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Ryan_J
Code Guru Moderator
USA
1854 Posts |
 Posted - 03/30/2005 : 12:23:57
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Transformer Inspections (part 2 of 2)
Last month I discussed a few of the issues that face the inspector when looking at transformer installations. Some of these issues included the (earth) grounding of transformers, and the overcurrent protection of the transformer and its conductors. This month I would like to discuss some of the rules about the bonding of transformers, as well as some general rules about transformer installations.
The bonding of transformers and separately derived systems is an area that is often done incorrectly. Since transformers are a separately derived system, the rules found in 250.30 must be followed. When I inspect, the first thing that I look for is the “system bonding jumper”, which is a new term in the 2005 NEC for the conductor that makes the connection between the metal enclosure and the grounded (neutral) conductor. This is the conductor that creates the “effective ground fault current path”, discussed in 250.4(A)(5), and is crucial for a safe system. This connection can be made at the transformer or at the first disconnecting means of the new system, typically a panelboard. This connection must be made at either one these two locations, but not both. When installed at the transformer, this connection is typically a conductor installed by the electrician. The size of the conductor is very often incorrect. 250.30(A)(1) tells us how to size this conductor, and it ultimately leads you to table 250.66. The size of the secondary conductors is what determines the size of this system bonding jumper. For example, if the secondary conductors are 600 Kcmil, the system bonding jumper would have to be 1/0 copper. For very large transformer installations, you may have to use the 12.5% rule found in 250.28(D), as referenced by 250.30(A)(1), but this is a rare occurrence.
The bonding of the new system is also an area that I find myself having to fail on a regular basis. Many electricians use the wrong table to size the bonding conductor that goes from the transformer to the new system’s first disconnect. This bonding conductor, like the system bonding jumper, is sized based on table 250.66, as per 250.30(A)(2). Using the same example as above with the 600 Kcmil secondary circuit, the equipment bond would once again be a 1/0 conductor. Many installers and designers use table 250.122 to size this bonding conductor, which would result in an undersized bonding conductor (3 AWG).
One of the “general requirements” that I see violated often is found in section 408.36 of the 2005 NEC (408.16 in the 2002). This rule states that, generally speaking, a lighting and appliance branch circuit panel must have overcurrent protection on the supply side of the panel. This protection must consist of not more than two circuit breakers or fuses. What this means is that since the transformer isn’t protecting the new panelboard, a main breaker or other overcurrent device (such as a fused disconnect) must be installed in or before the panel.
Another “general requirement” worth mentioning is the rule of section 450.21, which governs the physical location of transformers. For transformers smaller than 112.5kVA, there must be at least 12” of separation between the transformer and any combustible materials. Be mindful that the transformer may require more (and sometimes less) than this 12”. If that is the case, 110.3(B) applies, which means that those requirements must be followed. For transformers rated 112.5kVA or greater, the transformer must be installed in a room constructed of at least one hour fire-resistance rated construction, unless one of the two exceptions to this section is met.
Once again, I hope this helps, and as always, please feel free to contact me with your questions or concerns at:
Ryan@Draper.ut.us
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Ryan Jackson, Salt Lake City |
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apj
Active Member
USA
97 Posts |
Posted - 01/19/2006 : 18:34:10
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thanks for the info. My questions are based on subtransformers set up inside buildings off of the main disconnect panelboard. (Sorry there are so many of them ).
1. You state that the system bonding jumper should not be inside both of the transformer and the first main disconnect panel fed by it. Why?
2. would the exception to 450.21A (2002) reduce the 12" clearance requirement due to the transformer primary being 480VAC or less?
3. Does drywall qualify as a fire resistant barrier in 450.21?
4. You mentioned the bonding conductor from the transformer to the first new system disconnect. Is this the Grounded conductor?
5. Is an equipment grounding conductor required between the transformer and first main disconnect, or just the phase wires and grounded conductor(if utilized)? |
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BrianJohn
Avid Member
271 Posts |
Posted - 10/23/2008 : 14:11:20
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Ryan: I want to thank you for this post not only have I found it informative I have forwarded this to several friends eith the link or the text. I did credit you and hope this was acceptable to you.
"As much as I would like to claim responsibility for this I must give credit to a top notch electrical inspector from Salt Lake City Utah, Ryan Jackson" |
Brian John
Leesburg, VA |
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Ryan_J
Code Guru Moderator
USA
1854 Posts |
Posted - 10/23/2008 : 17:10:35
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| Brian: Its my pleasure to help, and I do appreciate the compliment. I trust that you also saw part one of this article? |
Ryan Jackson, Salt Lake City |
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BrianJohn
Avid Member
271 Posts |
Posted - 10/24/2008 : 06:29:36
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| Ryan: I utilize Part I and Part II. Great for so many of us that are confused by this particular section of the NEC. |
Brian John
Leesburg, VA |
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luckyshadow
Avid Member
USA
241 Posts |
Posted - 03/04/2009 : 20:01:06
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Ryan-
Just to let you know,That it has been almost 4 years to the day that you posted this and it is still be handed out!
I have printed out a few copies and have given them to some of the contractors I have come across with transformer issues.
All good things last the test of time!
Thank you (four years later) |
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Ryan_J
Code Guru Moderator
USA
1854 Posts |
Posted - 03/06/2009 : 16:09:41
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| Wow! Thats kind of cool to read, thanks! |
Ryan Jackson, Salt Lake City |
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Transformer article, part 2
