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Extension Cords – Part I

Extension Cords – Part I

By Mike Orzechowski

Extension cords, also referred to as portable cords, are used for temporary power connections requiring a flexible cord. They are not intended to be used as a substitute for fixed wiring of a structure, however they may (in some circumstances) be used as a temporary wiring solution—The Occupational Safety & Health Administration (OSHA) defines temporary as less than 90 days. The National Electric Code (NEC) Article 400 addresses flexible cords as a separate category. The code does not consider extension cords to be a wiring method. The key factor for the NEC in using a temporary flexible cord is the need to move the equipment i.e. temporary use with portable equipment.

Extension cords must be properly sized and rated for the electrical load and must be suitable for the conditions of use and location. The United States Department of Labor, OSHA defines acceptable extension cords for use in construction through OSHA 29CFR 1926 Subpart K – Electrical.

These extension cords are constructed of multiple strands of wire in each conductor in the cord to permit flexibility. The diameter of the strands and the number of strands that comprise each conductor will define the tightest bending radius of each cord before damage to the conductors internal to the cord outer jacket occurs. The overall size of the conductors (American Wire Gauge, AWG), will define the electrical load rating for the cord (Ampacity) and based on the resistance voltage drop on the cord will determine the maximum acceptable length for the cord for the intended use.

The AWG (also referred to as the Brown & Sharpe wire Gauge) has been used in the United States since the mid-1800s. The dimensions of the wire are called out in ASTM B258-02. The AWG was originally based on a number of steps to draw the wire to a specific diameter so the larger the number AWG is the smaller the diameter of the conductor. Stranded cables have the equivalent current carrying capacity as a solid wire but might be physically bigger in overall diameter of the conductor due to the airspace between the individual strands of wires. The AWG is typically followed by the number of conductors in the cord, for example a 10AWG (also written #10 or No.10) extension cord for 120 volts having three conductors (hot, neutral and ground) be called out as 10/3. Extension cords in the concrete cutting industry are limited to 18AWG to 2AWG.

Plugs and receptacles are defined by NEMA, the US National Electrical Manufacturers Association, such that the mating parts are independent of any proprietary manufacturer and have a universal fit. There are also special connectors used on specific equipment that may have unique or proprietary pin configurations that are designed into the product for a specific reason, but most follow the NEMA standards. For extension cords in the concrete cutting industry contractors deal with two basic categories: straight blades and Locking. The mating fits on the plugs and receptacles are strategically placed to match the voltage and current ratings on the plug. Some plugs and receptacles have a waterproof cord grip or strain relief to seal the electrical connections (the ultimate seal is molded plugs and receptacles on the cords), and some do not have any waterproof or water resistant seals. Depending on the application of the cord, the sealing of the cord may be important. All the extension cords meeting the OSHA requirements for construction will have a clamping arrangement so that forces pulling on the cord will not strain the actual terminal connection of the wire.

One of the main defining characteristics of OSHA-approved extension cords is in the outer jacket. OSHA references the NEC and calls out hard or extra-hard rated cords. The S is the extra-hard service grade and the SJ is hard service. The difference between the S and SJ can be significant. The SJ (also referred to as junior) has a thinner outer thermoset jacket than the S, so it is limited to 300 volts maximum but it is significantly lighter weight to carry. The S cord has a thick outer rubber thermoset insulation jacket and conductors that is rated up to 600 volts (important for 480-volt equipment) and it is very heavy. The S cord also offers much more abrasion resistance and cord durability.

  • If the S or SJ is followed by an O then the outer jacket is oil resistant. If it is a double-O (OO), then both the outer jacket and the insulation on the individual conductors are oil resistant.
  • The SJT or ST is basically the same ratings as the SJ and S respectively. The T denotes a thermoplastic insulated conductors and outer jacket.
  • The W (like SOOW) at the end would signify that the cord is rated for outdoor use with W for wet or water but it also includes sunlight UV protection.
  • Remember per OSHA, the extension cords are not a substitute for fixed wiring of a structure. They are not to be run through holes in walls, ceilings or floors, doors, windows or similar openings. They are not to be concealed behind building walls, ceilings or floors.
  • Extension cords are not under rugs or carpet because (among other things) it can lead to overheating or can conceal damage.
  • Extension cords are not attached to building surfaces. Do not staple or hang from nails or suspend by wire. Attaching them to the building can cause damage to both the inside cords and the outer jacket.

