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#95111 09/03/05 07:45 AM
Joined: Feb 2005
Posts: 44
B
Member
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.

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#95112 09/03/05 08:31 AM
Joined: Feb 2002
Posts: 2,233
H
Member
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.

#95113 09/03/05 08:46 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
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).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#95114 09/03/05 12:43 PM
Joined: Jan 2004
Posts: 1,507
G
Member
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) [Linked Image]

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


George Little
#95115 09/03/05 01:18 PM
Joined: Jul 2004
Posts: 9,923
Likes: 32
G
Member
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.


Greg Fretwell
#95116 09/03/05 02:18 PM
Joined: Feb 2005
Posts: 44
B
Member
George sorry I miss the OCP on the original post. [Linked Image]
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. [Linked Image]

#95117 09/03/05 11:39 PM
Joined: Jul 2004
Posts: 625
S
Member
Quote
The design engineer at that point is putting his lic. on the line.

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

#95118 09/05/05 08:33 PM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
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)

#95119 09/05/05 08:51 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Quote
The code never says "just breakers", it talks about ALL overcurrent protective devices.

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

No.


Quote
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.

That is not what the UL 2004 general directory says.

CIRCUIT BREAKERS, MOLDED-CASE AND
CIRCUIT BREAKER ENCLOSURES (DIVQ)
Quote
Unless otherwise marked, circuit breakers should not be loaded to
exceed 80 percent of their current rating, where in normal operation the
load will continue for three hours or more.

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


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#95120 09/06/05 12:43 AM
Joined: Jul 2004
Posts: 9,923
Likes: 32
G
Member
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 Fretwell
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