Voltage Drop Test

The purpose of the voltage drop test is to verify that Rule 8-102 of the Canadian Electrical Code, Part I (CSA C22.1), has been met for branch circuits in all health care facilities (including hospitals and clinics). The explanation that follows is provided as a general guide and should only be performed by qualified personnel using the appropriate test equipment. Please consult the current applicable electrical safety codes in effect for your area. The complete test procedure is outlined in clause 5.4.3 “Voltage drop test for solidly grounded systems” within the CSA Z32 standard and should be consulted.

The test requires that a load is connected between the poles of the receptacle under test for anywhere between one to five seconds. This load is adjusted so that 80% of the rated current of the circuit is flowing throughout the duration of the test.

As no conductor or connection is perfect, there will be a measurable drop in voltage when the load is connected (SW closed) compared to a “no load” condition (SW opened).

Voltage Drop Test Configuration
Voltage Drop Test Configuration

Recommended Test Procedure

  1. A load of 80% of the rated current of the circuit is applied at the receptacle. As the supply side of the consumer’s service (or equivalent) can vary during the test, the load may require adjustment.
  2. After five seconds has passed, the measured voltage is recorded as VLOAD and (in quick succession) the switch (SW) is opened, and the measured voltage is recorded as VOFF.
  3. Use the recommended Allera Systems Voltage Drop Test Limit (3%) when determining if the branch circuit meets the criteria of Item (a) Subrule (1) of Rule 8-102 in CSA C22.1. Note that this differs from the limit specified in CSA Z32. Meeting the Allera Systems Voltage Drop Test Limit will also meet the requirements of the limit specified in CSA Z32.

Test Results

When the entire test is complete, you will end up with three measurements:

  • The load current, IL
  • The loaded voltage, VLOAD
  • The unloaded voltage, VOFF

The voltage drop is calculated from the measured results:

Voltage drop (in volts) = VOFF – VLOAD
Voltage drop (as a percentage) = 100 × (VOFF – VLOAD) / VOFF

The voltage drop (as a percentage of VOFF) shall not exceed 3% (Allera Systems voltage drop test limit).

Example

Suppose you perform the test on a CSA 5-15R receptacle configuration (120 V nominal and 15 A rated current) and record the following measurements:

IL = 12 A (this is 80% of the rated current, or 0.8 × 15 A = 12 A)
VLOAD = 118.6 V
VOFF = 119.89 V

VDROP = VOFF – VLOAD = 119.89 V – 118.6 V = 1.29 V
% drop = 100 × (VOFF – VLOAD) / VOFF = 100 × (119.89 V – 118.6 V) / 119.89 V = 1.08%

Example – Analysis of Results

As 1.08% is less than 3%, this branch circuit meets the requirements of Item (a) Subrule (1) of Rule 8-102 in CSA C22.1 providing the feeder circuit voltage drop is not greater than 2% of the voltage at the point-of-supply.

It is important to note that the 1.08% voltage drop is not the voltage drop in the branch circuit as a percentage of the voltage at the point-of-supply. It is a value that is used to compare against a test limit in determining the performance of the branch circuit in delivering power to a receptacle.

To sum up, a voltage drop under 3% ensures that the branch circuit meets the requirements of Item (a) Subrule (1) of Rule 8-102 in CSA C22.1 with the provision that the feeder circuit not exceed 2% of the voltage at the point-of-supply.

Meeting the Allera Systems Voltage Drop Test Limit means that the branch circuit also meets the requirements of clause 5.4.3.2 in CSA Z32.

What to watch out for…

  • Care should be taken to avoid interruptions at the receptacle (connecting or disconnecting the test plug when the supply is energized) as it may result in pitting of the contacts inside the receptacle.
  • Item (a) Subrule (1) of Rule 8-102 in CSA C22.1 states that the voltage drop shall not exceed 3% of the voltage at the point-of-supply. The voltage drop test outlined in CSA Z32 clause 5.4.3 sets a test limit of 5% of the unloaded voltage (VOFF) for the branch circuit. See Allera Systems Voltage Drop Test Limit for a recommended test limit.
  • The time between measurements is critical as the voltage may be fluctuating during the test. Minimizing the time between measurement of VLOAD and VOFF will reduce the probability of an erroneous result.
Voltage Drop Measurement

How often should this test be performed?

This test is performed upon completion of new construction or major renovation or additions to existing installations. Although not stated within CSA Z32, it is recommended that this test also be performed when a receptacle is replaced.

What test equipment do I need?

The ALTS2400 CSA Z32 Test System will perform voltage drop tests quickly and easily for grounded and isolated 120 V circuits (15 A and 20 A).