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#95101 08/30/05 04:21 PM
Joined: Feb 2005
Posts: 44
I have an inspector telling me that the 12 volt transformer feeding low voltage track has to be figured at 125%. I have 296 VA on a 300 VA transformer and the inspector is saying that it is no good, that it needs to be at least 125% larger which would be 370 VA. I thought lighting transformers were designed to run at 100% load continuously?

[This message has been edited by boggerbutt2454 (edited 08-30-2005).]

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#95102 08/30/05 08:13 PM
Joined: Jan 2004
Posts: 1,507
I would be interested in the code section on this one. I can't think of any restriction of loading on transformers unless the manufacturer's specs call it out. Continuous load is only an issue with overcurrent devices and their enclosures. Transformers are rated in va or kva and they are fused accordingly.

George Little
#95103 08/30/05 10:15 PM
Joined: Jul 2004
Posts: 9,828
Likes: 22
This is an interesting topic. I know a listed item is not supposed to catch fire if loaded to 100% (and perhaps even a bit more) but, as a guy who came from the "broken electronics" biz, I also know things used at the "red line" don't last very long even if they don't fail in a flaming conflagration, like a switchboard in the movies.

I do agree it is a design issue, not a code problem.

Greg Fretwell
#95104 08/31/05 05:34 PM
Joined: Feb 2005
Posts: 44
Inspector said since it was a continuous load we had to figure the transformer at 125%. I've tried discussing it with him but he jsut keeps saying the same thing. I think he is wrong but I don't want to cause more trouble down the line with him as I have 6 stores that he is inspecting but the customer is looking at 4 more 600 VA
transformers and 10 connectors to split some of the track. Also while there is only 8 track head per 8' track the drawing showed 9 which would have overloaded it but a electrical note took care of that as it stated only 8 heads per track. His threory is that when he leaves they will add another head, to which I said if they do that then the internal overload will keep tripping out or the transfomer will burn up which wouldn't make any sense for them to do that.

Am I wrong in my thinking? Is the inspector? Is it just a matter of interpretation?

[This message has been edited by boggerbutt2454 (edited 08-31-2005).]

#95105 08/31/05 06:27 PM
Joined: Jan 2004
Posts: 1,507
Since you have two inspectors, myself and Gfretwell telling you it's a design issue and the design professional has you working off his specs, I wonder why the Inspector isn't following the plans number 1 and number 2, why isn't he quoteing code page and verse. In Michigan - It isn't a code violation unless the inspector quotes page and verse and does so in writing. In your situation I guess I'd look for some kind of violation in writing. As for overloading the xformer after you leave we can't prevent that but it's kinda self policeing.

George Little
#95106 09/01/05 07:16 AM
Joined: Jul 2002
Posts: 103
jes Offline
George, I'm sure you meant to include 'conductors' in your reply regarding the 125% for continuous loads, right? (210-19 '02 ed.)

I'm curious as to the specifics of the installation(s). Does it fall under Art 411 operating under 30 volts? What are the instructions provided with the system? Are the transformers part of the listed system? Certainly if it does the mfg can clear up the issue based on the listing.

#95107 09/01/05 12:58 PM
Joined: Sep 2003
Posts: 650
Conductors are permitted to operate at 100% loading for continuous loads, unless protection requirements force over sizing.

The issue is that _breakers_ are not in general permitted to operate at 100% loading for continuous loads. So for continuous loads you must increase the size of the breaker to at least 125% of the continuous load.

But if you have oversized the breaker to 125% of the load, it will now not protect a conductor that is sized to 100% of the load. For the conductor to be properly protected by this breaker, it is _also_ oversized to 125%.

You will note that the requirements for over sizing the conductors to 125% have an exception: when used with a breaker that is rated for 100% operation. If you use a breaker rated for 100% operation, then you can use conductors right up to their ampacity, even on continuous loads.


#95108 09/01/05 02:26 PM
Joined: Jan 2004
Posts: 1,507
Thanks Winnie- Yes, I agree and should have stated that. The issue really is the xformer per the original post. The conductors are listed for continuous load and while I can't find anything in the code that says xformers are rated for continuous load I assume somewhere there are guidlines.

The conductors need to match the breaker or just below and round up (usually) and the overcurrent device is sized at 125% for continuous load. See definition of ampacity in Article 100 as it uses the word continuous and the Tables 310.16 etc. are the continuous rated ampacities.

George Little
#95109 09/02/05 10:04 AM
Joined: Jul 2001
Posts: 599
JBD Offline
Step 1 - load calculations
The NEC does not address the issue of transfomer sizing. All parts of 450 simply talk about the full load rating of the transformer. It is up to the manufacturer to decide and state if a device is rated for continuous or non-continuous loading.

Step 2 conductor sizing
Conductors are sized based on the load. for example 215.2(1) and 210.19(A)(1)
non-continuous: 100% for loading up to 3hrs
continuous: 125% for loading longer than 3hr
combination: a total of the above loads

Step 3 overcurrent protection selection
Overcurrent protective devices are sized based on the chosen conductors, for example 240.4, or using the same general rules, for example 210.20.

The exceptions to 210.19(A)(1) (conductors) and 210.20(A) (protective devices) do not require the 125% factor when the "entire assembly" is rated for 100% operation. 100% rated assembly are very rare.

No where in the code does it say breakers are only rated for 80%. For normal conductor (full load current) protection the NEC treats and sizes breakers and fuses identically.

#95110 09/03/05 07:35 AM
Joined: Jan 2003
Posts: 4,391
No where in the code does it say breakers are only rated for 80%.

Actually it does say that for continuous loads in a round about way.

210.20 (A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

Also if you look in the UL General directory you will find that breakers unless otherwise marked, are only listed for 100% operation of loads lasting less than 3 hours.

A typical 20 amp breaker can be loaded to 20 amps for a period up to 3 hours or 16 amps forever. [Linked Image]

Bob Badger
Construction & Maintenance Electrician
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