Minimum distance requirement between bus bars and enclosure per NEC and ANSI? 1

[Benjamos86]

Hello everyone! This is my first post on eng-tips, but I've been a long time observer of numerous topics brought up here and have always found this website to be a useful resource.

I am wondering if anyone can help me determine the requirements for spacing between bus bars and a panel enclosure?

I am designing a panel assembly that will house a number of bus bars used for distributing 480 V 3 Ph power to a separate panel assembly that houses a number of breakers.

For the panel that houses the bus bars, power will be supplied via 500 MCM cable that ties to the bus bar via a number of amphenol connectors. Then we will use 6AWG insulated cable coming off these bus bars via ring terminal connections to feed power to the breakers in a separate panel.

Bus bar material is 110 copper, 3/8" thick X 8" wide. Each bus bar does have a 90 degree bend incorporated into it. The bus bars are mounted inside the panel via 1.25" tall insulator mounts. Each bus bar is spaced 1.5" away from the next closest bus bar and also separated via a sheet of 1/4" thick Garolite.

The closest distance I have between the bus bars and the panel itself is 0.6" with the panel doors closed. This dimension is the one that concerns me and has ultimately led me to posting here.

Should there be any reason for concern with the above mentioned distances? I have researched this topic for a few days and see a lot of suggestions on spacing, but I want to ensure that we won't have any issues with code regulations once the equipment is onsite. Install is in the US.

I've seen some info suggesting that there is no actual requirement per ANSI, and haven't been able to determine whether or not NEC has specific requirements that I am violating with my current design.

Hoping someone can confirm that we won't be violating any code restrictions given the above.

Thanks all for any input!!

 

[LionelHutz]

0.6" is probably too small. I don't know the code reference offhand but try something like UL508 or UL508A or UL840.

 

[Benjamos86]

Thanks LionelHutz for the reply!

I found this in UL508A in Table 10.1:

"Between any uninsulated live part and an uninsulated live part of opposite polarity, uninsulated grounded part other than the
enclosure, or exposed metal part. Between any uninsulated live part and the walls of a metal enclosure including fittings for conduit or armored cable."

And for general industrial control equipment, voltage range 301-600, shortest distance is shown as 1/2" with this same value being shown through oil or air over surface.

Table 10.2, for feeder circuits has different requirements:

Between live parts of opposite polarity, 251-600V, Through air gap is 1", Over surface is 2". Between live parts and grounded metal parts, through air and over surface: 1"

What exactly does "over surface" mean? This table seems to indicate what you suggested, that I'm out of spec with this 0.6" distance between the bus bar and panel face.

 

[Benjamos86]

Should have specified, I believe I would need to reference table 10.2, seeing as this is in fact a power feed circuit that I'm dealing with

 

[xnuke]

When there are many companies that make NRTL-listed distribution equipment, may I ask why you're designing something like this instead of purchasing something from a vetted manufacturer? They can do custom work that will comply with standards.

Also, without evaluation by a NRTL, an AHJ may not approve of the installation. I hope you're planning and budgeting for a field evaluation and are convinced that your design will be in accordance with the proper standard that will be used to evaluate it.

I guess what I'm trying to say is that it is often difficult and costly to get custom electrical equipment installed.

Also, this doesn't sound like an industrial control panel, so UL508A is probably not the correct standard to reference.

Finally, the NEC is generally concerned with how the equipment is installed and connected, and doesn't concern itself with internal equipment design, which is covered by NRTL standards.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Benjamos86]

xnuke,

We have used distribution equipment that carries the NRTL listing on similar projects. But the NRTL listed items we'd be looking to use for this project actually carry quite a hefty price tag. I can only imagine what the price would then look like if we were to make inquiries for a low volume order of custom made items. We have the capability to machine/fabricate raw material in house. So if I can determine the specific guidelines I should be referring to, we can easily manufacture the bus bars in house in order to manage cost/cut lead times.

I fully understand where you're coming from though, and that's essentially why I turned to this forum for some input; to see if this is something we can safely and legally accomplish. Not asking anyone to put this burden on themselves. Just looking for some guidance on where to look. I'm not 100% convinced that my design meets the necessary standards because I'm not really clear on what standards should be referenced here. But I can say that we have many pieces of equipment in our possession that utilize bus bar systems similar to what I am trying to accomplish.

To your comment on UL508A - perhaps you are correct this panel does not fall into the category of an "industrial control panel" - but it will be installed in an industrial environment. It's function is to provide power to another enclosure that houses a number of breakers which in turn provide power to an industrial sized electric boiler. My company specializes in boiler manufacturing, mobile boiler room construction, industrial heating/power generation equipment, etc.

 

[xnuke]

Can you please post a simple sketch of what you're trying to do with the currents shown?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Benjamos86]

I can do you one better. Check out the attached document. Has some screen shots of my modeled up panel assembly and some notes included. Thanks for taking a look!

