Does surface wiring have to be in conduit?

The question of whether surface wiring has to be in conduit is an important one for any electrical project. The National Electrical Code (NEC) provides guidelines on when conduit is required for surface wiring, but the specific requirements can vary depending on the type of occupancy, the specific location of the wiring, and other factors. Having a clear understanding of NEC conduit requirements can ensure your electrical system is code compliant and safe.

Quick Overview

In general, the NEC requires some type of protection for surface wiring in many (but not all) installations. Acceptable protection includes:

• Running the cables or wires inside a metal or nonmetallic conduit or tubing
• Covering the cables with metal armor or metallic sheathing
• Installing wires inside a surface metal raceway or wireway
• Using metal-clad cables that have an outer metal covering and armor

Some key exceptions to the conduit requirement include:

• Low-voltage wiring such as doorbell wires or thermostat cables in residential settings
• Temporary wiring for construction sites
• Cables protected from physical damage by the structure itself

So in many cases, surface cables and wires do need to be inside a conduit or other type of protection. However, rigid conduit is not necessarily required, and the specific type of raceway or armor needed depends on the details of each unique installation.

Where Does the NEC Require Conduit for Surface Wiring?

The NEC has several sections that spell out when a raceway or conduit is mandated for surface wiring methods. Some of the most common requirements are:

General Wiring

Article 300.37 requires protection for cables and conductors installed parallel to framing members and wall surfaces in accessible areas. Acceptable protection includes conduit, wireways, or other raceways. An exception is for cables that are not subject to physical damage.

Branch Circuits

Article 334.15(B) mandates that nonmetallic-sheathed cable (Romex) must be protected by conduit where installed on the surface of cinder block or other masonry walls or partitions. Conduit protection is also required where NM cable wiring passes through a foundation wall or under a concrete slab.

Places of Assembly

Article 518.4(A) requires that all wiring methods in places of public assembly be installed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, flexible metal conduit, liquidtight flexible metal conduit, metal wireways, rigid polyvinyl chloride conduit, or reinforced thermosetting resin conduit. This applies to venues like auditoriums, theaters, and stadiums.

Hazardous Locations

Articles 501, 502, and 503 have specific conduit requirements for wiring in areas with potential flammable or explosive vapors, gases, or combustible dusts. Rigid metal conduit or threaded steel intermediate metal conduit is often mandated.

Corrosive Environments

Article 300.6 restricts wiring methods in corrosive areas, like those with chemical fumes or moisture, to use approved corrosion-resistant raceways. Aluminum and ferrous metal conduits with proper protection are acceptable options.

When is Conduit Not Required for Surface Wiring?

While conduit is required in many surface wiring applications, the NEC does provide certain exceptions. Some key instances where conduit may not be mandated include:

Low-Voltage Wiring

Low-voltage wiring operating at less than 50 volts, like doorbell wires or thermostat cables, are not required to be in conduit when surface mounted in one-family dwellings (NEC Article 725.136). The cables must be properly rated and marked as suitable for external use.

Temporary Wiring

Cables for temporary electrical power and lighting during construction or emergencies do not require conduit when protected from damage or installed overhead (NEC Articles 590.4(J) and 305.6). Cables must be well supported and grounded.

Cables Secured Closely to the Surface

Armored cables, metal-clad cables, and nonmetallic-sheathed cables that run through holes in framing members and are clamped tightly against the structure do not need conduit (NEC 300.4(E)).

Wiring Inside Accessible Attics and Crawl Spaces

Nonmetallic-sheathed cables in attic and underfloor spaces with accessible framing do not require conduit as long as cables rise vertically through framing to distribution points (NEC 334.23).

Conduit for Exposed Outdoor Wiring

Outdoor electrical wiring is subject to damage from weather exposure, physical contact, and other hazards. As a result, the NEC requires effective protection:

Overhead Wiring

Overhead electrical service conductors typically must be installed in rigid metal conduit or intermediate metal conduit above the point of attachment at a roof or exterior wall (NEC 230.28 and 230.51).

