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!
According to CEC Rule 8-102:
Go beyond that, and your equipment might not work right—or at all. It's like giving a flashlight half-dead batteries
Meet your formula friends:
These characters team up to find either the voltage drop, the wire size, or how far the current can run!
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!):
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.
Bonus Rule:
Where:
Voltage Drop Formula (in percentage)
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.)
Given:
Step 1: Find Current (I)
Since it’s resistive (100% pf),
Step 2: Use CEC Table D3 for K
Step 3: Use VD Formula
Step 4: Calculate Percent Drop
Over the 3% limit.
Step 5: Use CEC Table D3 for K
Now the percentage! Remember, it has to be BELOW 3%!
Step 6: Calculate Percent Drop
This means that we have to use a #6 AWG
UNKNOWN CONDUCTOR SIZE
When the conductor size is known, length is found by:
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.
VD= By multiplying the Applied voltage by 3% (the maximum percentage allowed by the CEC), we can find the maximum voltage drop allowed.
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:
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