Transformer/Conductor Protection per Articles 450 and 240.

[vw55]

I understand the two methods of transformer protection as outlined in Article 450, but I have concerns when the transformer conductors are oversized to allow Method 2 where the secondary is to be protected by a panel's main circuit breaker. For instance, if a transformer has a primary rated current of 75A, and a secondary rated current of 150A, the contractor installs a 150A breaker in a distribution panel protecting the primary feeder per Article 240, runs #1/0 primary conductors to the transformer, no secondary overcurrent protection at the transformer, and then runs #3/0 secondary conductors that terminate in a 200A panel with a 200A main circuit breaker, this installation will be compliant with both articles 450 and 240. Although the conductors are protected, is the transformer truly protected?

What happens to the transformer windings, if for whatever reason, the transformer draws 99A on the primary for an infinite amount of time? Neither breaker would trip because primary breaker rating is not exceeded, but the transformer primary is subjected to 99A while only rated at 75A. Would the secondary be self-limiting and only allow 150A to flow during this time or would 198A actually flow?

I understand they are probably over-engineered and will withstand such conditions, but for how long?

 

[MarineTech]

To me, the failure characteristics of the load(s), in the context of transformer sizing and OC protection usually play a role.

I think the broader answer that I have heard is the NEC addresses fire and safety, and not perhaps how to design for transformer longevity. Especially, in environments with elevated ambient.

I know there are EEs on the forum, perhaps they have a more complete answer?

 

[iwire]

In your example the transformer is 'protected' from overload by Article 220 load calculations.

It is a code violation to connect more load to the transformer than it is rated for.


During a short circuit or ground fault the oversize breakers will still protect the transformer.

 

[jim dungar]

It is a code violation to connect more load to the transformer than it is rated for.

Where did you find this in the NEC?

If I protect a transformer using 450.3(b), Method 1 primary protection only, which code sections apply to the actual transformer secondary (220 sizes the conductors to the load, 240.21(C) protects the conductors based on their size)?

 

[BPoindexter]

I would say that 110.3 could be used as an argument for not connecting more load than the transformer is rated...

 

[iwire]

Where did you find this in the NEC?

If I protect a transformer using 450.3(b), Method 1 primary protection only, which code sections apply to the actual transformer secondary (220 sizes the conductors to the load, 240.21(C) protects the conductors based on their size)?

Interesting question, obviously I was winging it but how would you size a transformer?

Are you suggesting the NEC would allow me to load the transformer to the rating of the allowed OCPDs?

 

[rbalex]

Actually,the Code violation may be that the transformer isn’t protected consistent with Art 450. The key is to recognize that while Section 240.5(F) doesn’t care about the transformer, Section 240.21(C)(1)does.

 

[jim dungar]

I would say that 110.3 could be used as an argument for not connecting more load than the transformer is rated...

The transformer must be protected correctly, the primary conductors must be sized correctly, and the secondary conductors must be sized and protected correctly, therefore 110.3 is met.

The NEC makes no mention that the transformer must be able to supply the load connected to it.

 

[jim dungar]

Are you suggesting the NEC would allow me to load the transformer to the rating of the allowed OCPDs?

No, the OCPDS are sized based on the ratings of the equipment they are protecting.

This is one of the times the NEC is absolutely not a design manual. Remember what 90.1(B) says; compliance with the NEC does not necessarily mean an adequate installation.

 

[BPoindexter]

The transformer must be protected correctly, the primary conductors must be sized correctly, and the secondary conductors must be sized and protected correctly, therefore 110.3 is met.

The NEC makes no mention that the transformer must be able to supply the load connected to it.

I may be wrong but I believe that 100.3 basically says that equipment current rating is part of it's suitability for use. If the manufacturers instructions say that it is rated for 45 kVA then loading it with 60KVA would be in violation of 110.3(B).

 

[jim dungar]

I may be wrong but I believe that 100.3 basically says that equipment current rating is part of it's suitability for use. If the manufacturers instructions say that it is rated for 45 kVA then loading it with 60KVA would be in violation of 110.3(B).

You are correct, but nothing is ever that simple. The nameplate is only a part of the consideration.

If it is properly protected it cannot really be overloaded (just like a motor with proper running protection).

If we we never, ever, allowed to exceed a transformer's nameplate reading, how would we deal with load inrush currents?

Do we have a 110.3(B) compliance sizing issue based on the current actually flowing throught the transformer, or on the assumptions made when calculating the load in 220?

Have you ever seen manufacturer's data sheets that describe the loading capacity of transformers based on ambient air and loading profile? Something along the lines of: 'in a 25?C ambient the transformer can handle a 15% overload for up to 8 hours in any 24 hr period, as long as the loading is reduced to <65% for at least 8 hours'.

 

[BPoindexter]

Yes I have seen those types of manufacturers specs. I was assuming (maybe incorrectly) that we were talking a continuous load and not an overload based on inrush or similar issue.

 

[jim dungar]

Yes I have seen those types of manufacturers specs.

So do they allow loading beyond the nameplate?

I was assuming (maybe incorrectly) that we were talking a continuous load and not an overload based on inrush or similar issue.

You are correct, barring any specific language the NEC is based on nominal values and not inrush. I should have said starting currents which can easily extend into the several second range, and for which there are several protective device sizing adjustments in the NEC.