Is the neutral disconnected in an AC disconnect?

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SolarAlternativesDesign

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Soulsbyville, California, USA
In an external ac disconnect in the inverter output circuit between the inverter and utility service, is there a code requirement or any other reason to disconnect the neutral conductor? Is there a requirement to NOT disconnect the neutral conductor, since it is bonded to ground in the service panel?
 
SolarAlternativesDesign said:
In an external ac disconnect in the inverter output circuit between the inverter and utility service, is there a code requirement or any other reason to disconnect the neutral conductor? Is there a requirement to NOT disconnect the neutral conductor, since it is bonded to ground in the service panel?
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No, there is no reason to break the neutral in the disco. Yes, there is a requirement to NOT break the neutral in the disco.
 
ggunn said:
No, there is no reason to break the neutral in the disco. Yes, there is a requirement to NOT break the neutral in the disco.
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i've seen disconnects in health care facilities that open the neutral.
it's rare, and may not be allowed per code currently, but i've seen
it in critical care areas... however, the first, last and only time i've seen
it was 35 years ago.
 
Generally speaking you don't disconnect the neutral.

If this were a DC disconnect there is 690.17 (D) [2014 reference], but I do not believe that applies to AC disconnects. Otherwise the relevant code is 404.2 (B).
 
iwire said:
Can you cite it?
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Apparently I cannot. The requirement to not break the neutral is one of those things I have "known" for so long that I thought it was in the NEC, but if it is I cannot find it. It may be in the interconnection guidelines from my local AHJ.

But 705.20, 705.21, and 705.22 all speak to the disconnection of ungrounded conductors in a disco, and the neutral is a grounded conductor, so although there may not be a NEC proscription against breaking the neutral it is definitely not required.
 
ggunn said:
, so although there may not be a NEC proscription against breaking the neutral it is definitely not required.
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I agree, not required but not prohibited.


The only section of the NEC I know that required neutral disconnection is for fuel dispensers.
 
SolarAlternativesDesign said:
Doesn't 404.2 (B) [Switches] apply and prohibit breaking the neutral in an inverter output circuit disconnect? Jaggedben suggested this. Is there any reason why the disconnect in that circuit should not be considered a switch for purposes of 404?
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It may apply, but so does the exception

404.2(B)(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.

Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor can-not be disconnected until all the ungrounded conductors of the circuit have been disconnected.
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I had overlooked the exception.
In this case the contractor had called out a Square D DU322RB 3 pole 60A disconnect. If we wired L1 to one pole and L2 to another pole we would have a simultaneous disconnect and neutral could, optionally, be wired to the third pole. However Schneider tech support says that this disconnect is rated for only one conductor using all 3 poles. Following their rating we would need 2 disconnects, which seems unusual to me, and then the neutral could not be switched as in the exception.
I need to find out how to specify a disconnect that will handle two 60A conductors. I'll call tech support again.
 
SolarAlternativesDesign said:
I was trying to figure out why the contractor called for a 3 pole disconnect when we probably only need to disconnect the L1 and L2 out of a 240V split phase inverter.
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Maybe the contractor is used to working with three phase inverters, or maybe they don't understand how solar works. In any case, all you need to break is L1 and L2.
 
I called tech support back and got a different person and I think I got the right information now.
She said that each pole can take a 60A AC conductor.
The answer to why the contractor chose a 3 pole disconnect is that Square D doesn't sell an unfused (required by PG&E) outdoor rated 2 pole disconnect - not related to disconnecting the neutral conductor, which we agree isn't necessary.
Thanks.
 
SolarAlternativesDesign said:
I called tech support back and got a different person and I think I got the right information now.
She said that each pole can take a 60A AC conductor.
The answer to why the contractor chose a 3 pole disconnect is that Square D doesn't sell an unfused (required by PG&E) outdoor rated 2 pole disconnect - not related to disconnecting the neutral conductor, which we agree isn't necessary.
Thanks.
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PG&E requires unfused disconnects? All my projects use fused disconnects because the unfused disconnects only have a 10kAIC rating and most of the time on a supply side interconnection in a C&I project the fault current is more than 10KA.

Here is a link to the PG&E spec for disconnect switches: http://www.pge.com/includes/docs/pd...lity/electrictransmission/handbook/060559.pdf

It states that you cannot switch the neutral (item 7).

One of the functional requirements is:
  • A fusible ac disconnect switch is required for generators that do not have over-current protection (i.e., breakers, fuses) at the point of interconnection with the utility.

So if the generator does not have OCP you have to use a fused disconnect, for others it is an option.
 
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