Enter the supply voltage, load current, conductor size and one-way length, and the calculator gives the voltage drop in volts and per cent, the voltage left at the load, and whether the run is inside the 3% that Canadian Electrical Code Rule 8-102 allows for a feeder or branch circuit. Switch to find minimum conductor size and it works backwards from a maximum drop to the smallest standard copper or aluminium conductor that meets it. Lengths can be in metres or feet. The working is printed under the results in the form an exam answer takes.
Calculator
01What voltage drop is and why the code limits it
Every conductor has resistance. When current flows through it, some of the supply voltage is used up along the wire (Ohm's law: V = I × R) and the load at the far end sees less than the panel put out. That lost voltage is the voltage drop. It rises with the current, with the length of the run and with the resistance per unit length of the conductor, and it falls as the conductor gets bigger.
The lost voltage is not free. It is turned into heat in the conductor (P = I² × R), so a long undersized run wastes energy all day. At the load, low voltage makes motors draw more current and run hotter, dims lighting, and can stop electronic equipment and contactors from working reliably. That is why the Canadian Electrical Code puts a ceiling on it and why voltage drop is part of sizing any conductor longer than a few metres, alongside ampacity, derating and termination ratings.
Voltage drop is a separate check from ampacity. A conductor can be big enough to carry the current safely and still be too small to deliver an acceptable voltage at the end of a long run. On long runs, voltage drop is usually the check that decides the size.
02The CEC Rule 8-102 limits: 3% and 5%
The voltage drop rule in the Canadian Electrical Code, Part I (CSA C22.1) is Rule 8-102, in Section 8, Circuit loading and demand factors. In words, it sets two limits:
- 3% for a feeder or for a branch circuit, taken on its own.
- 5% in total, from the supply side of the consumer's service (or the supply of a separately derived system) to the farthest point of utilization on the installation.
Both limits are read together. A branch circuit that drops 2.5% is fine on its own, but if the feeder ahead of it already drops 3%, the 5.5% total fails. In practice the two percentages are split between feeder and branch circuit at design time, for example 2% on the feeder and 3% on the branch circuit, or the other way round, so that the total stays inside 5%. The calculator checks a single run against 3% and tells you when the run is also over 5% on its own; the total is yours to add up across the whole path.
The rule also has subrules on what load the calculation is based on and on when a calculation is required at all, and the exact wording changes between editions. Read Rule 8-102 in full in the edition you are studying rather than relying on the two percentages alone. Appendix D of the code holds the tabulated method: Table D3 relates conductor size, current and distance at a fixed drop, so that a length can be looked up instead of calculated. This page describes what that table is for and does not reproduce it or any other CSA table.
If a Canadian Electrical Code book is provided at your Red Seal sitting, Rule 8-102 is where the limits are; as our 309A exam guide explains, whether a code book is supplied and which edition is not settled by the Red Seal Program's published material, so ask your provincial or territorial apprenticeship office when you book.
03The formula and where K comes from
The calculator uses the conductor-constant form of Ohm's law that most Canadian and American textbooks teach:
- Single-phase or DC: VD = 2 × K × I × L / CM
- Three-phase: VD = 1.732 × K × I × L / CM
where VD is the voltage drop in volts, I is the load current in amperes, L is the one-way length of the run in feet, CM is the cross-sectional area of one conductor in circular mils, and K is the resistance of a conductor one foot long and one circular mil in area, in ohm-circular mils per foot.
K is a material constant. The values used here are the ones widely published for conductors at their 75 °C operating temperature: 12.9 for copper and 21.2 for aluminium. At room temperature (20 °C) copper is closer to 10.4 and aluminium to 17, which is why you will see both sets of numbers in print. Resistance rises with temperature, so the 75 °C figures are the conservative ones for a conductor carrying load.
Two things to keep straight about K. First, it is an approximation: the code's own conductor resistance tables and the Table D3 method are built from measured resistances for specific conductor constructions, so they give slightly different numbers than a single constant does, and stranded and solid conductors of the same gauge differ a little too. Second, an exam question will normally tell you what to use, either by giving K, by giving the resistance per unit length, or by saying which method to apply. Use what the question gives you, not what you memorized.
