Disconnect Switch Between ATS and Transformer

[Anwar Pasha]

Is it essential (as per code) to install a disconnect switch between transformer and ATS. The installed ATS has the ability to test the generator on load and no load without disconnecting power of the services main!!

 

[iwire]

Is it essential (as per code) to install a disconnect switch between transformer and ATS.

Directly, no.

But most times the conductors on the load side of a transformer need overcurrent protection within 10'

The installed ATS has the ability to test the generator on load and no load without disconnecting power of the services main!!

I am not sure why you are emphasizing that, that is very common and not a problem.

When the ATS tests with load it disconnects the building from the utility and connects the building to the generator.

There will be a very short moment when the building has no supply at all during transfer. (unless it is a closed transition switch but I doubt that is what you have)

 

[Anwar Pasha]

Thank you very much for your response. The reason being assigned for having a disconnect switch between TF and ATS by the consultant is to ensure ease in maintenance without any intervention from the Utilities . Would you please guide me where can I find in NEC stating that Disconnect Switch is not required between ATS and Transformer.

 

[iwire]

Would you please guide me where can I find in NEC stating that Disconnect Switch is not required between ATS and Transformer.

You will find no such wording, the NEC is a permissive document. You would have to find wording that requires a disconnect switch ahead of the ATS but there is no NEC section that directly requires that.

If this transfer switch is supplied directly from the utility it is required to be a 'service rated' ATS and that means it is required to have externally operated manual controls to lock both sources in the off position.

Of course these controls do not stop the power from entering the ATS cabinet and the NEC does not require that. It is a just a design decision.

I agree with you that servicing this arrangement is problematic but it is not an NEC violation.

 

[ceb58]

Thank you very much for your response. The reason being assigned for having a disconnect switch between TF and ATS by the consultant is to ensure ease in maintenance without any intervention from the Utilities . Would you please guide me where can I find in NEC stating that Disconnect Switch is not required between ATS and Transformer.

Back up. Is this transformer a utility transformer? and are the conductors that will be going to the ATS service conductors and what amp rating?

 

[Anwar Pasha]

This is an utility transformer 500KVA , the ATS rating is 1200Amp, 3ph,220V, 60Hz

 

[ceb58]

You will find no such wording, the NEC is a permissive document. You would have to find wording that requires a disconnect switch ahead of the ATS but there is no NEC section that directly requires that.

If this transfer switch is supplied directly from the utility it is required to be a 'service rated' ATS and that means it is required to have externally operated manual controls to lock both sources in the off position.

Of course these controls do not stop the power from entering the ATS cabinet and the NEC does not require that. It is a just a design decision.

I agree with you that servicing this arrangement is problematic but it is not an NEC violation.

Bob beat me to it but to add to what he has said since these conductors are the service conductors then 230.70 would apply. But as stated this can be accomplished with a service rated ATS.

 

[buddhakii]

You will find no such wording, the NEC is a permissive document. You would have to find wording that requires a disconnect switch ahead of the ATS but there is no NEC section that directly requires that.

If this transfer switch is supplied directly from the utility it is required to be a 'service rated' ATS and that means it is required to have externally operated manual controls to lock both sources in the off position.

I installed a couple of transfer switches last year that were service rated. They had the breakers inside and control switches on the outside, however in order operate manually you had to open the front door which exposed terminals. I thought it was a code violation and over all a crappy set up. I voiced my opinion, but no one listened and the AHJ was ok with the set up. I don't think he caught that section of the code that requires manual operation. I think the next one I do I will be more persistent. They are water pumping stations and we service them. I hate working one them cause I gotta suit up anytime i want to do something.