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Does the neutral wire float or go to the neutral bus on a sub panel?

  • Maybe I don't understand the question, but I don't see how the subpanel would work if the neutral was left to float? I don't think it's supposed to be bonded to ground at a subpanel, so if left to float, how would the panel work at all? Or is the question referring to bonding ground and neutral at the subpanel? – Johnny Oct 25 '13 at 21:14
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No. The grounded (neutral) wire should not float.

In a 120/240V single split phase system, a subpanel should be wired similar to this.

Subpanel Wiring

Main Service Panel

Notice that in the main panel the grounded (neutral), and grounding buses are bonded (electrically connected). This means that grounded (neutral), and grounding conductors of a branch circuit can be connected to either bus.

Subpanel

In a subpanel, the grounded (neutral) and grounding buses are not bonded. Which means grounded (neutral) and grounding conductors of branch circuits, must terminate at the appropriate bus.

Separate Building

If the subpanel is installed in a separate building, it might be supplied with a 3 or 4 wire cable, and there may or may not be a separate grounding electrode system installed. There are numerous factors that determine how a subpanel in a separate building is wired, which is beyond the scope of this answer.

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  • Earth and neutral are combiner in one rail? That is that US code? only I suspect. EU does not allow that at all. – Piotr Kula Oct 25 '13 at 11:57
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    @ppumkin "In a 120/240V single split phase system", yes earth and "neutral" are bonded at the main service disconnect. This is why the grounded (neutral) conductor is called the "grounded conductor". Not to be confused with the grounding conductor, which is earth. – Tester101 Oct 25 '13 at 12:13
  • I've always seen the neutral and ground wires on separate rails even on a main panel where they are bonded together - is that just an electrician's personal preference, or do some inspectors or local codes require (or prefer) the separation? – Johnny Oct 25 '13 at 22:09
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    @Johnny It's personal preference. Some inspectors and/or Electricians are pickier/more anal than others. Arguments can be made either way, but in the end the bars are bonded and should provide the exact same path to ground. Remember, this is only at the main service disconnect, which may or may not be the main service panel. – Tester101 Oct 25 '13 at 22:34
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    I think it is better to separate grounds and neutrals, because all too often a main service panel becomes a subpanel. At which point it would have to be wired that way. – Brad Gilbert Oct 26 '13 at 14:32
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The subpanel's neutral bus must be tied to the neutral on the master panel. Neutrals should never be allowed to float. That always causes problems down the road.

The only weirdity of a subpanel is that the grounds are a separate bus (apart from the neutrals) and that is independently connected to the main panel's ground/neutral bus. There is a screw which connects the bus to ground: For a subpanel, remove that screw.

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