Disribution Transformers & Power Panels

[richanton]

I am confused. 450.3(B) states that if I size my primary breaker at <125% of the transformer rating, that primary protection is adequate.
240.4F says that with a standard delta-wye transformer, I do need protection of the secondary conductors. The application here is a 30 or 45kva transformer in a room fed from a 480V distribution board. The transformer sits on the floor and a 120/208 volt panelboard is on the wall next to it. What is the correct way to install the transformer/panelboard combo. Does the panelboard need a main breaker? If it has a main bkr, does that meet 240.4F. Or does it need a separate secondary disconnect switch? The 480 volt source could be in the same room or not in the same room.

 

[fc]

Re: Disribution Transformers & Power Panels

240.21 (c)(1)Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single phase transformer having a 2-wire (single voltage) secondary, or a three phase delta-delta connected transformer having a 3-wire (single voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.
Single phase ( other than 2-wire) and multiphase (other than delta-delta 3 wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
You have 480 primary 120/208 secondary 3 phase 4 wire system you would need secondary protection.

 

[jim dungar]

Re: Disribution Transformers & Power Panels

Transformers, less than 600V primary, never need secondary protection. So the last place to look is 450.

Start at your panel and then work towards the primary.

Does your panel require a main device(s) per 408.36? Remember to use the definition in 408.34 not what the actual loads are. Also the main device does not have to be in the panel, it can be "remote".

Now look at your secondary conductors. According to 240.4F, you must supply overcurrent protection at the transformer unless you can use one of the tap rules in 240.21(C). If protection was required for the panelboard it may be used to meet requirements of 240.21.

Now if you have provided a protective device on the secondary, you can re-evaluate your transformer primary protection per 450.3(B).

 

[richanton]

Re: Disribution Transformers & Power Panels

OK thanks for your input. Based on looking at the referenced sections, the following is a summary.

In an industrial installation, if I have a 480-120/208 volt transformer feeding a 120/208 volt panel at the same location, the panel will require a main breaker per 408.36(B). Additional protection of the secondary conductors is not required as long as my conductors are less than 10' per 240.21(C)(2).

The only thing I am still confused about is why does 450.3(B) say that any xfmr 600 volts or less having primary protection <125% and greater than 9 amps current, has secondary protection listed as "Not Required". Is section 450 not applicable to dry type 3 phase xfmrs?

 

[templdl]

Re: Disribution Transformers & Power Panels

3ph delta-deltas and 1p transformers 2w secondaries with no neutrals.

 

[don_resqcapt19]

Re: Disribution Transformers & Power Panels

Rich,
You just have to remember that Article 450 only covers the transformer protection and that you have to look to Article 240 for the conductor protection.
Don

 

[lady sparks lover]

Re: Disribution Transformers & Power Panels

I just had this question the other day, and I wasn't sure. The code is very confusing on this issue. I always put a breaker in a panel or a secondary disconnect/breaker, because there's nothing that will protect the wire on the secondary side, so I'm glad this clears that up. JMO!

Lady

 

[pierre]

Re: Disribution Transformers & Power Panels

"You just have to remember that Article 450 only covers the transformer protection and that you have to look to Article 240 for the conductor protection.
Don"

Art 240 - conductor and equipment protection, 450 Transformer (windings) primary and secondary protection.

As Don has stated, this seems to confuse some. I know that in my classes some of the guys never knew this. I have them write this in the NEC so they do not forget.

I remember how it used to confuse the %$@#% out of me and I would get mad

 

[templdl]

Re: Disribution Transformers & Power Panels

Should you have a delta delta or a 1ph with a 2w secondary you can protect the secondard conductors with pri. protection only in proportion to the secondary conductors.
Whe primary will see the overload.
However, if there is a neutral, such as a delta-wye or a 1ph 480-120/240 1ph, 3w transformer you can have a L-N overload and the primary breaker will not sense it. That's why art 240 states that secondary protection is a must.
First take a look at 240.21(C)(1)where primary only is allowed.then go back to art 240.4(F).
It all revolves around that neutral. A L-N overload is not going to be seen by a primary OCPD which is looking at L-L currents on the delta primary.

 

[fc]

Re: Disribution Transformers & Power Panels

Article 450 gets confusing sometimes. The rule is that the primary MUST have protection within 125 percent of it's rating. After this, you go to article 240 for protection of the secondary conductors. When you look at section 240.21(C) you will see that the seconary MUST have conductor protection when the secondary has "more than two conductors". The protection of the secondary is dependant on the wire size, and has nothing to do with the primary. The primary OC device can be located anywhere, but the secondary must be NO MORE than 25 feet of conductor length. See 240.21(C)(6). Table 450.3 where it says primary and secondary. This does not eliminate the secondary protection required, it means that IF you protect the secondary condcutors at 125 percent, then you protect the primary at up to 250 percent of it's rating. You must install conductors rated at this ampacity also. When you protect the primary at 125 percent, the secondary can have any size overcurrent device that you want to install.