Underground Wiring

Direct buried underground feeder and branch-circuit conductors that emerge from the ground must enter a raceway like Schedule 80 PVC conduit within 18 inches (NEC 300.5(E)).

Wiring Across Surfaces

Wiring run across the surface of roofs and sidewalls must be protected by electrical metallic tubing, intermediate metal conduit, rigid metal conduit, or a wireway (NEC 225.10 and 230.28).

Swimming Pools

All electrical wiring within 5 feet horizontally of a pool must be in rigid metal conduit, intermediate metal conduit or a nonmetallic raceway listed for pool use (NEC 680.21(A)(1)).

Conduit Fill Requirements

When conduit is used to enclose and protect surface wiring, the conduit must be sized appropriately for the number and type of wires inside per NEC guidelines:

Conduit Size

Trade size conduit dimensions are specified in NEC Chapter 9, Table 4. Allowable wires must not exceed conduit fill capacity.

Conductor Fill

Table 1 in NEC Chapter 9 lists maximum number of conductors of various sizes permitted in each conduit size based on wire type and insulation material.

Adjustment Factors

The maximum conduit fill allowed may be reduced based on factors like elevated ambient temperature, number of current carrying conductors, and ease of wire installation (NEC Annex C).

Derating Wire Ampacity

As the wires fill more of the allowable conduit capacity, the ampacity might need to be reduced to avoid overheating (NEC 310.15(B)(3)(a)).

Proper conduit sizing and fill for each run is critical to avoid code violations.

Conduit Installation Requirements

When conduit is used to protect surface cables and wires, the installation must meet NEC standards:

Securing and Supporting

Raceways must be securely fastened at intervals not exceeding 5 feet and within 3 feet of each outlet box, junction box, cabinet, or fitting (NEC 358.30 and 344.30).

Bends and Curves

Conduit bends and curves must not damage conductors or reduce the cable bending radius below the manufacturer specifications (NEC 344.28).

Box Connections

A bushing or adapter is required where wires enter a metallic enclosure like an outlet box, panel, or conduit body (NEC 300.4(G) and 312.5(C)).

Sealing and Drainage

Conduit exposed to moisture should slope to drain properly, with sealant applied to prevent water infiltration through connections (NEC 300.7).

Expansion Fittings

Raceways crossing building expansion joints require expansion and deflection fittings to allow conduit movement (NEC 300.7(B)).

Corrosion Protection

Ferrous metal conduit must have protective coatings or corrosion protection when in contact with concrete or exposed to moisture (NEC 300.6(A) and 345.10).

Types of Conduit for Surface Wiring

The NEC allows various conduit options for exposed electrical wiring:

Galvanized Rigid Metal Conduit (RMC)

Rigid steel conduit with zinc galvanized coating. Provides very good physical protection. Must use threaded fittings.

Intermediate Metal Conduit (IMC)

Steel conduit with zinc galvanizing or protective paint coating. Less rigid than RMC. Uses either threaded fittings or compression connections.

Electrical Metallic Tubing (EMT)

Thin-walled steel tubing with zinc, enamel, or plastic coating. Easy to install using compression fittings. Not suitable for wet or corrosive areas.

Nonmetallic Conduit

Includes Schedule 40 and Schedule 80 PVC, as well as fiberglass or liquidtight flexible conduit. PVC types must be UL listed for above ground use.

Flexible Metal Conduit (FMC)

Made from continuous flexible spiral steel. Primarily used for equipment connections in dry, protected indoor areas only.

Liquidtight Flexible Metal Conduit (LFMC)

Flexible galvanized steel core with liquidtight plastic jacket. Suitable for outdoor use and wet or damp locations when properly sealed.

Metal Raceways for Wire Protection

Raceways provide an alternative to conduit for shielding surface wiring:

Wireways

Sheet metal troughs with removable covers for housing and protecting cables. Available in surface-mounted or flush-mounted types.

Metal Surface Raceways

Flat or low-profile metal channels designed as a wiring method. Snap-on covers provide easy access for modifications. Often used for wall-mounted outlets and switches.