The circular-mil areas in the calculator are the AWG and kcmil standard, not a code table: 14 AWG is 4,110 cmil, 12 AWG 6,530, 10 AWG 10,380, 8 AWG 16,510, 6 AWG 26,240, 4 AWG 41,740, 2 AWG 66,360, 1/0 AWG 105,600, 4/0 AWG 211,600, and from 250 kcmil up the name is the area in thousands of circular mils. Every three gauge numbers the area roughly doubles, so going up three sizes roughly halves the drop.
04Single-phase and three-phase: 2 versus the square root of 3
The 2 in the single-phase formula is the round trip. Current leaves on one conductor and comes back on the other, so the resistance the load sees is two lengths of wire. The same applies to DC and to a 120 V circuit with a neutral return: two conductors carry the full load current, and both drop voltage.
In a balanced three-phase system the currents in the three conductors are 120 degrees apart, so at any instant the return current is shared between the other two phases rather than flowing back on a single conductor. The line-to-line drop works out to the square root of 3 (about 1.732) times the drop in one conductor, not twice it. That is the 1.732 in the three-phase formula, and it is why a three-phase feeder of the same size and length drops about 13% less than a single-phase one at the same current.
The Red Seal Program's formula sheet for Construction Electrician lists 1.73 as the constant for the square root of 3. Using 1.73 instead of 1.732 changes the answer in the third significant figure, which is never enough to move you between multiple-choice options. If a question gives you 1.73, use 1.73.
The three-phase formula assumes a balanced load. For an unbalanced three-phase circuit, or for the neutral of a single-phase three-wire circuit with unequal loads on the two legs, the drop has to be worked conductor by conductor.
05Metres in a feet-based formula, and the SI form
Canadian drawings and code tables are in metres, but the K formula is in feet because K is defined per foot. Convert first: 1 m = 3.2808 ft, so multiply the one-way length in metres by 3.2808 and carry on. The calculator does this for you and prints the converted length as the first line of the working. Convert the length, not the answer: the volts that come out of the formula are volts regardless of the unit you started in.
There is an SI form of the same calculation that avoids the conversion: VD = 2 × ρ × L × I / A (or 1.732 for three-phase), with L in metres, A the conductor area in square millimetres, and ρ (rho) the resistivity of the metal in ohm-square millimetres per metre. Metric areas are a physical standard too: 10 AWG is 5.26 mm², 6 AWG is 13.3 mm², 2 AWG is 33.6 mm². The resistivity of copper is about 0.0172 Ω·mm²/m at 20 °C and about 0.0214 Ω·mm²/m at 75 °C; aluminium is about 0.0283 at 20 °C and about 0.0352 at 75 °C.
Those two temperatures are the trap. K = 12.9 is a 75 °C value, and converted to SI it is the 0.0214 figure, not 0.0172. Run the calculator's default example in SI with 0.0172 and you get about 7.1 V; with 0.0214 you get 8.8 V, the same as the K method. Neither is wrong; they describe the conductor at different temperatures. On an exam, use the constant the question gives you and note what temperature it is stated for, if it says.
06Worked examples in exam style
Each of these is written the way a Red Seal question is: a situation, the numbers you need, and a single thing to find. Work them on paper with the K values above before reading the solutions, and time yourself. The 309A exam averages 2.4 minutes a question.
Example 1: voltage drop, metric length (the calculator's default)
Question. A 240 V single-phase circuit supplies a 24 A load through 10 AWG copper conductors. The one-way length of the run is 45 m. Using K = 12.9, what is the voltage drop, and does it meet the 3% limit for a branch circuit?
Solution. Convert the length: 45 m × 3.2808 = 147.64 ft. Then VD = 2 × 12.9 × 24 × 147.64 / 10,380 = 91,416 / 10,380 = 8.81 V. As a percentage, 8.81 / 240 × 100 = 3.67%. The voltage at the load is 240 - 8.81 = 231.19 V. That is over 3%, so the run does not meet Rule 8-102 for a branch circuit with 10 AWG. Going up to 8 AWG (16,510 cmil) gives 91,416 / 16,510 = 5.54 V, or 2.31%, which passes.
Example 2: voltage drop, length in feet
Question. A 120 V, 20 A load is fed by 12 AWG copper conductors with a one-way length of 100 ft. K = 12.9. Find the voltage drop and the percentage drop.
