[123]

rule 4-004 (14) makes sense as it is stated in the code. It compels us to avoid using wires with low temperature rating.
rule 4-006 rule is not offering any solutions, in case of long runs where it doesn’t make sense to run larger conductor just to keep a breaker happy.
rule 4-004 (1) and (2) (c) Four or more conductors in a conduit is wrong, including the table 5c.
It only works if all the conductors are the same size.
We can calculate resistance of conductors based on diameter. From the ampacity we can calculate the power (heat). From the volume and material of the raceway we can find a heat dissipation.
Aluminum rigid conduit will cool conductors significantly better than PVC conduit.

For an example a #12 AWG has 0.0055 Ohm/meter. At 10 A it dissipates 0.55W /m.
# 2 AWG has 0.00055 Ohm/m. At 100A it dissipates 5.5W/m.
A raceway with 9 wires #2 dissipate 50 Watt/meter.
A raceway with 3 wires #2AWG and 6 wires 12AWG dissipates 20 Watt/meter.
Both have to be de-rated by 0.7.
The raceway with mixed wire sizes is wrongly de-rated too much.
The rule and table also ignore the raceway material.

 

[KDC]

It's better to derate too much, than to throw umpteen billion more exceptions and tables to figure out.

While the current rules may be excessive from the physics standpoint, from the standpoint of safe installations, they're fine.

 

[jza]

They upped the ampere rating of #14, #12 and #10 in the latest release. That helps alleviate useless derating. A good start.

 

[FrunkSlammer]

Who even derates.. that's crazy talk.

Companies I worked for in the past, the rule was if you can fit the wires, it's good.

 

[123]

4-004 (1) c de-rating for more than 3 conductors.
Electricians know how to calculate a voltage drop, or use table D3 to find it.
Finding a power loss is just one more calculation.
Code should have a table like D3 for power loss. and it should be used instead of 5c.
Some de-ratings for amount of conductors are overdone.
Other times we should de-rate, even if the code doesn’t call for it.
Heat loss of fully loaded free air conductors is painful for anybody paying Ontario’s electricity rates.
In some instances they have to pay for the heat losses twice if they air-condition their plant.
4-006
When it comes to de-rating a 90 degree conductor because the circuit breaker is only 75 degree rated,
the conductor has to be de-rated only at the connection to the breaker. The rest of the run is fine.
I can splice the wires in a junction box 3 feet before the breaker and run larger conductor the last 3 feet.
I would like to see the code allow running the conductor without de-rating, and find a way to cool the conductors before the breaker. like running larger conduit last couple of feet, or have a longer loop of the wire in the panel, before it’s connection.

 

[BlackHowling]

. 4-006 When it comes to de-rating a 90 degree conductor because the circuit breaker is only 75 degree rated, the conductor has to be de-rated only at the connection to the breaker. The rest of the run is fine. I can splice the wires in a junction box 3 feet before the breaker and run larger conductor the last 3 feet. I would like to see the code allow running the conductor without de-rating, and find a way to cool the conductors before the breaker. like running larger conduit last couple of feet, or have a longer loop of the wire in the panel, before it’s connection.

There is a similar exception for motor circuits

28-104
(2) Where Table 37 requires insulation temperature ratings in excess of 75 °C, the motor supply conductors shall be not less than 1.2 m long and shall terminate in a location not less than 600 mm from any part of the motor except that for motors rated 100 hp or larger, their terminations shall be not less than 1.2 m from any part of the motor.