Note: This page uses the 2024 CEC codebook edition
A circuit loading/loading effect is the influences of an ammeter/voltmeter/other measuring device's internal resistance has on a circuit affecting its initial properties, such as voltage/current.
If a loading effect is enough, the results on the device maybe not accurate enough of any value.
E.g. For this circuit, expected voltages are:
VR1 = 8 V
VR2 = 16 V
This meter has a standard 10 MΩ internal resistance. The reading must be 8 V, so why does it read 5.7 V?--The meter reads the 6 MΩ resistor as if it's parallel to the meter's 10MΩ internal resistance--It's really only an issue with big resistive values.
If the resistive values are lower:
These values are closer to expected voltages.
An ammeter's resistance is very low.
This circuit has a meter at 5 Ω.
Expected:
IT = 36 mA
IR1 = 6 mA
IR2 = 30 mA
Loading effects are more pronounced if they measure current in a branch with a resistance value close to the meter's internal resistance value.
pronounced: noticeable
Material here is based on the 2025 CEC book.
Table 2 (T2; Pg 405), 4 (T4), 14 (T14; Pg 467) should be used.
The calculated load chart is needed for questions of 8-104.
Calculated load chart
Section 8 (Rule 8-000) is for needed codes for:
Max circuit loading
Loads for consumer’s services, feeders,
Branch circuits;
Use of demand factors;
Branch circuit positions required for dwelling units;
Car heater receptacles
Determining service/feeder/branch circuit's minimum allowable ampere rating, be sure the electrical installation's design meets the requirements for circuit loading and demand factors from Section 8 and other appropriate Sections of the Code;
takes into account some provisions for wiring changes that might be required as a result of future load growth and takes into account the customer’s budget.
Note: Numbers indicate the label.
Consumer's service conductors supply power from the connection point with the supply authority lines to the line side of main service disconnect.
Feeder conductors send electric energy from the main service switch's load side of the branch circuit overcurrent devices.
Branch circuit conductors send electric energy from last overcurrent protection device (fuses, circuit breakers) to outlets/device/electric equipments.
8-104 Max circuit loading (1) The ampere rating of a consumer’s service/feeder/branch circuit must be the ampere rating of the overcurrent device protecting the circuit/conductors' ampacity, whichever is less.
A circuit's calculated load must not pass the circuit's ampere rating.
1# A circuit has a 15 A circuit breaker with 10 AWG copper wire of 75 °C insulation. The conductor has 35 A.
What's the circuit's ampere rating?
Use T2: The breaker has a copper wire of 10 AWG. The conductor (the copper wire)'s insulation is 75 °C and has 35 A.
But the 15 A breaker limits the max current of the wire.
Its ampare rating is 15 A.
2# A circuit has 2 poles, 25 A circuit breaker with 12 AWG copper of 75 °C insulation. The conductor has 25 A.
What's the circuit's ampere rating?
2-Pole circuit breaker is a circuit using 2 hot wires.
25 A rating means the breaker trips if current exceeds 25 amps on the circuit's either pole.
"The wire used is 12 AWG copper with 75 °C insulation, which has 25 A"--means the wire safely carries 25 A current safely.
Use T2: Ampacity of 12 AWG copper wire with 75°C insulation is also 25 A.
Sice the circuit breaker is also rated 25 A, the rating is 25 A.
3# Circuit 3 has 3 poles, 70 amp circuit breaker with 6 AWG copper of 75 °C insulation. The conductor has 65 amps.
What's the circuit's ampere rating?
Use T2: Ampacity of 12 AWG copper wire with 75°C insulation is 65 A.
As the conductor only safely carry 65 amps, the circuit's ampere rating 65 A.
Calculated load in a circuit mustn't pass a circuit's ampere rating.
If the calculated load passed the circuit's ampere rating, overheating and fire may occur.
The calculated load in consumers’ service/feeder, or branch circuit is a continuous load unless in a normal operation it won't persist for a total of: 1+ h in a 2-hour period or 3+ h in 6-hour period if it doesn't exceed 225 A.
If a pot is turned on, it boils water, but if the pot is turned on for a few minutes then turned off for a while then turned on, it only gets warm.
Continuous loads makes more heat.
Note: We assume load is continuous, but it isn't.
Feeders/services for single dwellings aren't continuous loads (8-200 Single dwellings).
Feeders/services for dwellings units in an apartment aren't continuous loads (8-202 Apartment and alike buildings).
Switch ratings are max electric current/voltage a switch (e.g circuit breaker) can safely handle and disconnect without damage.
Switches and breakers are marked as suitable for either:
Continuous operation at 100% of ampere rating of overcurrent devices; or
Continuous operation at 80% of ampere rating of overcurrent devices.
A not-marked switch/circuit breaker is rated 80%.
Difference between 80% and 100% equipments
Overcurrent devices make heat if loaded.
To carry full rated load continuously an equipment must be built to handle the resulting heat.
100% rated equipment is “tougher" and more expensive.
Note: Use T14 - pg 467
How much power is needed for a new bank, a 3,000 square foot house with electric heat, a 950 square meter machine shop? -
The CEC has rules in section 8 to calculate the power required, based on an estimate for lighting and small loads with the building's area plus identified load (e.g. air conditioning).
Where a fused switch or circuit breaker is marked for continuous operation at 100% of ampere rating of overcurrent devices, the continuous load as determined from calculated load mustn't pass the continuous operation marking on the fused switch or circuit breaker.
a) Except as needed by Item b), mustn't exceed 100% of allowable ampacities of conductors chosen in accordance with Section 4.
