August 20, 2025
Conductor Derating & Sizing Guide for CEC 2024
Red Seal Exam Question Guides
August 20, 2025
Red Seal Exam Question Guides
Why Do We De-rate Wires? Because wires are like athletes — they can carry a lot of current (their job), but only if they don’t overheat. If the environment is already hot, or if they’re crammed in a group, they can’t cool off properly… and that’s when we need to ease up the load.
Wires don’t like heat — they already get warm when current flows, and if the air around them is hot too, they can’t release that heat.
So, in hot environments (like attics or rooftops in summer), we de-rate the wire — meaning we lower the amount of current it’s allowed to carry safely.
If it’s 25°C out, the wire jogs with no sweat.
But at 40°C? It’s like running in a sauna — better slow it down (carry less current).
When you bundle a bunch of wires together in a conduit or raceway, they start to share heat like people packed in a crowded elevator.
No space = no airflow = no cooling.
So again, we de-rate — each wire gets a smaller current limit so nobody overheats.
CEC Rule 4-004 gives us tables with multipliers.
Wires heat up when they work.
If the world is hot, or they’re packed in tight, they can't cool off.
So we de-rate them = they carry less current safely.
W- Where are the wires? What is the ampacity? (Tables 1-4)
T- Temperature (ambient, Table 5A)
F- Fill (how many conductors are in the conduit, Table 5C)
And remember: Rule 4-006 applies here too.
What is the ampacity of seven #12awg TW75?
W- (where): Table 2 → 20A
T- (temperature): n/a
F- (fill): Table 5C (7 conductors) → 0.70 multiplier
20A × 0.70 = 14A
Note: Since copper/aluminum not specified, assume copper. Since conduit/free-air not specified, assume conduit → Table 2.
What is the ampacity of #12awg TW75 in a room that is 42°C?
W- (where): Table 2 → 20A
T- (temp): Table 5A, 42°C → 0.71 multiplier
F-: n/a
20A × 0.71 = 14.2A
Note: Use the same temperature column from Table 2 when applying Table 5A.
What is the ampacity of 7 #12awg TW75 in a room that is 42°C?
W- (where): Table 2 → 20A
T- (temp): Table 5A, 42°C → 0.71 multiplier
F- (fill): Table 5C (7 conductors) → 0.70 multiplier
20A × 0.71 × 0.70 = 9.94A
What is the R90 conductor size feeding a 75A 3ø 4-wire load in a room that is 50°C? Equipment Rating = 75°C.
Step 1: Start with 75A load (Table 2, 75°C column).
Step 2: Apply Table 5A, 50°C multiplier → 0.75
Step 3: Apply Table 5C, 4 conductors multiplier → 0.80
75A × 0.75 × 0.80 = 45A → too low, conductor must be upgraded.
To upgrade, reverse the math:
75A ÷ 0.75 ÷ 0.80 = 112.5A
Looking at Table 2, 75°C column → #2 AWG is the correct conductor size.
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