NEC Article 450.3 Overcurrent Protection of Transformers - HELP

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chevywaldo

chevywaldo

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roselle, il
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CONTROL SYSTEMS ENGINEER - BUILDING AUTOMATION
For applications < 600V....

Need clarification of this code section. Very confusing.

I was just at several jobsites and saw control transformers installed with a variety of scenarios with regards to overcurrent protection. This is very confusing. Let's just stick with the NEC (< 600V) - nevermind chicago code for now.

I saw a hot water boiler control panel with a control transformer where only the secondary was fused. The primary (3-phase 208) was NOT fused. It even confirmed it on the wiring diagram. No overcurrent protection on the primary side. I see similar scenarios all the time.

I also see hundreds of control transformers (typically 120/24V) under 100 VA, installed with NO primary overcurrent protection. I called (2) transformer companies and talked to their tech support people and both of them said they DO NOT provide primary side integral overcurrent protection on the transformers they make. Some modyels they offer are built with "secondary" overcurrent protection (via circuit breaker), but NO primary overcurrent protection. The tech support guys said, it's up to the contractor and the NEC code to determined if and when primary overcurrent protection is required. The way I interpret NEC 450.3 is that Primary overcurrent protection is "always" required for transformers. Yet, when I am on jobsites looking at installed transformers, in most cases there is NO overcurrent protection on the primary side. Why?

I have to make a presentation on this subject to our engineering staff and field electricians soon, and I'm trying to get to the bottom of this.

When is Primary overcurrent protection required for transformers and when is Secondary overcurrent protection required for transformers. Similarly, when is it NOT required? Again, were talking <600V, and in my case, small control transformers only (less than 500VA, typically with 24V secondaries)

;):huh::slaphead:
 
jusme123 said:
read 430.72
Click to expand...
That section applies specifically to control transformers in a motor control application. If the load being controlled is other than a motor, they do not apply.

And if the transformers are part of a UL listed product, they would not apply at all, yes?
 
More XFMR Overcurrent Protection Devices Questions

More XFMR Overcurrent Protection Devices Questions

1) So if a transformer meets the requirements of Class 2, but is NOT listed as
"Class 2", is it a Class 2 device even though it carries no listing from the
mfr?
2) Is a transformer with no built in circuit breaker or overcurrent
protection considered an "inherently limited power source" ? I would think the
answer is no.

3) If the answer to #2 above is "NO", then per table 11(A), if a transformer
is 120/24 VAC, then per table 11(A) the maximum overcurrent protection is
100/Vmax = .83 amps (primary side). Is this correct?

4) If #3 above is correct, is "secondary" overcurrent protection required?
See table 450.3(B). I think the answer is NO.

5) Can anybody answer this question: When is "primary and secondary
overcurrent" protection required for a transformer, and where in the code does
it state this?
 
chevywaldo said:
1) So if a transformer meets the requirements of Class 2, but is NOT listed as
"Class 2", is it a Class 2 device even though it carries no listing from the
mfr?
2) Is a transformer with no built in circuit breaker or overcurrent
protection considered an "inherently limited power source" ? I would think the
answer is no.

3) If the answer to #2 above is "NO", then per table 11(A), if a transformer
is 120/24 VAC, then per table 11(A) the maximum overcurrent protection is
100/Vmax = .83 amps (primary side). Is this correct?

4) If #3 above is correct, is "secondary" overcurrent protection required?
See table 450.3(B). I think the answer is NO.

5) Can anybody answer this question: When is "primary and secondary
overcurrent" protection required for a transformer, and where in the code does
it state this?
Click to expand...

Duplicate of same question at http://forums.mikeholt.com/showthread.php?t=157299
 
GoldDigger said:
Duplicate of same question at http://forums.mikeholt.com/showthread.php?t=157299
Click to expand...

so.............throw me in jail and sentence me to 30 years hard labor. It's posted in two forum categories because it's relevant to both, and some forum members only read postings in one forum. Regardless, it's apparent you don't know the answer since you didn't answer the question. Leave the "forum etiquette" to the Moderator and don't post a response unless you can make a legitimate contribution. :happysad:
 
chevywaldo said:
so.............throw me in jail and sentence me to 30 years hard labor. It's posted in two forum categories because it's relevant to both, and some forum members only read postings in one forum. Regardless, it's apparent you don't know the answer since you didn't answer the question. Leave the "forum etiquette" to the Moderator and don't post a response unless you can make a legitimate contribution. :happysad:
Click to expand...
Unlike a Moderator, who would tell you that this Forum strongly discourages duplicate postings, my only intent was to make it easier for other members who respond to your questions to collect all of the replies in the one place. That saves duplication of effort among members and often leads to a more coherent discussion.

Enjoy....
 
chevywaldo said:
so.............throw me in jail and sentence me to 30 years hard labor. It's posted in two forum categories because it's relevant to both, and some forum members only read postings in one forum. Regardless, it's apparent you don't know the answer since you didn't answer the question. Leave the "forum etiquette" to the Moderator and don't post a response unless you can make a legitimate contribution. :happysad:
Click to expand...

Well I am a mod and we don't like duplicate postings.
 
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