Tandem breakers for PTAC units

[joshfromoshoto]

I was told by an engineer that there's a code preventing shared neutrals for PTAC units. The inspector told me it was fine. Basicly were re modeling a motel and are trying to get away with using 12/3 to power 2 units.

Sent from my SM-A037U using Tapatalk

 

[jaggedben]

I don't believe the engineer is correct about shared neutrals. There is certainly nothing specific to PTAC units although there might be something about disconnecting means for motor equipment that trips you up indirectly. I gather the units are 120V.

But why does your subject line mention tandem breakers? You cannot put the ungrounded conductors for a multi-wire-branch circuit on a tandem breaker. If that's what you did and the engineer called you out then they were correct.

 

[retirede]

By way you worded this, I’m guessing the units are 120V?

Using a tandem, you can’t share a neutral - it will overload the neutral.

You could use a two pole breaker, but in that scenario, if one PTAC fails and trips the breaker, the other room also shuts down.

 

[norcal]

The only twin breakers that can be used for multiwire or 240V circuits are GE, only other ones are the now obsolete Zinsco, Challenger, & Crouse-Hinds.

 

[jaggedben]

The only twin breakers that can be used for multiwire or 240V circuits are GE, only other ones are the now obsolete Zinsco, Challenger, & Crouse-Hinds.

I don't consider the GE ones tandems, they are two-pole breakers, just skinny. Can they be installed on only one phase? (Can't remember.) Same with the Challenger, aren't they 2-pole common trip?

Not familiar with the Crouse Hinds.

 

[norcal]

I don't consider the GE ones tandems, they are two-pole breakers, just skinny. Can they be installed on only one phase? (Can't remember.) Same with the Challenger, aren't they 2-pole common trip?

Not familiar with the Crouse Hinds.

With GE, & the obsolete C-H, & Challenger, it possible for 240V without a quad.

 

[kwired]

Thread title and OP content don't really go together - maybe some misunderstanding of terminology?

True tandem breakers will connect both poles to the same supply line (phase). If you share a neutral for this it will carry twice the current as the ungrounded conductors.

If you correctly share that neutral across two/three supply lines then the neutral only carries unbalanced current that will not exceed any one ungrounded conductor. But in such case you also need handle ties or a common trip multipole breaker for the multiwire branch circuit.
That said I know that for say QO series tandem breakers they do make a handle tie that can be used between adjacent breakers, so that would would work here. You must use two adjacent tandems and use the inner two handles to derive that circuit though.