Overload on Xfrm secondary Code clarification

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haze10

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240.4(B) Devices Rated 800 Amps or less, permits the next higher standard overcurrent device rating if (2) the ampacity of the condutors does not correspond with the standard ampere rating of a fuse or a circuit breaker. 240.4(F) Transformer Secondary Conductors, for single phase transformers only, permits the secondary conductors to be protected by the primary overload provided that value does not exceed the value determined by muyltiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

Question: For the purpose of selecting a secondary conductor, are you permitted to use the rounding up rule, or, does the value of transformer ration x primary protection create an absolute threshold that can't be crossed.
 
Re: Overload on Xfrm secondary Code clarification

Just my opinion (in other words, I may be wrong), but I don't think you can use the rounding up rule here. I think the same thing applies to some of the tap rules. For example, one tap rule basically says "wires have to be protected at their ampacity". It doesn't make any reference to 240.4(B).

Steve
 
Re: Overload on Xfrm secondary Code clarification

Actually Steve, I believe you can. I think my opinion may be re-enforced by the fact that the 2005 specifically states that you can't use 240.4(B), but the 2002 did not. You also have the note to table 450.3(B) that says you can.
 
Re: Overload on Xfrm secondary Code clarification

Ryan, I didn't fully understand your response, how do you have 2005 Code, you see a preview, and are you sure that 450.3(B) note 1 applies to the secondaries, the note is only listed along the primary protection only. Sounds like this exception is only for the primary conductors.
 
Re: Overload on Xfrm secondary Code clarification

Haze10, the 2005 National Electrical Code? was out on Sept 15, so it is more than a preview.

Roger
 
Re: Overload on Xfrm secondary Code clarification

I haven't gotten the 05 NEC yet. But based upon the responsed I got from Ryan...

I think my opinion may be re-enforced by the fact that the 2005 specifically states that you can't use 240.4(B), but the 2002 did not.

The 05 code is stating that 240.4(b) is not applicable for 240(F) and the value calculated by primary overload rating x xfrm ratio is a threshold that can not be exceeded?
 
Re: Overload on Xfrm secondary Code clarification

I don't have my 2005 here at work, it is at home. I will look into it and post this weekend.

I got my 2005 Last Monday, the 14th, but as Roger stated, they weren't supposed to be available until Tuesday the 15th.

Charlie E: A little help here?..... :D

[ September 17, 2004, 12:40 PM: Message edited by: ryan_618 ]
 
Re: Overload on Xfrm secondary Code clarification

In my opinion, no matter what code edition you are using, if the code section requires that the conductors be protected at their ampacity, you cannot not use 240.4(B) to round up to the next standard size OCPD. 240.4(B) does not change the conductors ampacity, it just permits you to use an OCPD that exceeds the conductors ampacity. If the code section requires the conductor to be protected at its ampacity, then the OCPD must have a rating equal to or less than that ampacity.
Don
 
Re: Overload on Xfrm secondary Code clarification

Ryan,
You also have the note to table 450.3(B) that says you can.
Click to expand...
450.3(B) does not apply to the protection of the secondary conductors and that is what the original question asked about.
Don
 
Re: Overload on Xfrm secondary Code clarification

Article 450 is concerned about the transformer protection only. Article 240 is concerned about the protection of the conductors and gives permission for Article 450 to protect the transformer.

Ryan, panel 10 did not change any intent but it did clarify that section and that is what you are looking at in the 2005 NEC. If memory serves me correctly, that was a clarification that was submitted by Phil Simmons (he is as close to an official interpretation of the NEC on two feet that I know of). :D
 
Re: Overload on Xfrm secondary Code clarification

Ryan, you should edit you post and take that embarrassed icon off. You have nothing to be embarrassed about (you are too sharp for that). :D
 
Re: Overload on Xfrm secondary Code clarification

This thread has got me wondering:
What does the Secondary protection column apply to? (in table 450-3(b))

Note 2 to the Table talks about "Where secondary protection is required...."
So when is secondary protection required for a transformer? :(
 
Re: Overload on Xfrm secondary Code clarification

The protection required in Article 450 is for the transformer itself.
Don
 
Re: Overload on Xfrm secondary Code clarification

Transformer secondary protection is required whenever the primary OCPD is greater than the size required for "primary protection only", for most applications this is 125%.
 
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