Extension Cords – Part II
By Mike Orzechowski
(Click Here for Part I of this article)
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 (OSHA defines as less than 90 days). The National Electric Code (NEC) Article 400 addresses flexible cords as a separate category. They do not consider it 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.
Labels and Identifying marks
Listed and/or certified items (cords, adapters, boxes, plug ends, etc.) must be installed and used in accordance to the labeling, listing or certification.
Never use an ‘indoor only’ cord in an outside environment. The extension cord should be labelled such that you can identify it as a cord suitable for outside use.
Be aware of counterfeit cords that bear official markings as if they are properly listed, labeled or certified. The CPSC (Consumer Product Safety Commission), CSA and UL post recalls on their respective websites, including info on counterfeit marked cords. Use established vendors and legitimate distributors. Read labels carefully, the text should be grammatically correct and free of conflicting information.
It is important to note that individual components that make up an assembly can be marked by a Nationally Recognized Testing Laboratory (NRTL), but that does not make the entire assembly approved by the NRTL.
Extension cords should be marked about every foot on the cord with the service type, appropriate NRTL, the size and the number of conductors.
Inside the extension cord, each of the individual conductors are insulated and marked using a color code. The green conductor (sometimes bare wire but not in these type extension cords) is the Ground (earth Ground or grounding conductor), the white or grey conductor is the neutral (near ground potential, also referred to as the grounded conductor) and the colored wires are the hot conductors (also referred to as the ungrounded current carrying conductors). On single phase, the conductor is typically black but it can also be red, orange or blue. On three phase systems, per NEC Section 110.15 High-Leg Marking, the High leg delta (aka Red Leg or Wild Leg) systems that have the midpoint of one phase grounded are required to have that phase conductor marked with orange.
Connectors, Cord Ends and Strain Reliefs
Using the correct pin out for the voltage and current as defined by NEMA standards or a custom configuration specific to a unique piece of equipment provides a good means to be certain the correct connection is made to the appropriate voltage and current source.
Cable protection from physical damage like a wrap or sleeve is available but putting additional thermal barriers on a cable may cause it to get hot and may exceed the ratings of the cord during normal use. It is better to protect the surface of the cord where it will be dragged across to minimize any damage to the cord and to extend the useful service life of the extension cord.
It is important to use strain reliefs or molded connectors to prevent pull being transmitted directly to the terminal screws.
Proper Sizing of Extension Cords
At a very basic level, extension cords need to be sized appropriately for the wire size (Ampacity). The essential function of the extension cord is to carry electrical current to the equipment being operated. The longer the length of the cord equates to higher resistance which creates a larger voltage drop. Per ANSI/NEMA MG 1-2011, electrical motors are allowed +/-10% of nameplate voltage, so the practical limitation of the length and American Wire Gauge (AWG) wire size is dependent on the allowable voltage drop.
It is important to identify the influences of real world de-rate factors such as heavy inductive loads (induction – power factor), increased resistance with elevated temperatures in the heated copper conductors, heat generated by a high number of motor starts, local Resistance Points (loose lugs, connections, arc burned contact points), variable speed motors running with reduced cooling fan speeds, lower supply voltage (208 vs 230), unbalanced legs on a 3-phase circuit, etc.
Voltage is an across variable, meaning it is measured as a different potential between two points (usually from hot to earth ground). For example, on a 240V single phase cord there will be two hots (the National Electrical Code—NEC—refers to this as the ungrounded current-carrying conductors) and one Ground. Sometimes the cord might carry a forth conductor (4-pole), which would be a neutral (NEC refers to the neutral as a grounded conductor) in addition to the earth ground (referred to as a grounding conductor by the NEC). The voltage between the two hots is supposed to be 240V and the voltage between one of the hot conductors and the Ground should be 120V.
It is important to note that the correct way to measure voltage drop is when the tool is loaded. The voltage at the end of the cord may appear to be within the +/-10% NEMA nameplate voltage band but once the tool is running, the voltage drop might be significant.
Current is a through variable so it is measured under full load. Typically, one conductor is isolated and a clamp-on ammeter is used to measure the current flow, but a better explanation is with a mounted ammeter that is placed in the path of the conductor.
As a rule of thumb, Amps is Watts divided by Volts (or Watts = Volts times Amps). So for example, if a tool draws 20A at 120V, the equivalent tool at 240V would only draw 10A. This is a useful rule of thumb for determining the amp draw on a cord if the tool is rated on the nameplate in watts. It is only a rule of thumb because it doesn’t take into effect power factors on inductive loads and motor startup current draw that is not identified on the motor nameplate like the running FLA (full load amps).
Cords that are hot to touch might be an overloaded situation and may require further investigation.