 

[Benjamos86]

I believe what I need to be referencing is the following from the NEC:

408.56
312.11a

UL should only apply if the enclosure were to be UL certified and needed to carry that label.

Someone please correct me if I'm wrong.

 

[Gr8blu]

The conductivity of air in best-case conditions (below 1000 m altitude, no more than 50% humidity, clean, etc.) works out such that you need to maintain 0.001 inch of clearance between live conductor and ground for each volt. If we're looking at potentially opposite polarities (+/- for example, or phase-to-phase) then it would be twice that. Note that this assumes the major face of the conductor and the ground plane are parallel - which is NOT what you say you have in your panel.

If we have an insulated conductor, the distance(s) are halved. If we have an edge (like you appear to have in your panel), the distances are DOUBLED.

With a 480 V-to-ground scenario, looking at conductor and panel at right angles, and assuming "stuff will happen" in terms of air quality ... I would START at 1.00 inch minimum clearance between the live edge and the grounded panel sheet. And probably go up from there.

Converting energy to motion for more than half a century

 

[che12345]

Dear Mr. Benjamos86 (Mechanical)(OP)
".... The closest distance I have between the bus bars and the panel itself is 0.6" with the panel doors closed. This dimension is the one that concerns me and has ultimately led me to posting here....."
1. Your design may have to comply with UL...or NEC .... and whatever "name" you call it; it has to comply with NEC; which is the " LAW " during inspection by the authority.
2. I think the following is relevant for your consideration. In NEC 373-11 (2) Doors reads " There shall be an airspace of at least 1 in. ( 25.4mm) between any live metal part, .... See also exception: where the door is lined with an approved insulating material or..... the airspace shall not be less than 1/2 in )12.7mm)..."
Che Kuan Yau (Singapore)

 

[Benjamos86]

Thank you to all who have posted thus far. I am using NEC as my guideline for spacing.

My last question relates to the wording the NEC uses for spacing requirements. There are two columns in this table under section 408.56 that indicate different spacing requirements.

One pertains to "opposite polarity where mounted on the same surface" and indicates a space requirement of 2" with nominal voltage not over 1000 volts.

The other column reads "opposite polarity where held in free air" and indicates a space requirement of 1" with nominal voltage not over 1000 volts.

When I read "opposite polarity" I think of a hot and a neutral in a single phase circuit. What about phase to phase in a 3 phase circuit?

What's the meaning of "held in free air" vs "mounted on the same surface"?

My bus bars are all mounted to the back face of the control panel via non-conductive insulator standoffs. Would this be considered as "mounted on the same surface"? My assumption would be yes. But the wording is vague enough that it raises some doubts on my end.

 

[xnuke]

Now that I've seen the diagrams, I strongly discourage you from proceeding further with your design. Buy a listed switchboard from a reputable manufacturer that serves your needs. I would never allow your unit to be installed for the following reasons:
[ul]
[li]Designed by someone unfamiliar with relevant codes and standards and the nuances of electrical design.[/li]
[li]There is absolutely no short-circuit protection for either the busbars or the load conductors. There is a reason why the NEC says that all conductors (with few exceptions) are to be protected at their source against over-currents. You're risking a significant fire.[/li]
[li] No place to attach grounding electrode conductors/equipment grounding conductors to maintain continuity of grounding system. [/li]
[li]Unit not likely to be tested in accordance with requirements established by an NRTL. No temperature rise test, short-circuit current test to find the overcurrent withstand rating, enclosure tests, dielectric tests, etc., among other things. [/li]
[li]Not likely to have the installation approved by an electrical inspector for the above reasons.[/li]
[/ul]

While I appreciate you looking here for assistance, I think there is just too much you don't know to complete this project in a way that protects the safety and welfare of the people at your facility.



xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[che12345]

Dear Mr. Benjamos86 (Mechanical)(OP)22 Mar 22 17:59
"... the wording the NEC uses for spacing requirements. There are two columns in this table under section 408.56 that indicate different spacing requirements.... One pertains to "opposite polarity where mounted on the same surface" and indicates a space requirement of 2" .... The other column reads "opposite polarity where held in free air" and indicates a space requirement of 1" ..... #1. When I read "opposite polarity" I think of a hot and a neutral in a single phase circuit. What about phase to phase in a 3 phase circuit? ... #2. What's the meaning of "held in free air" vs "mounted on the same surface"?..."
1. I do not locate in NEC any "official" definition of "opposite polarity", "held in free air" and "mounted on the same surface"
2. My guessing for your consideration:
a) "opposite polarity" is between a hot and neutral in single-phase, hot to hot in three-phase,
b) "held in free air" are "opposite polarity" held in free air. The separate distance is the "clearance " through air,
c) "mounted on the same surface" are "opposite polarity" held on any insulator. The separate distance is the "creepage" distance through the surface of the insulator.
3. Info: Refer IEC publications for "official" with detail definition mentioned above.
Che Kuan Yau (Singapore)