Tap Rule

[mityeltu]

I have been trying to understand the application of the tap rules as set forth in the NEC, so I decided to open the handbook 70H. There is an example of the rule I am trying to understand in Fig 240.9 (taps < 25' to transformer primary). It is attached for reference.

Here's the problem I see with this diagram. First, the transformer is rated 37.5 kVA with FLA=45A primary. While the #8 can carry the 45A, it would make more sense to me to upsize the cable for better protection of the cable during overload conditions. Secondly, why is the fuse so large? 110A? That fuse is not protecting the trasnformer or the cable. I know the inrush can be high, but at 6 times FLA, the fuse would blow assuiming it is not a TD fuse. It just seems to me that the fuse disconnect is rated too high for this circuit. Am I missing something here?

Thanks.

 

[ibew501ed]

the quick answer id the secondary ocpd protects the transformer from overload and the large primary ocpd protects the transformer from fault current yet allows for initial energizing

 

[augie47]

I don't recall ever seeing that tap rule applied, but the primary conductors are protected from overload by the transformer secondary overcurrent protection if the turns ratio so allows.
The 1/3 requirement gives the primary conductors short circuit protection.

 

[steve66]

I have been trying to understand the application of the tap rules as set forth in the NEC, so I decided to open the handbook 70H. There is an example of the rule I am trying to understand in Fig 240.9 (taps < 25' to transformer primary). It is attached for reference.

Here's the problem I see with this diagram. First, the transformer is rated 37.5 kVA with FLA=45A primary. While the #8 can carry the 45A, it would make more sense to me to upsize the cable for better protection of the cable during overload conditions. Secondly, why is the fuse so large? 110A? That fuse is not protecting the trasnformer or the cable. I know the inrush can be high, but at 6 times FLA, the fuse would blow assuiming it is not a TD fuse. It just seems to me that the fuse disconnect is rated too high for this circuit. Am I missing something here?

Thanks.

First of all, the NEC is not a design guide. So the point of the diagram is to show what is allowed. So they are showing a somewhat extreme case. That doesn't necessarily mean it is a good design. In most cases, I would normally use a smaller fuse on the primary. But in other cases (like keeping things coordinated), the increased fuse size might be necessary.

Anyhow, I wanted to point out the following:

The #8 wire is also the minimum size needed for the 45 amp current, as you noted. But it is also an example of the tap conductors not having an ampacity of less than 1/3 the fuse rating. #8 at 45 amps is more than 1/3 of 110 amps. But #10 would only be rated for 30 amps, so it wouldn't be allowed since 30A < (110/3).

Also, in using the tap rules with transformers, you also need to comply with tables 450.3. There you will see that the 110 amp primary fuse is the maximum size fuse that would be allowed to protect the primary of this transformer, no matter what size wire you ran to the primary.

And in table 450.3, you will also find a max. size for the breaker on the secondary side.

So my point is, with transformer tap rules, you have to comply with BOTH the tap rules and the transformer protection rules in 450.

 

[mityeltu]

That makes sense, but is it good design practice to go with the largest that is allowed? It has been my experience that it is best to keep protection devices as small as possible to mitigate damage and danger, while keeping conductors as large as possible for almost the same reasons.

There are so many rules that apply to so many different aspects of the same installation, how in the world does anyone ever follow them all correctly?

I know how to size the cable to provide current, limit vd, allow for sc interruption time without damage, but when it comes to protection.... these rules don't seem terribly intuitive. Just the personal observation of a new engineer.

 

[augie47]

If it were simple, they wouldn't need engineers

 

[Pierre C Belarge]

...And if you think this is complex, you have just started. There are a lot of other applications in our industry that make this seem like a walk in the park.

 

[iwire]

That makes sense, but is it good design practice to go with the largest that is allowed?

Everything is a trade off, yes larger conductors are generally safer, but if you make all conductors larger than needed you're adding job costs to the customer.

IMO (In My Opinion) a good engineer needs to use good judgment to determine when the minimum allowed by the NEC is fine and when it is time to do more than the minimum.

I strongly suggest you grab any recent NEC and take a look at 90.1(A), 90.1(B), and 90.1(C). They are all short but IMO very important code sections to keep in the back of you're mind.


To paraphrase

(A) says the purpose of the NEC is the practical safeguarding of people and property from hazards arising from the use of electricity.


(B) distinguishes from the adequacy concept (provisions necessary for safety) and other concepts. The Code is a minimum standard. Further effort may be required for an installation to be efficient, convenient, or adequate for good service or future expansion.

(C) clearly states the Code is not intended to be a design specification or instruction manual.


(D) well I don not really care about (D)

It has been my experience that it is best to keep protection devices as small as possible to mitigate damage and danger, while keeping conductors as large as possible for almost the same reasons.

It takes little skill to always overdue things. Good engineering has to take into account all the variables of an installation. Going 'big' may make the job a bit safer but it may start a progression, larger conduits, larger radius required to bend the conduit, larger j-boxes for these conduits, larger gear to handle these larger conductor etc. this can quickly mean you cannot fit the electrical installation in the space provided.

There are so many rules that apply to so many different aspects of the same installation, how in the world does anyone ever follow them all correctly?

Carefully and with training.

Just the personal observation of a new engineer.

Welcome to the electrical field, we are very glad to have EEs here at the forum and even happier when they want to understand the NEC.

I started my NEC training at about 15 years old in trade school, 30 years latter I still learn new things and find items that I need help with.