Cable Trays

Perforated or ventilated sheet metal ducts supporting cable runs. Used as an alternative to conduit for multiple large feeder circuits. Require metal covers in most locations.

Cellular Wireways

Metal enclosures made up of square or rectangular cells in a honeycomb structure. Filled cells provide protection while accessible cells allow cable changes.

Busways (Bus Ducts)

Prefabricated busbar power distribution systems enclosed in a metal housing. Allows tap-off connections to devices along the entire run.

Selection of Appropriate Conduit and Raceways

Choosing the right wiring method for surface installations involves several considerations:

Physical Protection

More rigid, thick-walled metal conduit provides greater durability and protection in high-traffic areas or where impact damage is possible.

Exposure to Moisture

PVC or liquidtight flexible conduit is better suited for wet or damp environments. EMT provides good moisture resistance outdoors when properly sealed.

Corrosive Conditions

Schedule 80 PVC or galvanized steel hold up better in corrosive settings. Stainless steel conduit may be required in highly acidic locations.

Heat Resistance

Rigid metal, IMC, or RTRC conduit provides greater heat tolerance compared to plastic types in very hot environments.

Ease of Installation

Thin-wall and flexible metal conduit involves faster, simpler installation. PVC and fiberglass are also easy wiring methods.

Expansion and Contraction

Use EMT with expansion fittings for long exposed conduit runs that may contract and expand with temperature changes.

Future Changes

Larger wireways, bus ducts, or cellular raceways simplify cable additions or modifications down the road.

Cost Comparison

Conduit costs vary based on material and conduit size:

Conduit Material Costs Per Linear Foot

Conduit Type	Average Cost Range
PVC Schedule 40	$0.40 – $1.50
PVC Schedule 80	$0.70 – $2.25
Rigid Metal Conduit	$2.50 – $6.70
EMT	$0.60 – $2.70
Flexible Metal Conduit	$1.30 – $2.90

Estimated Conduit Material Costs by Size

Conduit Size	PVC	RMC	EMT
1/2 Inch	$0.65	$4.50	$1.20
3/4 Inch	$0.85	$5.25	$1.60
1 Inch	$1.10	$6.15	$2.20
1-1/4 Inch	$1.65	$8.25	$2.75
1-1/2 Inch	$2.10	$9.60	$3.30

Labor costs, fittings, and conduit supports will also factor into the total installed cost. Larger conduit and more robust materials usually have higher costs.

Advantages of Conduit for Surface Wiring

Using conduit for exposed cables offers important benefits:

Physical Protection

Conduit shields wiring from damage from impact, moisture, dust, and other hazards. Rigid metal types provide the greatest durability.

Improved Safety

Conduit helps prevent exposed cables from being pinched, penetrated, or improperly modified, reducing shock and fire risks.

Visual Appeal

Straight conduit runs and consistent bends provide a neat, orderly appearance. Painted conduit can blend into surroundings.

Code Compliance

Conduit allows surface wiring installations to meet NEC requirements for many occupancy types.

Convenient Modifications

Conduit offers easy access to wiring to add new circuits or additional cables compared to concealed raceways.

Better Support

Conduit provides secure attachment points for improved cable support spacing.

Following code requirements helps ensure safe electrical systems. Conduit is well suited for surface runs where enclosed protection is mandated or recommended.

Conclusion

In summary, the NEC requires some form of conduit or raceway for many surface wiring installations as a critical safety measure. However, conduit is not necessary in all cases, as exceptions exist for some low-voltage wiring, temporary wiring, and cables closely secured to framing. Rules for specific occupancy types like public assembly areas must also be followed. Conduit selection factors include durability, moisture resistance, heat tolerance, and ease of installation. Costs range widely based on conduit material and size. Where conduit is used for exposed cables, proper installation and derating of conductors is key. Following the NEC standards helps guarantee proper protection for surface wiring. Discuss your particular project requirements with an experienced electrical contractor to determine where conduit may be required.