Solution. No conversion is needed. VD = 2 × 12.9 × 20 × 100 / 6,530 = 51,600 / 6,530 = 7.90 V. Percentage: 7.90 / 120 × 100 = 6.58%. The load sees 112.10 V. This fails both the 3% and the 5% limits on its own. It is a good number to remember: 12 AWG at 20 A loses more than 5% every 100 ft at 120 V, which is why long 120 V branch circuits are so often upsized.
Example 3: find the minimum conductor size
Question. A 120 V single-phase branch circuit carries 16 A over a one-way distance of 60 m. Using K = 12.9 for copper, what is the smallest standard conductor that keeps the voltage drop within 3%?
Solution. Find the allowed drop first: 3% of 120 V = 3.6 V. Convert the length: 60 m × 3.2808 = 196.85 ft. Rearrange the formula for area: CM = 2 × K × I × L / VD = 2 × 12.9 × 16 × 196.85 / 3.6 = 81,259 / 3.6 = 22,572 cmil. The smallest standard size at or above 22,572 cmil is 6 AWG (26,240 cmil). Check it: 81,259 / 26,240 = 3.10 V, which is 2.58%. The next size down, 8 AWG (16,510 cmil), would give 4.92 V or 4.10%, so it fails. Note that 6 AWG is far larger than the 16 A load needs for ampacity; the length, not the current, is setting the size.
Example 4: three-phase, aluminium
Question. A balanced three-phase, 600 V feeder carries 75 A through 2 AWG aluminium conductors over a one-way length of 90 m. Using K = 21.2, find the voltage drop and state whether the feeder meets the 3% limit.
Solution. Convert: 90 m × 3.2808 = 295.27 ft. Three-phase, so the factor is 1.732: VD = 1.732 × 21.2 × 75 × 295.27 / 66,360 = 813,144 / 66,360 = 12.25 V. Percentage: 12.25 / 600 × 100 = 2.04%. The feeder passes the 3% limit, and leaves just under 3% for the branch circuits beyond it before the 5% total is reached. Two things to notice: at 600 V a 12 V drop is a small percentage, and the aluminium constant (21.2) is about 1.6 times the copper one, which is why aluminium conductors need to be about two sizes larger to match copper.
Enter any of these into the calculator to see the same working printed step by step. The results match to the second decimal; a textbook or an exam key that uses 1.73, 3.28 or a slightly different K will match to the first.
07Common mistakes
- Doubling the length. L in the formula is the one-way distance from the panel to the load. The 2 (or 1.732) already accounts for the return path. If you measure the wire out and back and also multiply by 2, the answer is twice what it should be.
- Forgetting the 1.732 on three-phase. Using 2 for a three-phase circuit overstates the drop by about 15%, which is enough to pick the wrong conductor size on a question that is designed to sit near the 3% line.
- Mixing metres and feet. K is per foot. A length in metres dropped straight into the formula gives an answer about a third of the true value. If the question is in metres and gives K in ohm-circular mils per foot, converting the length is part of the question.
- Using the wrong K. Copper and aluminium constants are not interchangeable, and 20 °C values (about 10.4 and 17) are not the 75 °C values (12.9 and 21.2). Use the constant the question states. If it states none, say which one you used.
- Reading kcmil as AWG. The AWG scale ends at 4/0 (211,600 cmil). Above that, conductor sizes are named by their area in thousands of circular mils, so 250 kcmil is 250,000 cmil, not a gauge number. Below 4/0, remember that a smaller AWG number is a bigger wire.
- Percent of the wrong voltage. The percentage is the drop divided by the nominal supply voltage of the circuit, line-to-line for a three-phase or 240 V circuit, line-to-neutral for a 120 V circuit. Dividing by the voltage at the load, or by 120 when the circuit is 240, gives a different percentage.
- Rounding too early. Keep the converted length and the intermediate product to at least four significant figures. Rounding 147.64 ft to 148 ft is harmless; rounding 3.2808 to 3 is not.
- Stopping at 3%. A branch circuit at 2.9% still fails if the feeder ahead of it drops 2.5%, because the total is over 5%. When a question gives both feeder and branch circuit, add them.
