is a transfer switch required

[wyboy]

On a set of plans, the engineer shows legally required standby power (NEC 701 not NEC 700 for emergency systems). The main service conductors are “tapped” ahead of the service disconnect and terminate in a separate service disconnect per NEC 701.12 (G). I argued the need for a transfer switch but cannot find a specific requirement. The engineer said a transfer switch is not required. After reflection, I agreed that if NEC 701.12 (G) is used to provide legally required standby power, a transfer switch accomplishes nothing. In truth, I have seen this type of install before. My question: is a transfer switch required (give code reference).

Thanks!

 

[Terminator5047]

I think it will cuase nothing but problems having a new source of power come on out of phase such as tripping rooftop units elevators anything with a motor

 

[jap]

Where does the Legally required standby power come into play in this scenario?

There is nothing in this that indicates the source of the standby power.

If existing service conductors are tapped ahead of the service disconnect, and, terminate in another serviced disconnect,then, you've simply added an additional service disconnect.



JAP>

 

[don_resqcapt19]

If you are using the line side tap as your legally required standby system power source no transfer switch. There is no second power source to isolate, so no need for a transfer switch. This assumes that all of the legally required standby loads are directly connected to the second service disconnect.

I just cannot imagine any AHJ permitting the line side tap as the power source for a legally required standby system.
Doing that assumes that your utility source will never be out. Not an assumption that I would ever make.

 

[don_resqcapt19]

Where does the Legally required standby power come into play in this scenario?

There is nothing in this that indicates the source of the standby power.

If existing service conductors are tapped ahead of the service disconnect, and, terminate in another serviced disconnect,then, you've simply added an additional service disconnect.



JAP>

Exactly the legally required standby power is the same utility source that powers all of the other loads in the facility. However, with the approval of the the AHJ, this is permitted by 700.12(G).

 

[wyboy]

I agree. it just is another service disconnect. But it looks like it is allowed by 700.12(G). Why is this allowed? It makes no sense to me.

 

[texie]

Of course no transfer switch as there is not actually a second source. 701.12(G) used to be a pretty common practice back in the old days. I have not seen this as an acceptable source by an AHJ in at least 40 years or more.