Generator in a outside building and powering the panel...

[CobyRupert]

Is the inspector allowed to overrule this?

No. Unless the municipality has their own more stringent rules. Perhaps the inspector is referring to having to derate the cable's ampacity if there's more than three current carrying conductors and the existing conductors aren't suitably oversized to do this? But seriously, if you want a technicality to try and argue against this, see Code section 300.3(B).
(I want to see how many pages of post we can carry on about whether or not to run 1 extra wire in the conduit. Interesting topic though.)

 

[aczlan]

It's where they draw the line with the code as the exemptions become too complex.
The Inspector was not concerned about sharing the neutral or the ground.
It all revolved around the definition of two sources in the same conduit. Which can occur if the gen set is operating and energizing the wires up to the breaker at the 200amp panel even though the generator breaker is still locked open at the 200amp panel. It's due to cable insulation faults in conduit, in particular underground conduit. Now the generator and utility can cross connect via the short.
There are a number of incidents commented on this site about underground cables going" Ka-Pow" due to time and environment. So the concern is real.

So, does the inspector say that 702.9 is invalid? If so, why did the NEC specifically say that this scenario is allowable?
How would the danger he referenced be any different than 2 current carrying cables from the panel in the same conduit doing the same thing?

Aaron Z

 

[GPintheMitten]

300.3(b)
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment-grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

All the cables are in the same conduit. He already has a neutral and ground. Now he wants to add 2 hots. The wires making up the circuit for the backup power will be in the same raceway. So 300.3 b is satisfied.

 

[GPintheMitten]

I think the inspector is just trying to justify a position he is taking rather than enforcing code.

This happens in other trades with inspectors. You have to show them where they are wrong and you are right.

I had to do appeal a decision on framing and used a variety of sources and calculations to.make my case and won it.

My brother had to do the same numerous times in his heating and cooling business.

 

[aczlan]

Looking at prices on Ebay, I can get a Kirk Key setup for $120ish and not have to deal with pulling more wires into the conduit. If the inspector is ok with it, I might go that route.

Aaron Z

 

[CobyRupert]

All the cables are in the same conduit. He already has a neutral and ground. Now he wants to add 2 hots. The wires making up the circuit for the backup power will be in the same raceway. So 300.3 b is satisfied.

Shhh. Don't tell the other side, cause I'm on your side, but the neutral for the subpanel circuit isn't in the conduit anymore when the generator is running. Only the neutral current from the main panel being feed by the genny is using that conductor to return. So while the subpanel is still fed through that conduit, any neutral current from the subpanel wouldn't go through the neutral conductor in that conduit back to the main panel, it would go directly from the subpanel's neutral bus directly to the genny neutral that it is connected to it. - A technicality? Ehh? An interpretation? maybe..

 

[GPintheMitten]

Shhh. Don't tell the other side, cause I'm on your side, but the neutral for the subpanel circuit isn't in the conduit anymore when the generator is running. Only the neutral current from the main panel being feed by the genny is using that conductor to return. So while the subpanel is still fed through that conduit, any neutral current from the subpanel wouldn't go through the neutral conductor in that conduit back to the main panel, it would go directly from the subpanel's neutral bus directly to the genny neutral that it is connected to it. - A technicality? Ehh? An interpretation? maybe..

Interesting, but the neutral is common to both the panel and the genny. It doesn't matter. The neutral for the subpanel is in the same raceway as well.

Think of it as a real funny shape copper wire with white insulation. The wire goes where it goes even if the current is returning to the source (the genny).

How's that?

 

[CobyRupert]

Ok, seeing how no one from the other side will/can argue, I'll play devil's advocate:

Interesting, but the neutral is common to both the panel and the genny.-Common yes, but a conductor of the subpanel feeder circuit when in genny mode? It doesn't matter. Not in this application, that's why I say go for it (if not using a kirk-key with no new wires). But it can matter, I believe the fine print notes (FPN) of 303.3 (of the hard copy?) refer to the increased impedance, I believe especially in a ferrous metal conduit, due to the (imbalanced?) magnetic fields. For a 30 or 50 amp subpanel with a couple lights as load, the unbalanced neutral current is diddley. It's the same reason you need to run three phases together and not in 3 conduits, same reason that when you run a (single conductor) lightning ground cable through metal conduit you have to bond the conduit to the conductor on both ends. Otherwise, (as I understand) the conduit acts as a "choke" to the magnetic field around the current The neutral for the subpanel is in the same raceway as well. -Wait, we're talking about just having 1 common neutral right? It's only a neutral "for the subpanel" when on utility power.

Think of it as a real funny shape copper wire with white insulation. The wire goes where it goes even if the current is returning to the source (the genny).

How's that?

 

[GPintheMitten]

Hmm. Buried conduit. It's probably PVC.

The reason I say it is common is that it the neutral wire from the generator is "spliced" in the subpanel via the neutral buss to the white wire in the conduit that is connected to the neutral buss in the main panel.

You made a good point that the (120 volt) current from the sub returns to the source to the generator via the white neutral from the generator when using it and returns to the main panel on the existing neutral wire when on the local utility.

Anyway I think I'm over my head. I better dog paddle to the shallow end now.

 

[MossRoad]

Looking at prices on Ebay, I can get a Kirk Key setup for $120ish and not have to deal with pulling more wires into the conduit. If the inspector is ok with it, I might go that route.

Aaron Z

Why do you want to use a kirk key setup? Its overly complicated(in my opinion, of course). Why do you need a captive key? All you need is an interlock like this Generator InterLock Kit Manual Transfer Switch to be perfectly safe. You can kill the grid power, then turn on the genny breaker, then start the genny, or you can start the genny, kill the grid power, then turn on the genny breaker. Either way, the genny and the grid power can never meet. Its not physically possible.

Can you show me a schematic as to how you'd not need two more hot wires with the kirk key setup? I'm not getting it.