Wire Size to a transformer

[tobyk]

Table 450.3(b) permits me to size my Overload Protection on the primary side of a transformer to 250% of it Full load rating in a Primary Secondary Protection Situation.
Is there any place in the code that permits one to Size the transformer feeder Conductors based on the Full load Current of the transformer or do I need to size them according to the size of the Over current Protection?
I want to keep the Primary protection size High to avoid Nuisance tripping due to Inrush.

 

[bob]

Re: Wire Size to a transformer

Note the Table 450.3B says "Maximum Rating or Setting of OC protection for the Transformer 600 volts or less"
The secondary conductors must be protected according to 240.20(C)

 

[templdl]

Re: Wire Size to a transformer

You picked up on something not used by many as it does require a larger breaker and a panl that has provisions for it.
This is the way I interpret it. Consider the 250% and apply it exactly the same as you would a 25' tap rule. The 25' tap distance can be measured through the transformer to the location of the sec. OCPD. Of course the primary cable is sized in proportion to the secondary OCPD which would then establish what that 250% rating would be.
Overloads on transformers are caused by connected loads which should be protected by a properly sized sec. OCPD. A pri. OCPD sized a 125% is really redundant. What is considered with pri. protection is to take the transformer of like if the transformer fails, i.e. short circuit protection where you are really relying on the instantaneous (magnetic) element of the breaker.
What is really being done is to take the transformer off line and protect the rest of the distribution system should the transformer fail.

 

[tobyk]

Re: Wire Size to a transformer

Thanks for the responses,
are you saying that since my Secondary protection is sized to 125% of the rated current then my primary Conductors only need to be sized to 125% as well. For example on a 300KVA transformer At 480V the FLA is 361A, at 125% that would be 451A
I am just wondering if this is permitted?
Electrically I don?t think it's a problem, but was wondering what the NEC has to say.
Any Advice welcome.

 

[bob]

Re: Wire Size to a transformer

Toby
Size your primary protection first using the rules of 450.3B. Many installations use 125% for the primary protection.
Then choose the conductors for the secondary current using 250.3B. Generally they are at 125% of the secondary amperage.

 

[templdl]

Re: Wire Size to a transformer

I was going by your last statment Toby, regarding using the 250% rule regarding inrush and avoiding nuisance tripping. Naturally you are allowed to use a lower rated breaker and most of the time one rated 125% is where you normally would be. Going less than 125% can lead to nuisance tripping problems on energization.
With some transformers inrush currents can be significantly higher where breakers sized at 125% may not have a high enough magnetic trip nuisance trip upon energization in which cas you may consider a higher rated breaker up to 250% depending upon where that breaker is located. But, size and cost go up along with it.

 

[tobyk]

Re: Wire Size to a transformer

Thank you for your help, the Code reference's and advice are Appreciated.

Later