizes, protective-earth connections, and any required barriers. Allow access for tightening, inspection, and replacement without disturbing adjacent circuits.
Keep 120 VAC terminals clearly separated from 24 VDC control wiring using the equipment's required spacings and wiring arrangements. Label the source of the switched voltage. If an external source can energize relay contacts, identify that additional supply and its disconnecting means.
Define cable-entry locations and sealing requirements before releasing the enclosure drawing. The cutouts, glands, and HMI windows guide covers the mechanical details that affect the finished entry.
Allow for transformer weight, heat, and wiring space
Transformer selection comes before enclosure sizing. Its footprint alone does not establish the space needed around it.
Include mounting hardware, terminal covers, manufacturer-required clearances, conductor bends, protective devices, and the field terminal strip. Keep the transformer off the door unless the door and mounting arrangement are designed for that load.
For a catalog starting point, the 16 x 12 x 8 in 304 stainless wall-mount enclosure has those outside dimensions in H x W x D order. Its nominal CAD inside dimensions are 15.88 x 11.88 x 7.88 in. It includes a removable mounting panel and wall-mount feet.
Those inside dimensions are not a guaranteed usable mounting envelope. Subtract panel stand-off, hardware, door interference, and wiring clearances. Confirm the selected transformer and populated panel against the drawing before ordering. Use the electrical enclosure sizing guide for the underlying layout method.
Transformer VA is not transformer heat loss. At a defined operating point, a measured input of 90 W and output of 80 W means 10 W becomes heat. Use manufacturer loss data or suitable measurements, including the energized no-load condition.
Count power-supply losses, the internal relay coil, and other internal dissipation separately. The external actuator's full power consumption does not become enclosure heat merely because its wiring passes through the box.
The transformer heat loss at partial load article explains why low actuator duty does not eliminate transformer heating. Check the worst combination of ambient temperature, energized time, and simultaneous loads.
Keep temperature-sensitive electronics out of the transformer's rising warm-air path. Follow documented mounting orientation and clearance requirements. The component layout inside a sealed enclosure article covers that placement decision.
What to specify before releasing the enclosure order
Send the electrical design basis with the mechanical layout. A note saying “isolated relay output” is ambiguous and should not reach the final drawing.
State whether the required boundary is coil-to-contact isolation, primary-to-secondary isolation, or both. Identify the incoming supply and the actuator's exact electrical requirements.
Include these details in the drawing package:
- Transformer model, ratings, loss data, mass, mounting pattern, and required clearances, if a transformer is needed.
- Relay and socket ratings for the actuator load, including the suppression arrangement.
- Primary and secondary protection, disconnecting provisions, and secondary grounding design.
- Field terminal schedule, conductor sizes, protective-earth terminals, and cable-entry positions.
- Backplate layout, service access, thermal design conditions, and required enclosure dimensions.
- Environmental rating, material, and any project requirement for third-party evaluation.
If the selected equipment cannot fit with its required clearances, change the enclosure dimensions rather than compressing the layout. We build custom enclosures with the specified material, width, height, depth, and cutouts, from one piece.
The order should describe a resolved circuit and a verified layout. That prevents an unnecessary transformer from consuming space and heat allowance, while ensuring a required transformer is accommodated properly.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a 24 VDC relay coil isolate the actuator from 120 VAC mains?
No. A suitably rated relay can isolate its coil circuit from its contact circuit. If the contacts switch incoming 120 VAC, the actuator remains connected to that supply when energized. Supply isolation requires a separate, appropriately specified isolation device, usually a transformer when the design calls for one.
Does a 120-to-120 VAC isolation transformer make the output touch-safe?
No. Separate windings provide galvanic separation, but 120 VAC remains hazardous. Contact with both secondary conductors can cause electric shock. Secondary grounding, protection, insulation, disconnecting means, and protective bonding still need to follow the equipment design and applicable installation requirements.
Should the transformer secondary always be left floating?
Not as a general rule. The grounding arrangement depends on the circuit purpose, equipment instructions, and applicable requirements. An intentionally ungrounded circuit needs an appropriate protection strategy. Never remove protective-earth bonding from the enclosure or actuator to create a floating supply.
How much enclosure space should be reserved for an isolation transformer?
Use the selected transformer's dimensions, mass, terminal arrangement, required clearances, and loss data. Then add protection, field terminals, conductor bends, and service access. There is no reliable enclosure allowance based only on VA rating, and nominal inside dimensions do not equal usable mounting space.
