ECN Forum Forums General Topics of the Trade General Discussion Area conductor insulation

sid123456:



As Bryan (BPHgravity) pointed out, it is Code Compliant on that Residential Feeder to use 2/0 THHN cu for a 200 Amp Circuit.
This is due to the fact that the Feeder will rarely draw anything close to the 200 Amp level, and extremely rare to draw that much of a load for 3 Hours or more.

Even if the example 200 Amp Circuit / Service Feeder was in a Commercial Environment, if there are only 3 CCCs, the Maximum OCPD for the 3/0s may be selected from the 75º column - or Terminated at 200 Amps.

sparkyinak:



Thanks!!!

It has been covered here a few times over the years, so the writing gets easier each time

SolarPowered:

*** Per: (if you have questions why 8 CCCs instead of 6, please let me know) ***



The reason behind this is that most of the time the Common Grounded Conductor will be carrying >= 70% of the highest L-N Load Current value - and more commonly, 100% the highest L-N value.

If the MWBCs are driving Loads which are nearly Pure Resistance, or >.95 PF with very low Harmonic Distortion, then the Common Grounded Conductor will carry a much lower Load Current.
This is not too common - especially in my line of work, so I always consider the Common Grounded Conductor to carry 100% of the highest L-N Load value - when applied to MWBCs (4 Wire Multiwire Branch Circuits).

Of course, any 2 wire L-N Circuit does not have a "Common Neutral", only an Ungrounded Conductor + a Grounded Conductor - so each Conductor is carrying 100% Load Current, and figured as such.

Similar results are encountered with 1 Phase 3 Wire MWBCs from a 3 Phase 4 Wire Wye System.
An L-L-N Circuit is more like a 3 Phase 3 Wire "Open Wye" Circuit - even though it contains a Grounded Conductor from the Star Point of the Secondary Windings.

These MWBCs typically have the higher Load Current value of one given Ungrounded Conductor flowing on the Grounded Conductor.

I can post more information later.

Scott