Look for other important ratings on the extension cords like outdoor rated cords suitable for wet areas and sustained sunlight. Cords with O or OO labels are for use around petroleum products like oils. The heavy Jacket can make the cord more crush-resistant and abrasion resistant. The voltage rating is important also for 300V SJ or 600V S cords if the equipment is running on 480V. Also, the electrical insulation usually means more thermal insulation, so sometimes heavy jacket cords don’t dissipate heat as effectively. Finally, the overall weight of the extension cord can be a factor on some jobs that require manually hauling the cords and equipment to remote locations or elevations.
Proper Care for Extension Cords
Cover and protect cords so they are not walked on or driven over. Hand trucks and push carts can also damage the inner insulation of the extension cords because the high concentration of forces on a smaller tire.
Flexible cords and cables shall be protected from damage. Sharp corners and projections shall be avoided. Flexible cords and cables may pass through doorways or other pinch points, if protection is provided to avoid damage.
When not in use, unplug the cord, coil it up with a generous loop larger than the practical bend radius of the flexible cord and store in a cool place not in direct sunlight (UV may damage the outer jacket).
Inspect the extension cord before use and take out of service if it is damaged. Be certain all the contact pins are intact and that the electrical insulation is not compromised. Check for visual signs that the cord was crushed and might need additional analysis to determine if there was any internal damage to the cord inside the outer jacket.
Polarity and Electrical Flow
On single phase 120V circuits there is a polarity established between hot and the neutral that must be preserved to keep the hot as the hot and the neutral at near ground potential. If the polarity is reversed it can have deadly consequences. Many switches are Single-Pole Single-Throw (SPST) and effectively only break the hot side of the circuit leaving the neutral intact. With reversed polarity, the switch will break the neutral (and without a continuous circuit, it will shut off the equipment) but it will leave the hot energized and the potential on the equipment to earth ground can be the full 120V.
Some switches are Double-Pole Single Throw (DPST) and effectively break both the hot and the neutral leaving the Ground intact (as required by OSHA).
It is important that cords are wired to the correct polarity and tested before putting them into service.
To reduce the incidences of having a reversed polarity situation, many plugs and receptacles have larger blades and receivers for the neutral side so they can’t get inserted incorrectly. Never modify the size of the blade to fit into an outlet to defeat this important safety design feature. They also color code the terminals such that the hot terminals are bright shiny brass colored and the neutral terminals are silver (the ground is green).
On 240V circuits, the potential is split between two 120V hot conductors with a 240V potential difference between them. If equipment uses both 120V and 220V circuits, the extension cord needs to have four conductors to carry a neutral for the 120V circuit side.
On 3 phase circuits the rotations of the motor can be established by switching any two of the three hot conductors.
GFCI, Circuit Breakers and Fuses
Surge arrestors (also known as lightning arrestors) are designed to protect the equipment from sudden excessive electrical surges in the power line. Fuses and circuit breakers are designed to protect the circuit from excessive current (unlike a fuse, the circuit breakers can be reset). Thermal overloads are designed to protect the motor from overheating (may have manual or automatic reset). An Arc Fault Circuit Interrupter (AFCI) is a circuit breaker designed to detect a non-working electrical arc to prevent fires, and Ground Fault Circuit Interrupters (GFCI) are designed to protect the operator from electrocution by detecting current balance from hot to ground.
Some equipment or power tools are designed with built-in GFCI protection and some extension cords also have built in GFCI protection.
Some nuisance tripping of the circuit breakers or fuses can be caused by excessive motor starts in a short duration of time.
GFCI protection can have nuisance tripping in damp conditions where electricity can leak across moist surfaces.
For safety reasons, it is important that the path to ground must permanent and continuous.
Repairing Damaged Extension Cords
Tag damaged equipment and remove from service.
Flexible cords have many strands, so pulling and bending can loosen the terminal connection.
Cords can be spliced or undergo major repairs if rated 12AWG to 2AWG, provided the repair meets the same insulating properties and usage characteristics as spelled out in OSHA 1926.
Placing electrical tape on the outer jacket of an electrical cord is ok for superficial nicks on the outer jacket, but is not recommended. Tape hides any damage from inspection. It is possible that damage to the cord is more extensive than it looks on the jacket, so the application of tape may mask a potentially dangerous situation. The tape may also decrease the flexibility of the cord. If the damage is bad enough to tape, it is also possible that the cord could be considered too damaged to tape.
It is important to note a relax in restrictions from OSHA in that extension cords are to be used for temporary electrical power, so OSHA 29CFR 1926.405(a)(2) may be less than on a permanent installation. The full requirements of NRTL do not apply, as long as they are equivalent to factory assembled cords. Assemblies should be marked appropriately as per 1926.405(g)(2)(iv) and 1926.403(b)(1)(i) with the appropriate strain reliefs. The assemblies should be made by a qualified person and the grounding conductor should remain intact.
Repairs 1926.404(b)(1)(iii)(C) but need to be compliant with 1926.403(a)
