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).]

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.

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.

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).]

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, 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.

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.

-Jon

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.

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.

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



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.

Spent the day getting info from manufacturer.
They didn't have anything in their instructions about derating of the transformer or using it at 100% load continuously so they sent me a letter stating that it was designed to run at 100%.
The engineer I spoke to said if you weren't able run it at 300 VA continuously on a 300 VA transformer they would have labeled it 240 VA Max.
The secondary is protected by a 25 amp overcurrent device in the transformer. The system was designed by the engineer with a load of 24.6 amps on the transformer(8-37 watt lamps).
The customer is going to put the extra transformers in as they have a deadline to meet. This was deceided after I got the engineer to talk to the inspector, and he still wouldn't change his mind.
I still think this was a acceptable installation. I don't think anything I said or did was going to get the inspector to admit he made a mistake or that he was wrong.

This is just my 2 cents, but if an EC got a letter from the design engineer, then as an AHJ, I would have to accept the installation. The design engineer at that point is putting his lic. on the line.

boogerbutt, the transformer can be run at 100% continuously, the OCP protecting it can not.

The 25 amp OCP protecting the circuit can not be run continuously over 20 amps.

IMO the inspector is correct.

[This message has been edited by iwire (edited 09-03-2005).]

While I agree with iwire, The orginal post said nothing about the overcurrent. The question had to do with the loading of the xformer and if it could handle continuous loading. The Inspector may well be right when I hear all the details of the job.

Edited to add this comment: Inspectors are right - (Sometimes)

[This message has been edited by George Little (edited 09-03-2005).]

If the OCP is part of the transformer and the enginerer says the "transformer" is OK at 100% how can you separate the issue to a part of the listed assembly?

I agree it is a poor design but I still don't see the code issue.

George sorry I miss the OCP on the original post.
I didn't think it was an issue since it was listed and labeled for a Max. load of 300 VA and the manufacturer said it was designed to run at 100% load continuously.

The more I listen to the differing opinions I can see the inspector's point.
It wouldn't be first time I was wrong.

License? What license? You don't need a license to design this stuff.

iwire,

The code never says "just breakers", it talks about ALL overcurrent protective devices. Actually, all fuses and breakers are tested by UL at their 100% rating. The "3hour derating" comes in when the protective devices are mounted into enclosure.

My question: are protective devices designed to protect wires sized for continuous and non-continuous loads OR are wires sized because of the performance of protective devices? (i.e. which is the chicken and which is the egg)

Simple question does the NEC allow loading a typical circuit breaker 100% for more than 3 hours?

No.



That is not what the UL 2004 general directory says.

CIRCUIT BREAKERS, MOLDED-CASE AND
CIRCUIT BREAKER ENCLOSURES (DIVQ)

The area UL brings the enclosure into question has to do with the 60 or 75 C rating.

I'm confused. Isn't the O/C device in question part of a listed assewmbly that the manufacturer says is good for 100% of labelled capacity. If so why are we looking further at that? I'm sure if we open up any listed equipment we will see things that don't look Kosher but they got listed.

Greg as far as my posts are concerned I think we have moved beyond the original question and moved on to a general discussion of breakers.

I do understand your point and most times I would agree.

However it sounds to me like even the manufacturer was confused when asked the question.


I bet that is correct as far as the transformer, I still question if the breaker manufacturer would back that up. I doubt highly this engineer had anything to do with the design of the breaker.

But a letter is a letter so all is good.

Bob

I don't think we have a question about breakers "not listed for 100%" but this one is a recognized part of a listed assembly. If it wasn't good at 100% why did the NRTL list it?

I agree this might just be a mistake at the NRTL, there are plenty but I am not sure I have the authority to dispute that with an installer. I could go to the NRTL or the manufacturer but these fights do not belong on Bogger's job.
I might ask him if he really likes the "design", but if that is what they are going for I am not sure I can stop them.

Maybe I am just too nice to be an inspector

Greg there is nothing wrong with being nice.

I do not believe an inspector working under the NEC has to accept NTL listed equipment. (I do think they would be on slippery ground refusing it)

However here in MA we have an amendment that directly states inspectors must accept NTL listed equipment if used per listing and labeling.

I talked to the inspector the other day just to let him know we were adding the extra transformers and balancing the load on the them. After talking to him for a while and bring up some of the issues discussed here his stance is that he feels the design is wrong and as the last one to look at an installation and design and that it his responsiblity as the AHJ to make sure it is to code.
All drawings go thru a plan review here and get red stamped and are required to be on site. If there is a mistake or the reviewer misses something, i.e. wrong size OCP or wrong size feeders, just because an engineer designed it and it got thru plan review doesn't mean he is required to pass it. In fact he said he is the "last line of defense" and he reads the code that the OCP on the secondary is only good for 80% continuous load.
I agree with him after talking with him and after listening to the others here.
I think I'm ready for a big piece of humble pie.