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#210167 - 05/31/13 10:14 AM Transformer rating  
cgw  Offline
Joined: Jul 2004
Posts: 133
Rochester NY
1. When the NEC refers to a Class 155 transfomer is that the same as a manufacturer's temeperature rise?

2. When the NEC requires that the transformer be installed in a transformer room does that mean a separate room from the electric switchgear and other mechanicals?

2017 / 2014 NEC & Related Books and Study Guides

#210169 - 05/31/13 05:23 PM Re: Transformer rating [Re: cgw]  
ghost307  Offline
Joined: May 2005
Posts: 907
Chicago Illinois USA
1. No. The temperature rise is stated as the maximum temperature rise above the room ambient. I think what you are referring to as 'Class 155' is a rating of the temperature at which the insulation system fails and the transformer becomes a giant flash bulb.
2. Yes and No. It needs to be in a room separate from things that are not directly associated with the electrical system. That means no chillers, fans, pumps, phone equipment, etc.


#210194 - 06/02/13 06:32 PM Re: Transformer rating [Re: cgw]  
cgw  Offline
Joined: Jul 2004
Posts: 133
Rochester NY
(I figured 1. out right after I posted)
Is your typical low voltage (say 480X208/120) dry type transformer considered totally enclosed except for the ventilating openings?

#210201 - 06/02/13 10:17 PM Re: Transformer rating [Re: cgw]  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,859
Brick, NJ USA
The term 'totally enclosed' does not specifically appear in Art. 100 or 450 (2011 NEC)

Mention is made within 450.21
"450.21 Dry-Type Transformers Installed Indoors.
(A) Not over 1121⁄2 kVA. Dry-type transformers installed
indoors and rated 1121⁄2 kVA or less shall have a separation
of at least 300 mm (12 in.) from combustible material unless
separated from the combustible material by a fireresistant,
heat-insulated barrier.
Exception: This rule shall not apply to transformers rated
for 600 volts, nominal, or less that are completely enclosed, with or without ventilating openings.
(B) Over 1121⁄2 kVA. Individual dry-type transformers of
more than 1121⁄2 kVA rating shall be installed in a transformer room of fire-resistant construction. Unless specified otherwise in this article, the term fire resistant means a construction having a minimum fire rating of 1 hour.
Exception No. 1: Transformers with Class 155 or higher
insulation systems and separated from combustible material
by a fire-resistant, heat-insulating barrier or by not less
than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically.
Exception No. 2: Transformers with Class 155 or higher
insulation systems and completely enclosed except for ventilating openings."

IMHO, a basic 480/120-208 transformer is 'enclosed' by def.; and 'completely' enclosed as the wording within exception 2 above.

A 'totally enclosed' transformer IMHO, is one that is encapsulated and only the pri & sec conductors are visable.


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