Extension Cord Voltage Drop Calculator

The Extension Cord Safety Calculator determines the voltage drop through a reviewed extension cord from its load current, one-way length, conductor resistance, and system voltage. It also compares the calculated voltage-drop percentage with an Entered drop comparison value and compares the reviewed load with an Entered cord rating.

Extension cords add resistance between the branch-circuit receptacle and the connected utilization equipment. As cord length and load current increase, conductor voltage drop increases. The resulting voltage at the equipment is lower than the source voltage shown by the System voltage input.

The calculator produces these electrical values:

• Total circuit length in feet
• Circuit resistance in ohms
• Voltage drop in volts
• Voltage drop as a percentage of system voltage
• Entered drop margin in percentage points
• Entered rating margin in amperes
• Record comparison for the selected cord-condition documentation state

This calculation supports a cord-record review or electrical measurement workflow. It does not issue an installation conclusion, establish ampacity, approve cord use, or determine NEC compliance.

Cord Circuit Resistance

An extension cord has an outgoing conductor and a return conductor. For a single-phase load, current travels from the source to the load on one conductor and returns on the other. The calculator therefore converts One-way length into the full current path:

\(\displaystyle \text{Total circuit length} = \text{One-way length} \times 2\)

The Conductor resistance input is entered in ohms per 1,000 feet. The calculator applies that resistance to the total circuit length:

\(\displaystyle \text{Circuit resistance} = \left(\frac{\text{Total circuit length}}{1000}\right) \times \text{Conductor resistance}\)

The conductor-resistance value should correspond to the reviewed cord conductor. Do not substitute an assumed AWG value when the record identifies a different conductor construction, material, temperature condition, or resistance basis.

A longer cord has more circuit resistance. A smaller conductor generally has greater resistance per 1,000 feet than a larger conductor. For example, a cord identified by a higher AWG number commonly has more resistance than a lower-AWG cord of the same conductor material and construction.

Voltage-Drop Calculation

The calculator multiplies the entered Load current by the calculated circuit resistance:

\(\displaystyle \text{Voltage drop} = \text{Load current} \times \text{Circuit resistance}\)

It then expresses the voltage loss as a percentage of the entered System voltage:

\(\displaystyle \text{Voltage drop percent} = \left(\frac{\text{Voltage drop}}{\text{System voltage}}\right) \times 100\)

For the selected Circuit type, the worksheet uses the single-phase circuit basis and doubles the one-way length. The result is a conductor voltage-drop calculation, not a motor-performance calculation, branch-circuit load calculation, or feeder design calculation.

The calculated percentage can be compared with the value in Entered drop comparison:

\(\displaystyle \text{Entered drop margin} = \text{Entered drop comparison} - \text{Voltage drop percent}\)

A positive margin means the calculated drop is below the entered comparison value. A zero margin means the two values are equal. A negative margin means the calculated drop exceeds the entered comparison value.

Cord Rating Comparison

The calculator also compares the reviewed Load current against the Entered cord rating:

\(\displaystyle \text{Entered rating margin} = \text{Entered cord rating} - \text{Load current}\)

This result is an arithmetic difference between two entered values. It is not an ampacity determination. A cord rating printed on a cord, tag, packaging record, or inspection record must be evaluated separately from the connected equipment load, plug and receptacle configuration, overcurrent protection, duty cycle, ambient conditions, conductor temperature, and manufacturer instructions.

The Cord condition record field documents whether the cord condition is recorded. Its result is shown as Record comparison. A recorded condition does not establish that the cord is undamaged or suitable for continued use; it identifies the documentation state of the reviewed record.

Calculation Example

A reviewed cord record contains the following values:

Input	Entered value
Load current	12 A
One-way length	50 ft
Conductor resistance	1.588 ohm/1000 ft
System voltage	120 V
Circuit type	Single phase
Entered drop comparison	3%
Entered cord rating	15 A
Cord condition record	Recorded
Source reference	cord-record

The calculator first doubles the one-way cord length:

\(\displaystyle 50\text{ ft} \times 2 = \mathbf{100\text{ ft}}\)

It then calculates the total circuit resistance:

\(\displaystyle \left(\frac{100}{1000}\right) \times 1.588 = \mathbf{0.1588\text{ ohm}}\)

Voltage drop at 12 A is:

\(\displaystyle 12 \times 0.1588 = \mathbf{1.9056\text{ V}}\)

Voltage-drop percentage at 120 V is:

\(\displaystyle \left(\frac{1.9056}{120}\right) \times 100 = \mathbf{1.588\%}\)

The comparison results are:

Result	Value
Total circuit length	100 ft
Circuit resistance	0.1588 ohm
Voltage drop	1.9056 V
Voltage drop	1.588%
Entered drop margin	1.412%
Entered rating margin	3 A
Record comparison	Entered values comparable

The 1.412% entered drop margin comes from:

\(\displaystyle 3\% - 1.588\% = \mathbf{1.412\%}\)

The 3 A entered rating margin comes from:

\(\displaystyle 15\text{ A} - 12\text{ A} = \mathbf{3\text{ A}}\)

Field Verification

Verify the values used in the cord record before relying on the arithmetic:

• Confirm that One-way length represents the actual source-to-load route, including the deployed cord length rather than a straight-line measurement.
• Confirm the Load current reflects the reviewed operating load. Motor starting current, changing process loads, and intermittent loads may not match a single observed current value.
• Confirm that Conductor resistance matches the cord under review and is stated in ohms per 1,000 feet.
• Confirm the System voltage is the voltage basis required by the reviewed measurement or record.
• Confirm the selected Circuit type matches the circuit basis used by the worksheet.
• Preserve the Source reference so the calculation can be tied to the cord record, inspection entry, or measurement source.

The calculated result does not evaluate cord damage, insulation condition, plug and connector condition, grounding continuity, GFCI protection, receptacle condition, overcurrent protection, equipment listing, branch-circuit loading, or compliance requirements enforced by the AHJ. Those determinations require separate field inspection, equipment documentation, applicable code review, and the conditions of the actual installation.