The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Forum Software Upgrade coming soon...
by Webmaster
42 minutes 17 seconds ago
Re: Fabricating Guards out of Lexan?
by frank
Today at 04:11 PM
Would ELV branch circuits be a good idea?
by gfretwell
Today at 01:26 PM
Why cables look like they do
by Texas_Ranger
Today at 12:33 PM
100% breakers?
by Texas_Ranger
Yesterday at 06:13 AM
New in the Gallery:
Desk-mounted "power-board"
Shout Box

Top Posters (30 Days)
HotLine1 20
Webmaster 16
Trumpy 12
ghost307 12
frank 11
Who's Online
1 registered (Webmaster), 0 Guests and 66 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#210167 - 05/31/13 07:14 AM Transformer rating
cgw Offline

Registered: 07/29/04
Posts: 133
Loc: 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 02:23 PM Re: Transformer rating [Re: cgw]
ghost307 Offline

Registered: 05/09/05
Posts: 897
Loc: 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 03:32 PM Re: Transformer rating [Re: cgw]
cgw Offline

Registered: 07/29/04
Posts: 133
Loc: 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 07:17 PM Re: Transformer rating [Re: cgw]
HotLine1 Offline


Registered: 04/03/02
Posts: 6829
Loc: 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.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals