310.15B(4)(a)
- Thread starter NICK D
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Good question. I have no idea.
It has no correlation with structure type. Subpart (a) refers essentially to a full-complement Multiwire Branch Circuit for whatever system is in use.NICK D said:
In a single phase system the circuit conductors would be L1-L2-N.
In a mid-tap-grounded 3? Δ system: A-C-N.
In a 3? Y system: A-B-C-N, where subpart (c) is the "exception".
That is a given... yes it counts. I guess the CMP figured it goes without mention if you are electrically qualified to be doing this adjustment. Basically, the neutral is not counted if the sum of the current on each conductor of the multiwire branch circuit will likely never exceed the sums of the circuits' ampacities. In the case of dedicated neutral, i.e. one hot, one neutral circuit, the sum of each conductor's current is double the current of the circuit.NICK D said:
NICK D said:
I think so for either a corner or mid tap grounded delta. Say you have a 100Amp service with all three phase wires from the delta. The most current you can have flowing at any point in the conduit would 300 amps (100 amps on each wire).
Now you add another neutral wire from the delta transformer. If you have 25 amps on this wire, the current on the other wires has to decrease by 25 amps. If you think about it, there is still no way to get more than 300 amps at any point. So we only count the 3 wires.
Steve
winnie
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The way to approach this is to ignore the question of 'is the neutral current carrying' (it almost _always_ carries some current), and instead ask the question: how many net 'conductors worth of heating' are in the conduit.
With a corner grounded three phase system, the grounded conductor most certainly counts as a current carrying conductor. If the load is balanced, then the grounded conductor carries as much current as the other phase conductors. All three conductors are producing heat.
With a center tapped delta system, the loads on the 'center transformer' are _single_ phase loads. If you have three conductors (the hots and the neutral from the center transformer) in a conduit, then the worst case is that two of those conductors are fully loaded, and one is not loaded at all. In any other im-balanced situation where all three conductors are carrying some current, the total heat produced is less than with two conductors fully loaded and nothing on the neutral. There are at most 2 net conductors worth of heating.
But if you bring the third leg in, then you can have situations in which all conductors are carrying full current, and you need to determine if the circuit configuration is such that the neutral is actually only carrying imbalance current.
Say you feed three hots and the neutral to a heating system that has one full current line voltage heater connected between two of the phase legs, and one full current half voltage heater connected between the third phase leg and neutral. Not a very realistic example, but consider it an extreme example of possible situation. In this case, all 4 conductors will carry full current, and the current flow on the neutral is _not_ the imbalance current between the other supply legs; in this extreme case all the supply legs are carrying the same current, yet there is full current on the neutral.
-Jon
With a corner grounded three phase system, the grounded conductor most certainly counts as a current carrying conductor. If the load is balanced, then the grounded conductor carries as much current as the other phase conductors. All three conductors are producing heat.
With a center tapped delta system, the loads on the 'center transformer' are _single_ phase loads. If you have three conductors (the hots and the neutral from the center transformer) in a conduit, then the worst case is that two of those conductors are fully loaded, and one is not loaded at all. In any other im-balanced situation where all three conductors are carrying some current, the total heat produced is less than with two conductors fully loaded and nothing on the neutral. There are at most 2 net conductors worth of heating.
But if you bring the third leg in, then you can have situations in which all conductors are carrying full current, and you need to determine if the circuit configuration is such that the neutral is actually only carrying imbalance current.
Say you feed three hots and the neutral to a heating system that has one full current line voltage heater connected between two of the phase legs, and one full current half voltage heater connected between the third phase leg and neutral. Not a very realistic example, but consider it an extreme example of possible situation. In this case, all 4 conductors will carry full current, and the current flow on the neutral is _not_ the imbalance current between the other supply legs; in this extreme case all the supply legs are carrying the same current, yet there is full current on the neutral.
-Jon
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