Transformer secondary conductors

[infinity]

Here's is (what I thought) was a typical installation:

112.5 KVA transformer, secondary conductors 500 kcmil feeding a 400 CB. Connected load 300 amps. Load conductors 500 kcmil. I've been told that under the 2005 NEC this is a violation. Any idea as to what the violation is?

 

[templdl]

Re: Transformer secondary conductors

What should be assumed here. That the secondary is 208v, 3ph4w? Then the sec FLA is 312a.
Also, what would be the best guess as to what the pri OCPD rating is since it could affect the sizing of the secondary cable as well as the sec. OCPD rating.
As a shot in the dark with 312FLA and 500MCM cable rated 380a@75degC you have it covered as well as th 400a breaker.
The pri. OCPD probably is 200a, the 500MCM covers the 312a, then for the breaker 312x125%=390a which is not a std. rating. Art 450.4 and table 450.3(A) foot note 1. allows the breaker to be sized as the next closest higher rating of 400a. So whats the problem?
The only problem that I see is the 300a load of a transformer that has a 312a secondary rating. If that's continuous transformer, although not to be considered overloaded, life still may be an issue.

 

[bob]

Re: Transformer secondary conductors

Temp
What you said may work for the transformers but not the conductors.
Check the code book(i don't have mine) on over current. There is a section for transformer secondary. It requires that the?capacity of the conductor must be equal to or greater that the OC device. In this case 500 kcm = 380 amps and the OC = 400 amps. You will need to use 600 kcm.

 

[infinity]

Re: Transformer secondary conductors

Originally posted by bob:
Temp
What you said may work for the transformers but not the conductors.
Check the code book(i don't have mine) on over current. There is a section for transformer secondary. It requires that the?capacity of the conductor must be equal to or greater that the OC device. In this case 500 kcm = 380 amps and the OC = 400 amps. You will need to use 600 kcm.

This is exactly what I've been told. As a code change for the 2005 NEC you can no longer go up to the next higher OCPD for the conductors on the secondary of a transformer even though the load is less than, in this case 380 amps. The conductor ampacity would have to be greater than or equal to the OCPD it feeds. This would require 600 kcmil or 2 sets of #3/0 secondary conductors.

 

[bob]

Re: Transformer secondary conductors

infinity
This is not a 2005 change. There are other types installations that allow the upgrade to the next OC rating. This is special for transformer secondary conductor protection. It goes back several code changes. I don't recall the first time it appeared.

 

[infinity]

Re: Transformer secondary conductors

Originally posted by bob:
infinity
This is not a 2005 change. There are other types installations that allow the upgrade to the next OC rating. This is special for transformer secondary conductor protection. It goes back several code changes. I don't recall the first time it appeared.

I would disagree that this isn't a new code requirement. Here is 240.21 from the 2005 NEc:

(C) Transformer Secondary Conductors. Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

The section in bold print is new for 2005. It does not appear in the 2002 NEC. Prior to 2005 we always used the next higher rule found in 240.4(B) when sizing transformer secondary conductors.


Trevor

 

[bob]

Re: Transformer secondary conductors

Infinity
Look at 240.21(C)(1) through (C)(6). One of those
I think requires the conductor to match the ampacity of the OC device.

The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

You are right about this statement. It is an add on but the requirement stated above was in the other NEC requirements.

 

[don_resqcapt19]

Re: Transformer secondary conductors

Trevor,
While the wording is new for the 2005 code the requirement is not. The "tap" rules in the previous codes required that the conductors be protected at or below their ampacity. The "round up" rule in 240.4(B) does not change the conductor ampacity, it only increases the maximum permitted size of the OCPD. The "round up" rule was often being seen as changing the ampacity of the conductor, and the code was changed to make it clear that you cannot us 240.4(B) for taps.
Don

 

[infinity]

Re: Transformer secondary conductors

Bob and Don,

Thanks for the responses. After reading the sections again you guys are right. The requirement was there. The new wording seems to be a clarification of an old rule. Makes it simpler for guys like me who may don't always peruse the wording of the rules properly. Thanks for the input.

Trevor

 

[wyatt]

Re: Transformer secondary conductors

I'm no expert but dose not 240.29(c) apply to more than one load like secondary feeding 2 panels with 2 sets of secondary conductors those condutors could not be less than the ocpd at each panel. I think that table 450.3(b) note 1 allows rounding up
(C) Transformer Secondary Conductors. Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary

[ February 19, 2006, 07:11 AM: Message edited by: wyatt ]

 

[don_resqcapt19]

Re: Transformer secondary conductors

Tony,

I'm no expert but dose not 240.29(c) apply to more than one load like secondary feeding 2 panels with 2 sets of secondary conductors ...

I assume that you are talking about 240.21(C), and it applies to all transformer secondary conductors. There could be one set or multiple sets, and the rules in this section apply to each set, even if there is only one set.

I think that table 450.3(b) note 1 allows rounding up

Article 450 has nothing to do with the overcurrent protection of conductors. It only covers the transformer itself.
Don

 

[wyatt]

Re: Transformer secondary conductors

(C) Transformer Secondary Conductors. Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without over current protection at the secondary
Don
I my be reading in to this to much but. If you come of the trans and hit a breaker 400amp as in the post above, that is the secondary protection. the above quote says without OCPD. I know that 2002 did not say separate loads. If in the case as the post if he had hit a mlo panel what would be the conductor size, asumeing that the primary is 125%.

 

[don_resqcapt19]

Re: Transformer secondary conductors

Tony,

I my be reading in to this to much but. If you come of the trans and hit a breaker 400amp as in the post above, that is the secondary protection.

That is the protection for the secondary conductors and maybe for the secondary of the transformer, but the problem is that the secondary conductors must be protected by a device with a rating no higher than the ampacity of the conductors. The 500 kcmil has an ampacity of 380 amps and cannot be protected by a 400 amp OCPD in this application.
Don

 

[bob]

Re: Transformer secondary conductors

Wyatt

(C) Transformer Secondary Conductors. Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without over current protection at the secondary

provided you follow the requirements of (1) thru
(6).

 

[infinity]

Re: Transformer secondary conductors

That is the protection for the secondary conductors and maybe for the secondary of the transformer, but the problem is that the secondary conductors must be protected by a device with a rating no higher than the ampacity of the conductors. The 500 kcmil has an ampacity of 380 amps and cannot be protected by a 400 amp OCPD in this application.
Don

Don I agree. The new sentence in the 2005 NEC says just that. A friend showed me Mike Holt's 2005 code change book and it has an illustration showing exactly what you've described. The text describes this as a code clarification, which would lead me to believe that the requirement was present somewhere else in another code article.


Trevor

 

[don_resqcapt19]

Re: Transformer secondary conductors

Trevor,

The text describes this as a code clarification, which would lead me to believe that the requirement was present somewhere else in another code article.

It was present, but misapplied by many code users. The code rule in the previous codes said that these conductors must be protected at or below their ampacity, but many times 240.4(B) was used for the installation. The problem is that 240.4(B) does not change a conductor's ampacity, it just permits the use of an oversized OCPD. The new wording makes it clear that you can't do this for taps.
Don

 

[wyatt]

Re: Transformer secondary conductors

Don thanks for the info I'm trying go get a good understanding on trans. so by puting out there what I think and being shown the right way helps me learn.