Three Related Questions: #3

[charlie b]

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars. Breakers serve loads both inside and outside the adjacent building. Primary is over 600 volts, secondary is 120/208V.

Question:
I am reviewing a drawing. The transformer is 500 KVA. With a secondary of 120/208V, the rated current is 1389a. 125% of that is 1736, and the next higher standard rating is 1800. There are four breakers connected to the busbars: 1800a, 100a, 20a, and 15a, for a total of 1935a. Does this not violate Note 2 of Table 450.3(A)? Would I have to call this a supervised location, and use the 250% limit, in order to call this installation acceptable?

 

[david luchini]

Yes, I'd say you have to apply the "Supervised Location" in 450.3(A).

 

[augie47]

Yes, I'd say you have to apply the "Supervised Location" in 450.3(A).

agree

 

[RUWired]

Charlie, does the transformer serve the prerequisites of supervised installations.

Supervised Industrial Installation. For the purposes of Part VIII, the industrial portions of a facility where all of the following conditions are met:
(1) Conditions of maintenance and engineering supervision ensure that only qualified persons monitor and service the system.
(2) The premises wiring system has 2500 kVA or greater of load used in industrial process(es), manufacturing activities, or both, as calculated in accordance with Article 220.
(3) The premises has at least one service or feeder that is more than 150 volts to ground and more than 300 volts phase-to-phase.
This definition excludes installations in buildings used by the industrial facility for offices, warehouses, garages, machine shops, and recreational facilities that are not an integral part of the industrial plant, substation, or control center.

 

[kingpb]

I assume your HV side is greater that 600V? Although Note 2 is the same for 450.3(A) and (B).

Even if it is considered a supervised location, as soon as you put overcurrent protection on the secondary side, arn't you obligated to meet the 125% OCP requirement?

 

[charlie b]

Supervised Industrial Installation. For the purposes of Part VIII, the industrial portions of a facility where all of the following conditions are met:

Table 450.3(A) mentions "supervised locations," and Note 3 to that table explains the phrase. It does not include the word "industrial." May I ask where your quote comes from?

 

[charlie b]

I assume your HV side is greater that 600V?

Primary is fed from a 6,600 volt, 50 Hz, delta configuration.

Even if it is considered a supervised location, as soon as you put overcurrent protection on the secondary side, aren't you obligated to meet the 125% OCP requirement?

I think not. There are two rows in Table 450.3(A) that says, in a supervised location, the secondary protection can be as high as 250%.

 

[RUWired]

Table 450.3(A) mentions "supervised locations," and Note 3 to that table explains the phrase. It does not include the word "industrial." May I ask where your quote comes from?

Article 240.2 definitions

 

[RUWired]

Is it possible to adjust the 1800 breaker to 1665 amps?

 

[charlie b]

Is it possible to adjust the 1800 breaker to 1665 amps?

I suspect so. That would be my recommended resolution, if there really is a problem in need of a resolution. But first I want to ascertain if there is a problem.

This situation is an existing building that was recently rennovated under a different engineering company's design. I get to do a similar building. I am more interested in not making their same mistake than in putting them on the spot for having made a mistake. So when I issue my preliminary design with a breaker set lower than 1800, I want to be able to give the owner a clear answer as to why. I'll let the owner deal with the other project and the other engineering company.