I was wondering what protects the neutral wire feeding a subpanel from over amperage draw? Near as I can tell, the only circuit breakers are on the two 'hots' in the main panel, thus it would seem that they are providing this protection? After giving this a little thought, it seems that the neutral wire feeding the subpanel should be twice the amperage rating of each ‘hot’ feeding the panel?

Here's a specific example for this question: a 50 amp subpanel is wired next to the main panel using #6 copper and a 50 amp breaker in the main panel. The subpanel is wired with eight 20amp 120volt branch circuits, each protected with a 20amp breaker. The 20amp breakers are set up in the subpanel with four on each 'hot' feed coming from the main panel. I believe that 50 amp breaker in the main panel allows EACH hot wire to pass 50 amps of current before tripping (that is to say, two 50 amp single pole breakers are 'ganged together' at the factory for this purpose). Now the question: in the subpanel, if each of the eight 120volt branch circuits were to draw 10 amps at the same time I believe the 50amp breaker in the main panel would NOT trip as each 'hot' is only drawing 40 amps. However, the neutral (which is servicing both ‘hots’ for 120volt wiring) would be carrying an 80amp load? (which would be bad using 6 gauge wire :-)

My first question, is my logic all catawampus in some way, have I missed something obvious in this supposition?

Second, if this is the case, should the neutral wire feeding the subpanel be gauged for twice the amperage of each ‘hot’ wire feeding the subpanel?

And third, if my logic is sound, has this been addressed by someone already (the national electric code or similar)?

Thanks in advance for any thoughts you may be able to offer.

tkrussell

Read this:

Split phase - Wikipedia, the free encyclopedia

To help you understand how this works, and to correct your thinking.

And a neutral is to never have overcurrent protection, in the USA.

Proper wiring and load balancing will prevent any overcurrent in a neutral.

stanfortyman

Yes, your logic is flawed. In fact it is the exact opposite of what you think.

The neutral does NOT carry the combined current. It carries the imbalance of the two hot legs. So theoretically the neutral can carry "0" current if both legs are carrying the exact same amperage.

For example:
If leg A were carrying 25 amps
and
Leg B were carrying 13 amps
The neutral would be carrying 12 amps.

Get it?

Credendovidis

Electrical supply is based on a circuit. A circuit made by the supply transformer on a "hot" phase and a neutral via a fuse panel to a load.
The wiring of that circuit has to be based on the maximum current in that circuit.
So a 60 Amps circuit has be wired from transformer to fuse panel with AWG 3/0 or AWG 4/0 wiring, depending the distance between transformer and panel.
But once in the panel, the load is split up into sub loads.
And each of these sub loads have to be wired accordingly to the rules. AWG 4, 2, 1, etc. depending on the load and the distance from panel to load.
Now if we split the entire main lot into two, using a second (sub) panel, the wiring between these panels has to be related to the max current involved in that circuit.
And in every case each sub load has wiring depending on load in the circuit. So that mean both the "hot" and the neutral wires have to be of the same size.
Never and nowhere within a circuit a neutral is bigger or smaller that the "hot" wire.
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In two or three phase systems the conditions differ from what I understand you are dealing with.
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In two phase systems there is 180 degree phase shift between phases. The current through neutral is the difference between the currents in the phases wires.
If the phase wires carry the same current (a 220 Volt only system), the neutral does not carry any current.
If the two phase system has 220 volt circuits and 11o volt circuits, the neutral carries the difference between the phase wires. So the neutral current can be anything between nil and the highest current in any of these phase wires, but never higher (because the difference in current in the phase wires equals the current in the neutral wire).
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Note that in a three phase system that is different again, as the currents are here 120 degrees out of phase.But I think we better leave that to the specialists!
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But back to sub load circuits : equal wires in each circuit, regardless if it is a "hot" or a neutral wire, or is a 1, 2, or 3 phase system! So you logic is not sound, as it is based on the wrong assumptions.
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I hope this helps you!