Use 18 AWG MTW for low-current internal control circuits when the governing requirements, protection and terminals permit it. Choose 16 AWG when current, voltage drop, mechanical handling or the project specification justifies the larger conductor. Neither gauge is a universal cabinet minimum. Select each circuit first, then decide whether standardizing several circuits on one gauge actually simplifies the build.
How the two gauges compare
The useful comparison is not a single ampacity number. Circuit classification, installation conditions and protective-device coordination determine whether either conductor is acceptable.
| Selection factor | 16 AWG MTW | 18 AWG MTW |
|---|---|---|
| Nominal copper cross-sectional area | About 0.823? No: 1.31 mm² | About 0.823 mm² |
| Approximate copper resistance at 68°F [20°C] | 4.02 ohms per 1,000 ft | 6.39 ohms per 1,000 ft |
| Typical reason to choose it | Lower voltage drop, higher current demand or a specified minimum | Low-current I/O and compact wiring where permitted |
| Terminal acceptance | Must fit the terminal's approved range and conductor construction | Must fit the terminal's approved range and conductor construction |
| Routing and bending | Usually needs more routing space for comparable insulation construction | Usually easier to route through dense terminal groups |
| Wire-duct occupancy | Often larger, but actual insulated diameter decides | Often smaller, but actual insulated diameter decides |
| Overcurrent protection | Must be selected under the applicable circuit requirements | Must be selected under the applicable circuit requirements |
| Compliance shortcut | Does not automatically satisfy every power-circuit requirement | Does not become acceptable merely because the load is small |
These resistance values are nominal copper references, not guaranteed specifications for a particular stranded wire. Use the selected wire manufacturer's data for final calculations.
MTW identifies a wire construction and intended use. It does not assign one allowable current to every installation. Check the actual insulation markings, temperature rating, voltage rating and environmental suitability.
A component or assembly label can impose a separate minimum conductor size. Treat that as a distinct requirement, explained in junction box minimum wire size and label requirements.
Where 16 AWG earns its space
Choose 16 AWG when the circuit needs its lower resistance or the approved design requires its conductor size. It contains about 59% more copper cross-sectional area than 18 AWG. For equal lengths at equal temperatures, its resistance is about 37% lower.
That difference matters on supply trunks, shared returns and circuits with limited voltage margin. A common conductor serving several simultaneously energized loads carries their combined return current. Do not size that conductor from the current of one input or one relay.
Consider a 24 VDC circuit with a 10 ft [3.05 m] outgoing conductor and an equally long return. At 2 A, the total copper path is 20 ft [6.10 m]. Using the reference resistances above:
- 18 AWG: 2 A × 20 ft × 6.39 ohms / 1,000 ft = approximately 0.256 V drop.
- 16 AWG: 2 A × 20 ft × 4.02 ohms / 1,000 ft = approximately 0.161 V drop.
Those drops are approximately 1.07% and 0.67% of a nominal 24 V supply. This calculation demonstrates voltage drop only; it does not establish an allowable current for either gauge.
Include terminals, protective devices and external cable in the complete voltage budget. Copper resistance also rises with temperature. Check the load's minimum operating voltage against the lowest expected supply voltage, not just its nominal value.
Inrush can be more demanding than steady operation. A solenoid or other inductive load may briefly draw enough current to cause unreliable pickup. Evaluate the manufacturer's starting-current information and minimum operating voltage before deciding that short wiring makes conductor resistance irrelevant.
A shop may also standardize on 16 AWG for selected circuit families. That can simplify wire schedules, provided every terminal accepts it. Standardization does not remove the need to identify circuits requiring larger conductors.
Where 18 AWG makes the layout easier
Choose 18 AWG when a permitted low-current circuit gains nothing useful from the larger conductor. Discrete inputs and low-current control connections are common candidates, subject to their equipment instructions and governing requirements.
The practical benefit is routing. Smaller insulated conductors can ease bends near terminal rows and reduce congestion at wire-duct fingers. They can also leave more room for identification sleeves and later maintenance.
Do not estimate duct occupancy from copper area alone. Insulation thickness, voltage rating and wire construction determine the finished outside diameter. Two different MTW products may have less diameter difference than their gauge numbers suggest.
For a round insulated wire, occupied cross-sectional area is calculated from its outside diameter:
Wire area = π × outside diameter² / 4
Use consistent units, then add the areas of all conductors in that duct segment. Include protective conductors, common returns and wiring that crosses between circuit groups. Apply the duct manufacturer's fill guidance and the requirements governing the panel.
The backplate and DIN rail layout guide covers duct planning and service access. Use it for the layout fundamentals rather than treating a smaller gauge as permission to crowd every routing channel.
A smaller conductor is not automatically the best fit for every connection. Check its stripping length, ferrule compatibility and approved termination method. Unsupported wiring also needs proper restraint; conductor gauge is not a substitute for strain relief.
Choose by circuit and cabinet environment
First establish which requirements govern the assembly. UL 508A, NFPA 79 and the NEC address different equipment and installation contexts. Apply the requirements relevant to the actual panel and machine, together with component instructions and project specifications.
