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Voltage Drop: The Sneaky Energy Thief

Imagine electricity as a marathon runner carrying voltage from the panel to the load. Along the way, it gets tired running through long wires — that tiredness is called voltage drop.

Just like water loses pressure in a long pipe, voltage loses strength over distance. The longer or thinner the wire, the more it drops!

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How Much Can It Drop?

According to CEC Rule 8-102:

  • Branch circuits: Max 3%
  • Total drop (feeder, through the panel, branch): Max 5%

Go beyond that, and your equipment might not work right—or at all. It's like giving a flashlight half-dead batteries 

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The Voltage Drop Formula Team

Meet your formula friends:

  • V = Voltage at the source (in Volts)
  • VD = Voltage lost (drop) (in Volts)
  • K = Resistance per km (from Table D3)
  • f = System factor (1.73 for 3-phase, 2 for single-phase)
  • I = Current (amps)
  • L = Length of the wire (meters)

These characters team up to find either the voltage drop, the wire size, or how far the current can run!

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How to Hunt Down the “K” Factor (Voltage Drop Style)

To figure out your K factor — the secret for voltage drop — you need four key ingredients. Just remember the acronym PICA (like a cool code name!):

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P – Power Factor / How “resistive” is your load? (100% is a perfect heater, but motors usually vary.)

I – Installation Type / Are the wires in a Raceway or a Cable Tray? This affects resistance!

C – Conductor Size / What AWG size are you using? (Smaller number = thicker wire)

A – AC or DC / Voltage drop behaves differently for alternating vs. direct current.

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Bonus Rule:

  • If you don’t know the power factor, no problem — just be safe and choose the highest resistance value from the table.
  • All the factors stay the same…..but the formulas change depending on what you’re missing.

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Voltage Drop Formula (when wire size is known):

VD = K × f × I × L 1000

Where:

  • VD = voltage drop (in volts)
  • K = resistance from CEC Table D3 (ohms/km)
  • I = current (amps)
  • L = one-way length (meters)
  • f =  2 for single-phase (or use 1.73 for 3-phase)

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Voltage Drop Formula (in percentage)

VD% = VD × 100 V

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Example 1 – Electric Heater

We’ve got a 12 kW, 240V electric heater that’s going in a detached garage, 30 meters away from the panel in the house. The circuit runs through a raceway using #8 RW90 copper conductors.

The heater is purely resistive (so power factor is 100%), and all the equipment is labeled for a 75°C max termination temperature.

Now the big question is:
Will these wires keep the voltage drop within CEC Rule 8-102’s 3% limit for branch circuits?‍

And if they don’t, what’s the fix? (Spoiler: you might need to either reduce the heater size or upgrade to #6 copper.)

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Given:

  • Load = 12 kW
  • Voltage = 240 V
  • Distance = 30 m
  • Power Factor = 100%
  • Wire = #8 RW90 copper in raceway
  • Equipment = 75°C rating

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Step 1: Find Current (I)

Since it’s resistive (100% pf),

I = P E

I = 12,000 240

I = 50A

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Step 2: Use CEC Table D3 for K

  • #8 copper, 75°C, in raceway, 100% PF
  • K = 2.54 ohms/km

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Step 3: Use VD Formula

VD = K × f × I × L 1000

VD = 2.54 × 2 × 50 × 30 1000

VD = 7.62V

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Step 4: Calculate Percent Drop

VD% = 7.62 × 100 240

VD% = 3.175%

Over the 3% limit.

  • Solution: try #6 AWG wire.
  • Let’s try upsizing to a #6 AWG!
  • Everything stays the same, except for the K FACTOR!

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Step 5: Use CEC Table D3 for K

  • #6 copper, 75°C, in raceway, 100% PF
  • K = 1.59 ohms/km
VD = K × f × I × L 1000

VD = 1.59 × 2 × 50 × 30 1000

VD = 4.77V

Now the percentage! Remember, it has to be BELOW 3%!

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Step 6: Calculate Percent Drop

VD% = 4.77V × 100 240

VD% = 1.988%

This means that we have to use a #6 AWG

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UNKNOWN CONDUCTOR SIZE

K ≤ VD × 1000 I × L × F

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When the conductor size is known, length is found by:

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Example 2 - AC Unit

We need to find the correct conductor size to keep the voltage drop under to 3% for a single-phase 240V circuit supplying a 40-amp air conditioner that’s 21m from the panel. There is no marked Equipment termination temperature.

  • V = 240V
  • VD = 3% (Will need to do an extra calculation to turn it into Volts)
  • K = Resistance per km (from Table D3)
  • f = 2  (single-phase)
  • I = 40 A
  • L = 21m

VD= By multiplying the Applied voltage by 3% (the maximum percentage allowed by the CEC), we can find the maximum voltage drop allowed.

  • VD=240V * 0.03%
  • VD=7.2V

In order to find our Conductor size, we need to use Table D3.

So….. that K factor!

This is a VERY tricky part you need to remember!!!!

First, size the conductors at 125% [CEC Rule 8-104(3)(a) and (6)(a)] of the air conditioner load:

  • 40 x 1.25 = 50A minimum size conductors are required.
  • For equipment that is not marked with a maximum conductor termination temperature, CEC Rule 4-006 (2)(a)(i) applies; use the 60°C column of CEC Table 2.
  • Therefore, go to CEC Table 2, use No. 6 AWG copper conductors.