In addition to the OSHA 29CFR 1929 Subpart K standards, OSHA publishes clarifications in the form of letters from time to time and posts them at

Double Insulated Handtools (Using Two-Prong Tools with a Three-Prong Extension Cord)
A common misconception is that if the tool does not have a three-conductor plug then it will not be approved for use on a construction site, as per OSHA rules. If the tool is double Insulated, properly rated and marked as such, the double insulated tool does not require a grounding wire.

If a tool has a ground wire, the ground must remain intact i.e. one must not remove the ground prong on a three-prong plug so it can plug into a two-prong outlet. This can cause a safety issue, because if the tool is not properly insulated or grounded the operator may be at risk of electric shock or electrocution.

Double Insulated type tools are designed with a second level of insulation inside the tool to reduce the risk of contact on an energized outer case of the tool if an electric short circuit or wire defect/damage happens inside the inner workings of the tool. The Double Insulated tools will be marked on the handle or data label with the words ‘Double Insulated’ or marked with a graphic symbol of a square box inside another square box.

It is important to maintain the tools to be certain the insulation of the outer case of double insulated type tools is not compromised.

Regulatory and Controlling Authorities

NEMA defines the plugs and receptacles to maintain consistency regardless of the manufacturer. The mating fits on the plugs and receptacles are strategically placed to match the voltage and current ratings on the plug. For example, a typical household 120-volt u-ground plug rated at 15 amps can plug into a 20-amp receptacle but a 20-amp plug cannot plug into a 15-amp receptacle.

National Recognized Testing Laboratory (NRTL), such as Underwriters Laboratories (UL), MET Laboratories, Inc. (MET), Intertek Group (Formerly ETL), Canadian Standards Association (CSA), or FM Approvals (FM). Over 50 volts must be approved by one of these laboratories. All extension cords used in construction should bear the identifying mark from one of these laboratories. Beware of counterfeit marked cords.

National Fire Protection Association’s NFPA 70 NEC national electric code (formally identified as ANSI/NFPA 70). It was first published in 1897 and is revised on a three year cycle. The NFPA NEC is not a regulating code by itself, but is written such that it is frequently adopted by regulating bodies. The NEC makes a distinction between low voltage and high voltage (over or under 100 volts) with different safety criteria. Presumably, skin will protect a body to 100 volts but that might not be an entirely accurate assumption. OSHA 29CFR frequently references NEC. At OSHA’s request, the NFPA created NFPA 70E, which addresses safe electrical work practices to comply with OSHA 29CFR 1910 Subpart S and 1926 Subpart K.

OSHA 29CFR 1926 (mostly Subpart K – Electrical), is a key tool to OSHA’s enforcement on construction jobsites for use of extension cords.

The International Electrotechnical Commission (IEC) publishes numerous international standards, including standards for plug and sockets, electrical power cords, weather protection on connections and electrical enclosures.

The Institute of Electrical and Electronics Engineers (IEEE) has many standards for electric insulation and testing.

InterNational Electrical Testing Association (NETA) has many published standards for testing international codes, state codes, local ordinances and permit requirements, and governing the use of extension cords. NETA frequently references NEC codes.

Manufacturer requirements or recommendations for proper use are also important to recognize. The instructions of acceptable use on the label of the cord must be followed.

Read part II of this Tech Talk in the September issue, which will cover:
  • Labels and Identifying Marks
  • Connectors, Cord Ends and Stain Reliefs
  • Proper Sizing of Extension Cords
  • Proper Care of Extension Cords
  • Polarity and Electrical Flow
  • GFCI, Circuit Breakers and Fuses
  • Repairing Damaged Extension Cords
Mike Orzechowski is the engineering manager for equipment of CSDA member DITEQ Corporation, based in Lenexa, Kansas, and has been in the industry for 19 years. He is the chair of the CSDA Standards & Specifications Committee, Finance Committee and is also involved with other committees within the association. Mike can be reached at 866-688-1032 or by email at

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