Do not use a building-wiring ampacity table as the sole answer for internal control wiring. Equally, do not borrow an internal control-circuit allowance for unrelated field wiring or power circuits.
Classify the load before assigning a gauge
A 24 VDC supply does not automatically make every connected circuit power-limited or Class 2. Establish the source classification and the conditions under which the circuit qualifies.
Separate low-current signal branches from distribution conductors feeding several devices. Where several loads share one supply, map the branch protection and return paths. The article on one power input for multiple devices addresses that distribution arrangement.
A fan or heater remains a load requiring its own wiring assessment. Switching it through a thermostat or relay does not automatically establish its circuit classification or permit smaller conductors. Check its instructions, operating current, starting characteristics and protective arrangement.
Coordinate the protective device
Low normal current does not prove that a conductor is adequately protected during a fault. Check the applicable conductor-protection requirements against the upstream fuse, breaker or electronic protective device.
Record the device type, rating and operating characteristic. Confirm its suitability for the circuit voltage, AC or DC service, and available fault current. A current-limited power supply does not automatically establish compliant protection for every downstream wire.
Where a design relies on a specific exception or special circuit allowance, document that basis. Do not replace it with an undocumented shop rule such as a fixed current limit for each gauge.
Include heat and exposure
Check conductor temperature ratings against the actual cabinet conditions, including warm duct sections and nearby heat-producing equipment. Higher-temperature insulation does not override terminal temperature limits or other applicable restrictions.
A sealed cabinet's internal temperature may exceed room temperature substantially. Establish the operating conditions using control cabinet temperature limits, then apply the relevant wiring requirements.
Also identify oil, coolant or other exposure inside the cabinet. Verify the selected wire's markings and suitability instead of assuming every product marked MTW has identical additional approvals.
Verify the terminals before releasing the drawing
Terminal acceptance can decide the gauge even when current and voltage drop permit both choices. Check each connection against the component manufacturer's instructions.
Confirm all of the following:
- The accepted conductor-size range includes the selected gauge.
- The terminal accepts the conductor material, stranding and flexibility class.
- Ferrules are permitted where proposed, with the correct ferrule and crimping method.
- The connection accepts the intended number of conductors.
- Required strip length, tightening torque and temperature limits are documented.
A ferrule does not make an undersized conductor acceptable in a terminal outside its approved range. Likewise, two conductors fitting physically into one clamp does not establish that the connection permits them.
Show transitions explicitly when a distribution conductor is larger than its outgoing branches. Use suitable terminals or distribution components rather than trimming strands to force a fit.
The junction box terminal drawing requirements provide a useful basis for documenting terminal identities and conductor assignments. Apply that drawing discipline to cabinet terminal rows as well.
What to specify for the finished panel
Specify conductor size by circuit family, with exceptions identified individually. Avoid a blanket statement that every internal wire must be 16 or 18 AWG.
| Drawing or order item | What to record |
|---|---|
| Governing requirements | Applicable panel or machine standard, project specification and equipment instructions |
| Wire construction | Copper MTW, required markings, voltage rating and temperature rating |
| Circuit schedule | Function, voltage, operating current, starting current and conductor gauge |
| Protective arrangement | Source classification and upstream protective-device details |
| Voltage-drop basis | Complete outgoing and return length, current and minimum load voltage |
| Terminations | Terminal range, conductor construction, ferrules, conductor count and torque requirements |
| Routing | Wire outside diameters, duct selection, fill basis and spare routing allowance |
| Enclosure layout | Backplate arrangement, terminal access, cable entries, heat loads and required dimensions |
Choose the enclosure after laying out the components, terminal rows and ducts. Outside cabinet dimensions alone do not establish usable wiring space. Allow for door-mounted devices, internal hardware, cable bends and access to terminal screws.
Our wall-mount enclosures include a removable mounting panel for the internal assembly. For custom enclosures, specify the material, width, height, depth and each cutout against the completed layout.
The final decision is circuit-specific: use 18 AWG where permitted and sufficient, and 16 AWG where justified. If neither meets the circuit requirements, specify a larger conductor rather than forcing the design into a two-gauge choice.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Should all internal control wiring be 16 AWG?
Neither gauge is a universal requirement for internal control wiring. Use the applicable panel or machine requirements, equipment instructions, load current, protection and terminal acceptance. An 18 AWG conductor can suit a permitted low-current circuit, while another circuit may require 16 AWG or larger.
How much current can 18 AWG MTW carry inside a cabinet?
No single current limit applies to every 18 AWG MTW installation. The applicable requirements depend on circuit classification, installation conditions, wire and terminal ratings, and protective arrangements. Use the relevant rules for the actual assembly rather than treating a shop convention as a universal ampacity.
Does 18 AWG always save wire-duct space compared with 16 AWG?
Usually, but compare the actual insulated outside diameters. Copper cross-sectional area is not the same as occupied duct area. Calculate occupancy using the selected wire dimensions, then apply the duct manufacturer's guidance and the requirements governing the panel.
Can a thermostat-controlled enclosure heater use the same wire as PLC inputs?
No. A thermostat controls operation but does not by itself establish the heater circuit's classification or conductor requirements. Check the heater instructions, circuit voltage, load current, protective device and applicable assembly requirements before choosing either gauge.
