NEC 2005 - Transformer Seconday Conductors

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kbowman

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My understanding of 250.21(C) is that since 2005 you can no longer use the "next size up rule" for transformer secondary conductors. This means you need 1-600 mcm or 2-3/0 CU per phase on a 400A service and double that on an 800A service. However, a standard 400A cb has one lug per phase good for up to 500 MCM and a standard 800A cb has 2 lugs per phase good for up to 500 MCM. Do I have to order special lugs on a cb to meet the new requirement?
Am I missing an exception somewhere?
 
This is not a code change...the wording was changed to make the sure that users understood the rule. Most breakers have optional lugs that can be either ordered when you order the breaker or ordered later and field installed.
The reason why this is not a change in the rules is that the tap rules have always said that the tap conductors must be protected at their ampacity. Many code users were trying to use the "round up" rule and use the next larger size OCPD. That was not protecting the conductors at their ampacity and was a code violation even prior to the wording change in the 2005 code. 240.4(B) does not change the ampacity of a conductor...it only permits the use of an oversized OCPD.
Don
 
Thanks for the reply Don. From what I see, 1-500 mcm per phase has been the norm for a 400a service and 2-500s per phase on an 800a service. If this is incorrect, I wonder why cb industry doesn't design thier standard cb's to meet the conductor requirements?
 
In cases other than tap conductors you can use a single 500kcmil on a 400 A breaker and parallel 500's on an 800 A breaker. Maybe with the code wording change to make the code rule very clear the breaker people will change their standard lugs. I just looked a few breakers from one of the major brands. It appears that their standard lug for 250 thru 600 amp breakers will take 1 #8-600kcmil or two 2/0 thru 500 kcmil.
Don
 
NEC 2005 - Transformer Seconday Conductors

Section 240.4(B) does permit the use of the next up breaker for calculated loads and services. If you?re calculated loads is 380 ampere for your service or feeder 240.4(B) permit the use of the next size up breaker. The transformers used by the serving utility are not covered by the NEC. The service transformers are covered by the NESC.
I feel that the manufacturer feel that most of the breaker sold will be used on services or feeders, and for those used on secondary transformer conductors they can make little more money by selling the required lug kit.
 
I discussed this issue with a utiliy provider and thier take was that the NEC takes jurisdiction starting at the transformer secondary lugs for 3 phase services.

240.21(C) specifically states that 240.4(B) cannot be used for transformer secondary conductors.

Doesnt this also include service entrance conductors?
 
Transformer secondarys have their own rules. Not a tap, not a feeder, and mostly not a service (unless a POCO transformer)
 
Tom,
If it is a POCO transformer with customer owned service entrance conductors,are the conductors sized by 240.21(C) or 240.4(B)
 
(Ithink) it ws hidden amongs 11 or so exceptions.

I would think if they are truley service entrance conductors they are sized IAW article 220. and protected per 230.90 along with it's exceptions.

The application of 240.21 (C) as I see it leans towards SDS on premises that are not classified as SE or Service. In other words the primary conductors are branch circuits But seeing that your brought up the question I can see where you might try to apply it to a utility xformer, I don't think that was the intent.

This language has always been in the NEC as Don mentioned but untill 99 it was hidden amongst the 11 or so exceptions.

As the 02 and 05 code worked to clear these things up the language to stop rounding up was introduced.

I never read the languge that was previously in this section to allow 240.4 B to apply to transformer secondary conductors.
 
230.90 (exception 2) does allow service entrance conductors to use the "round up rule" 240.4(B)

240.21(C) prohibits using 240.4(B) for transformer secondary conductors.

Most service entrance conductors are also transformer secondary conductors.

I'm having trouble trying to figure out which article supercedes the other.
Is there any language in the NEC that tells you which rule to use if they contradict each other?
 
The tap rules in 240.21 don't apply to service conductors. Since the utility owns the transformer, you don't have to worry about 240.21(C).

240.21D is the paragraph that basically tells you this. The layout of 240.21 is basically this:

240.21 Location of OCP
(A) for branch circuits
(B) for feeder taps
(C) for transformer secondaries
(D) for service conductors
....


Steve
 
try 240.21 (D)

On second thought I do not think this section asnswers your question.

There probably should be some clarifying language to exclude service conductors from the application of 240.21 (C) It is my opinion that 240.21 is not intended to apply to article 230 conductors. 230.90 addresses that application in a practical manner which works for services. See my op
 
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240.3 tells us that the rules in Article 240 do not apply to service conductors. The rules in Article 230 apply to those conductors.
Don
 
I just completed a job that involved this details in this thread. 800A I Line panel fed with 2 sets of 600 MCM. The lugs in the breaker had to be changed. The lugs supplied were good for up to 500 MCM only. The secondary of the transformer was rated at 635A.
 
don_resqcapt19 said:
240.3 tells us that the rules in Article 240 do not apply to service conductors. The rules in Article 230 apply to those conductors.
Don
Click to expand...


Thanks Don
I couldn't find that section (although I was staring at it)
 
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