The question that was given to me is that if you have 6 two wire circuits each with it's own neutral in the same conduit, can he use 310.15(B)(5)(a) and not count the neutrals.

In this case, there are no "Neutrals" - only Grounded Conductors and Ungrounded Conductors.

George's scenario states "6 Two-Wire Circuits", which results in 12 Current Carrying Conductors.

Derating Factor will be 50%.

Installing (12) #10 THHN CU. Conductors with 20 Amp OCPDs will comply.

BTW, these Two-Wire Circuits could be any of the following flavors;

- One Ungrounded Conductor + One Grounded Conductor,

or; - Two Ungrounded Conductors.

In either case, there will be Two Current Carrying Conductors per Circuit.

The time a Grounded Conductor may be determined as a Common "Neutral" is when it is part of a Multiwire Branch Circuit.

For the Common Grounded "Neutral" Conductor of a Multiwire Branch Circuit to become a "Balanced Load Carrying Conductor", the System needs to be derived from a Single Phase 3 Wire Transformer - either via the Center Transformer of a 4 Wire Delta System, or from a "Stand-Alone" 1 Phase 3 Wire Transformer.

The "Center-Tapped" Secondary Winding will have only the Imbalanced Load flowing, when Two L-N Connections are made across the entire length of the Coil.

Simply stated, using L-N-L (AKA: 1 Phase 3 Wire Multiwire Circuit) with a 10 Amp Load between the "A" Line Output and the Center Tap, and a 15 Amp Load between the "B" Line Output and the Center Tap, the Center Tap Load will be the imbalanced Load value - which is 5 Amps.

This occurs due to the "Common Load" flow running across the entire length of the Secondary Coil - as if the connection was a 10 Amp, 240V Load; and the imbalanced "Half-Winding Section Load" flow being circulated between the Center Tap and one half of the entire Secondary Coil is 5.0 Amps.

In this case, we will say the Right-Hand side of the Secondary Winding's end will be Line "B", so the Winding Section between the Center Tap and Terminal "B" will have 5.0 Amps circulating, and the entire Winding will have 10 Amps circulating.

Let's say the Primary of this Transformer is 480V; the Primary Load will be 6.25 Amps - or 3.0 KVA

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With a Wye System, the Common Grounded Conductor will only "Balance" under certain conditions - mainly when the connected Loads are equal and at Unity Power Factor.

Three equally sized, equally loaded 1 Phase Motors across a 3 Phase 4 Wire Wye Multiwire Circuit will result in a low Percentage of L-N Load Current flowing.

Other than these Load types, the Common Star Point Derived Grounded Conductor of a Wye System will draw close to the same Load Amps as the Highest L-N Load value.

With Reflective Loads (Multiple Harmonic Frequency Load Currents Reflected back into the Power System), the Common Grounded Conductor will carry the highest L-N Load Amperes (Fundamental Amperes), along with 3 times the Percentage of Reflected Harmonic Amperes.

Example:

THD = 10%.

- Load's Fundamental Amperes = 10.0 Amps,
- Load's Harmonic Reflected Amperes = 1.0 Amps,
- Total Load Amperes = 11.0 Amps.
- (3) of these Loads connected L-N (Phase A-N, Phase B-N, Phase C-N),
- Load on "N" = 10.0 Amps Fundamental, plus 3 times the Harmonic Reflections (1.0 Amps x 3),
- Total Load on "N" = 13.0 Amps.

Using a 1 Phase 3 Wire Multiwire Circuit from the 4 Wire Wye creates an "Open Wye" Polyphase System, so once again the Common Grounded Conductor will be a Current Carrying Conductor - functioning similar to a 3 phase 3 Wire Branch Circuit.

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Lastly, not all Grounded Conductors are the same as a "Neutral", and vice-verse...

The Grounded Conductor of a 3 Phase 3 Wire Corner Grounded Delta is not a "Balanced Neutral" at all.

This Grounded Conductor is one of the three system phases. The only difference is it is intentionally ground bonded. Otherwise it functions exactly the same as any of the remaining Two Ungrounded Conductors on the System.

Will stop here and wait for feedback.

-- Scott