What's the calculated load?
Is this load continuous? Rules 8-104 (5 & 6) only apply to continuous loads. If the load is non-continuous less robust, less expensive wiring may be acceptable.
Continuous load: a load lasting 1+ h in a 2-hour period or 3+ h in 6-hour period if it not exceed 225 A.
switch rating: max electric current (in amps) a switch (e.g. circuit breaker) can safely carry.
wiring method: ways conductors (wires) are installed.
Overcurrent devices and conductors for non-continuous loads can be loaded to 100%.
T2 and 4 are ampacities for multi-conductor cables or wire in Conduit.
Continuous load musn't exceed rated continuous operation of the circuit breaker, if rated at 100% of its ampacity.
The load musn't exceed 100% of the used conductors' ampacity, as per Section 4 of the code.
With single conductors, load must be limited to 85% of ampacity for safety and overheat.
If a fused switch/circuit breaker is marked for continuous operation at 80% of ampere rating of its overcurrent devices, the continuous load found by the calculated load musn't exceed the continuous operation marking on the fused switch or circuit breaker, except:
As required by Item b), musn't pass 80% of allowed ampacities of conductors selected in accordance with Section 4; or
shall not pass 70% of allowed ampacities of single conductors selected in accordance with Section 4.
A panelboard has a calculated 238 amps load, fed from a circuit breaker with R90 copper conductors in conduit.
What's the breaker's min size for this load?
What's the conductor's min size for this?
The load is continuous, as it's 238 amps.
Switch rating is 100%.
The wiring method are single conductors, for T1 and 3, with conductors carrying more current for a given size and heat the overcurrent more.
The derating factor is 85%.
If the switch or breaker is rated for continuous operation at 80%, the derating is heavier.
A panelboard has a calculated 238 amps load and is fed from a circuit breaker with R90 copper conductors in conduit.
What's the min size of circuit breaker for this load?
What's the min size of conductor for this?
Step 1 Use the calculated load chart.
Calculated load is 238 A.
Step 2 Is the load non-continuous? - No, it is continuous.
Step 3 How's the switch rated?
- It has no rating, so it's 80% rated.
Step 4 What's the wiring method? - It's wired in conduit.
Conductor ampacity is from T2 or T4.
Step 5 Max circuit load = 80%
Step 6 calculated load = 80% x circuit ampere rating
= circuit ampere rating = calculated load 80% = 238/80% = 298 A
Min overcurrent device is 298 A.
MIn conductor ampacity is 298 A.
Step 7 Use T2
Step 8 Use T13
Step 9 The panelboard has a 238 amps calculated load and fed from a circuit breaker with 75°C copper conductors in conduit.
Min size of circuit breaker for load : 300 A
Min size of conductor for application: 300 kcmil R90 copper conductors in conduit
Notes: Since 2017, panel terminations are rated 75°C--even though we may use 90°C conductors, we use the 75°C column for ampacity. So now we need 350 kcmil.
1# A 158 amps load is non-continuous, since it doesn't last 3+ hours.
2# A 220 amp load only works for 20 minutes of an houe is non-continuous, becaise
3# A 3/0 AWG copper conductor in conduit protected by an 150 amp fuse. What's the circuit ampere rating?
Use T2: if R90 (90°C) insulation, ampacity is 225A.
Use Ovecurrent Protection Rule: CEC 14-104 says overcurrent device rating must not pass conductor ampacity unless exceptions.
Fuse is 150A, which is under conductor ampacity (225A), circuit ampere rating is foud by fuse.
4# A 3/0 AWG copper conductor in conduit protected by an 200 amp fuse. What's the circuit ampere rating?
R90 = 90°C
Use T2: if
UNSURE ANSWER 5# A panelboard is fed with 1 AWG, R90, single conductor copper cables in free air and protected by a 200 amp fuse. The switch containing the fuse is rated for continuous operation at 100% of its rating. What is the maximum continuous load on the circuit?
Use calculated load chart: The switch's calculated load is at 100%, so at its full 200 A, given these info:
a single conductor
rated continuous operation at 100% its rating
Use T2: Panelboard rated 90 (90 °C) and is 1 AWG, has 145 A ampacity, despite the 200 A fuse.
UNSURE ANSWER 6# What's the ampere rating of a branch circuit with conductors rated 310 A protected by a 300 A circuit breaker?
The breaker limits the current by 300 A, which is the ampere rating.
UNSURE ANSWER 7# A panelboard is fed with 2/0, R90, copper conductors in conduit and protected by a 200 amp fuse. What is the ampere rating of the circuit?
It's protected by a 200 A fuse, so 200 A is the ampere rating.
UNSURE ANSWER 8# A panelboard is fed with 2/0, R90, copper conductors in conduit and protected by a 200 amp fuse. What is the maximum non-continuous load on the circuit?
Use calculated load chart for the circuit: We need a non-continuous load, so the max circuit load is at 100%, so 200 A.
UNSURE ANSWER 9# A panelboard is fed with 2/0, R90, copper conductors in conduit and protected by a 200 amp fuse.
What's the max continuous load on the circuit?
Use calculated load chart for the circuit: We need a non-continuous load, so the max circuit load is at 100%, so 200 A.
[M1] CEC 2024 Code Book 26th Edition required (Codebook updates each year)
[M1.2] All CEC 2042 tables - Canva
[1]
[Q1] Flashcard
[Q2] Exercise