08What the 309A exam expects about voltage drop
Voltage drop sits inside the Construction Electrician standard's largest block. Our 309A exam guide sets out the Red Seal Program's breakdown: block C, wiring systems, is 30 of the 100 questions, and its task on raceways, conductors, cables and enclosures gets 9 of them. Block B, generating, distribution and service systems, is 28 more, and services and feeders live there. Conductor selection, including voltage drop, is the kind of sub-task those questions are written to.
The program says 60 to 70% of 309A questions are procedural and application questions, which its preparation guide says can include calculations and the interpretation of code books, and 20 to 30% are critical thinking questions that may need more than one step. A voltage drop question can be either: a straight calculation like examples 1 and 2 above, or a size-finding question like example 3, where you have to work out the allowed drop, rearrange the formula, and then pick a standard size, all before you reach the options.
Three practical points follow. First, the formula sheet provided at the sitting lists current, power, resistance and the three-phase relationships, with 1.73 as the square root of 3, but the voltage drop question will normally supply K or the method it wants, so read the stem for what you are given. Second, the preparation guide warns that a question will not announce that it is a code question; if a stem mentions a feeder and a percentage, Rule 8-102 is in play whether or not the code is named. Third, the guide also says the exam covers the trade as practised across Canada, so expect metric lengths and Canadian supply voltages (120, 208, 240, 347 and 600 V) rather than American ones.
Practise until the sequence is automatic: convert the length, pick the factor (2 or 1.732), pick K for the material, calculate, then compare with 3% and with the 5% total. The free 309A questions include conductor and code items in the same format as the exam.
09Sources
- Red Seal Program: Construction Electrician trade page the trade the 309A exam certifies, the occupational standard editions, and links to the exam information page with the block breakdown and the formula sheet.
- CSA Group: Canadian Electrical Code, Part I (CSA C22.1) the publisher's page for the code that contains Rule 8-102 and Appendix D. The code is copyrighted; this page cites it and reproduces none of its tables or rule text.
- TicketPrep: 309A Electrician Red Seal exam guide the block and task question counts, the question-type mix, the formula sheet contents and the open question of the code book at the sitting.
10Questions people ask
- What is the maximum voltage drop allowed by the Canadian Electrical Code?
- Rule 8-102 of the Canadian Electrical Code, Part I limits voltage drop to 3% for a feeder or branch circuit and 5% in total from the supply side of the consumer's service to the farthest point of utilization. Both limits apply together, so a 3% branch circuit on a 3% feeder fails the 5% total. Read the full rule in the edition you are studying, because it also sets out what load the calculation is based on.
- What is the formula for voltage drop?
- For single-phase or DC, VD = 2 x K x I x L / CM. For balanced three-phase, VD = 1.732 x K x I x L / CM. I is the load current in amperes, L is the one-way length in feet, CM is the conductor area in circular mils, and K is about 12.9 for copper and 21.2 for aluminium at 75 degrees C. Divide the drop by the supply voltage and multiply by 100 for the percentage.
- How do I use a length in metres with the K formula?
- Multiply the one-way length in metres by 3.2808 to get feet, then use the formula as normal. K is defined per foot, so a length left in metres gives an answer about one third of the true value. The calculator on this page converts for you and shows the converted length in the working.
- Why does the calculator give a slightly different answer from the code book method?
- Because K is a single approximate constant. The Canadian Electrical Code's tabulated method in Appendix D and its conductor resistance tables are built from measured resistances for specific conductor constructions, so they differ from the K method by a small amount. Exam questions normally state the K value or the method to use, and the difference is not enough to change which multiple-choice option is correct.
- Does the 309A Red Seal exam have voltage drop questions?
- The Red Seal Program does not publish question-level topics, but voltage drop is part of conductor selection in the Construction Electrician standard, and wiring systems is the largest block on the exam at 30 of 100 questions. The program says 60 to 70% of the questions are procedural and application questions, which can include calculations and code interpretation. A voltage drop question can ask for the drop, the percentage, or the minimum conductor size that meets a limit.
11Practice for this exam
Voltage drop is one calculation among many on the 309A exam. TicketPrep 309A practice tests follow the Red Seal Program's block-by-block breakdown, and every answer is explained, including why each wrong option is wrong. You can try free sample questions with no account.
Ten free questions per trade, no account: free practice questions. How our questions are written and checked: how our questions are made.