TRANSFORMER DISCONNECT/PROTECTION

[arvinolga]

Hi!

Can anybody help me regarding my query?

We are required to provide a new transformer to one of our buildings but the location of the nearest tappong point is a bit far from the location of the new transformer room where the new transformer will be located. It is about 300meters but the location of both mentioned above is only inside the same premises.

Question: Is it ok not to provide a disconnection/protection device if I already have a circuit breaker in the tapping point location? This means that the new transformer room will not have a circuit breaker in the primary of the transformer and will just use the circuit breaker in the tapping point, which is about 300meters away, as discussed.

Transformer: oil immersed, 1000kVA, 13.8/0.415kV dyn11

Appreciate any feedback and possibly some reference in the NEC where I can find the justification.

Thanks
Arvin

 

[rbulsara]

If I understood your setup, it is not ok.

Look up NEC Article 450 in its entirety.

A primary protection, fuse of circuit breaker, is always required.

The secondary side overcurrent protection device only protects the secondary conductors. In certain set up secondary device may help protect against overload on the transformer, but never protects the transfomer from short circuit.

 

[rcw retired EE]

If you have a circuit breaker or switch at the tapping point with protection (fuse or protective relays) set to protect the primary cables and the transformer, it may be OK.

Your local codes might prohibit having a transformer in the building without a local means of disconnecting it. Depending on the transformer location, the secondary protection device may serve as the building power disconnect and meet this requirement.

 

[l3city]

arvinolga: previous fellas are right. Also, I see you have some distance of 300 meters. I recommend taking a look at NEC 240.21(B)(5). This "feeder tap rules" section (NEC 240.21 (B)) will help you determine if you need a disconnect or not.
regards...

 

[rbulsara]

arvinolga:

I am sorry I misread your question earlier.

What is the primary voltage?

If it is less than 600V, the breaker at the tapping point will be acceptable and will meet the code for transformer protection. You do NOT need additional local disconnect at the transformer rated less 600V or less.

You do need local disconnet on primary side for transformers rated greater than 600V. (NEC 110.58)

 

[dpc]

rbulsara,

I believe NEC 110.58 only applies in tunnels. (Learned the hard way a while back.)

Regards,

dpc

 

[l3city]

dpc you wrote: "I believe NEC 110.58 only applies in tunnels.(Learned the hard way a while back.)"

Can you be more specific? I would to know why in tunnels only? I don't see the difference between a XFMR/electrical equipment disconnect "within sight" above ground than in a tunnel (except for enclosures or ambient conditions). Electrical equipment is electrical no matter where. I'm in no way challenging your opinon, but I will like to know/learn why you say "NEC 110.58 in tunnels only".
regards

 

[dpc]

It's not my opinion - that's what it says. Paragraph 110.58 is part of Section IV in Article 110. Section IV applies to Tunnel Installations over 600 Volts. It's an NEC requirement.

I really don't know why this specific requirement is in the NEC. Electrical equipment in tunnels probably poses a higher risk than equipment located outdoors or in a more normal environment due to limited access and means of egress/escape.

For equipment NOT in tunnels, there is no requirement in the NEC for local disconnect on transformer primary unless required to satisfy other requirements. There are many industrial facilities that have no local disconnecting means on transformer primaries, regardless of the voltage. There is not much in or around a transformer that anyone would be tempted to "service" in an energized state. Motors are a different matter, but even for motors there are some exceptions regarding the local disconnect requirement.

In general, the NEC does NOT require local disconnects for electrical distribution equipment. For example, a motor control center does not have to have local disconnecting means. It can be fed directly from an upstream circuit breaker in a 480V switchboard thousands of feet away. You may decide to provide local main breakers or disconnects for other reasons (I often do), but it is not a requirement of the NEC in most cases.

However, if this is serving a separate building, you are required to provide local disconnecting means for all feeders serving the building, similar to requirements for a service (six handle rule can be applied). See Article 225.

 

[l3city]

you got it!! thanks

 

[rbulsara]

dpc:

You are correct. Learn a thing everyday.

Thank you

 

[arvinolga]

Thanks very much guys, will look into the NEC, actually am in the Philippines and will try to look in our Philippine Electrical Code as well...

By the way, is there a way that I can upload drawings for better clarity of the query?