Ambient temperature
Ambient temperature
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Ambient temperature (or ordinary/baseline temperature) is the average temperature of an environment's air.
ambient: about one's surrounding
Derating
Derating
Derating is to reduce a component/device's power rating.
The derating/correction factors equations:
de-rated ampacity = ampacity x de-rating Factor
ampacity = de-rated ampacity / de-rating Factor
Ampacity
Ampacity
Ampacities are the max amount of electrical curent a conductor can carry without overheating. Ampacity is measured in amperes.
ampacity = ampacities x ampacity correction factor
Thousand circular mils
Thousand circular mils
[5] Thousand circular mils (Kcmil or MCM) is a measurement unit for area. It equals to a circle's area with a diameter of 1 mil (1,000 of an inch or 0.0254 mm), π/4 square mils, or ~5.067×10−4 mm2. It intends to refer to a wire's area with a circular cross section.
As its definition contains π, we can calculate area values in circular mils if we know the diameter in mils.
It's used for electrical conductors' cross-sectional area, used in North American electrical industry to identify conductors above 4/0 AWG.
1 MCM = 0.5067 square millimeters
1 mil = 1/1000 of an inch
A 200 mils wire in diameter = 40 MCM
[4] image
Insulation
Insulation
Insulation means protection, measured in R-value.
Insulation failure is when equipments/systems' insulation worsen and can no longer stop electric flow in wrong paths.
An insulated aluminium is an aluminum with a non-metallic insulation layer.
Ampacity charts
Ampacity charts
Ampacity charts/tables show the max electrical current amount conductors can safely carry.
[2] Engineers use them to tell a wire's size and insulation rating needed for an application and the max amperage capacity or the max amperage capacity of a bus bar's size.
insulation rating: rating of how well insulation resists heat transfer
bus bar: metallic bar, often in a switchgear
Ampacities under 3 insulated copper conductors, under 5000 V and unshielded, in cable, under 30°C ambient temperature:
T2 - Pg 405
Ampacities under 3 insulated aluminium conductors, under 5000 V and unshielded, in cable, at a 30°C ambient temperature:
ampacities for -3 insulated alluminium conductors, -5000 V - T4 - Pg 409
T5A - Pg 411
T5B, 5C, 5D - Pg 412
T19 - Pg 475
Using ampacity charts Example 1
Using ampacity charts Example 1
You run power cables through a boiler room's ceiling and measured an ambient temperature of 40 °C.
By how much would you need to de-rate the conductor ampacity if it had 75 °C insulation?
Ambient temperature is 40 °C and insulation temperature is 75 °C.
Use T5A: Derate the conductors by 0.88.
Using ampacity charts Example 1 Pt 2
Using ampacity charts Example 1 Pt 2
After derating, conductors must have 200 amps. What must the ampacity be prior derating?
de-rated ampacity = ampacity x de-rating factor
ampacity = de-rated ampacity ÷ de-rating factor
= 200÷0.88 = 227 A--Ampacity must be 227 A before derating.
Example 2
Example 2
A conduit has 7 conductors, which are 3 AWG copper with 60°C insulation (one is a bond).
What's each conductor's ampacity?
Use T2: 3 AWG copper with 60°C insulation has a base of 85 amps.
For 3+ conductors in a conduit: In Table 5C, 7 conductors, has a 70% or 0.7 derating factor.
85A × 0.7 = 59.5 A--Each conductor has has 59.5 ampacities.
Heat
Heat
A stove element is hotter than a pot-heat is sent to the pot.
A conductor's normal heating: With a wire piece with a load producing 10 watts of heat, its whole length dissipates heat.
The right thermometer indicatse conductor temperature.
Left thermometer idicatse air temperature.
The wire must be hotter than air.
The greenline is the max conductor temperature.
What happens if the air temperature increases?--The condcutor temperature reduces.
The wire must be hotter than air.
If the max conductor temperature is exceeded, what to do?--Reduce the produced heat.
Tables 1 - 4 Based on Ambient
Temperature of 30 ºC
What if the ambient temperature is above 30 ºC?--
Mutual heating
Mutual heating
3 conductors in a conduit under load make heat and heat each other.
More conductors leads to more mutual heating.
The temperature of the conductors goes up.
At some point there's insulation failure.
To manage this,
The method: 3+ conductors, if we lower the current through conductors, we lower the created heat.
Example 2
Example 2
The right conduit has 9 conductors with current.
How much must we de-rate the conductors for this?
conduit: a tube to protect wires/carry water
answer: T5C
9 = 7 – 24 range so we de-rate by 0.70
CEC Code Book Page 412
Example 3
Example 3
If Example 1's conductors are 8 AWG copper with 90° C insulation.
What's this installation's ampacity?
De-rated Ampacity
= Ampacity x De-rating Factor
= 55 x 0.7 (Table 2 x Table 5C)
= 38.5 A
Rule to count conductors
Rule to count conductors
For rule 4-004 only, we only count a conductor if it carries significant current and we don’t count:
neutrals if balanced
bonding conductors
grounding conductors
control conductors
Example 4 - T5C
Example 4 - T5C
The conduit on the left- side has 4 conductors. 2 live conductors, 1 neutral and a bonding conductor. What derating factor is applied?
Here only the 2 live conductors count and neutral if it's expected to carry current – count is 3.
The de-rating factor is 1 (no de-rating required).
Required materials
Required materials
[M1] CEC 2024 Code Book 26th Edition required (Codebook updates each year)
[M1.2] All CEC 2042 tables - Canva
[Q1]
[Q2] Practice questions
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