The NEC, stray voltage, and ductwork

[buddy91082]

The National Electrical Code (NEC) has a 6' vertical clearance requirement above electrical equipment (panelboards and switchboards) where no "foreign" equipment (ductwork, piping, etc.) can be installed. This requirement is for low voltage systems , 600v and below.

I An inspector told me that the 6' is in place because to keep stray voltages from energizing metallic objectives such as ductwork or piping. I find this hard to believe. You would need a "stray" current first to produce a "stray" voltage.

Yes, you could get a stray current from the magnetic fields from transformers, and perhaps from feeders that are unbalanced. But i just don't see ductwork and piping getting energized from nearby.

Anyone know where i might find some source of information where studies have been performed on this?

Thanks in advance.

b

 

[dpc]

Nothing to do with stray voltages i don't think. It's just to allow adequate space above for getting conduits in and allowing some working space. The panels are enclosed in a grounded enclosure, so electric field effect is minimal.

 

[waross]

I'm with dpc on this. Remember, even the best and most knowledgeable inspector does not know everything.
In Canada I check the Handbook to The Canadian Electrical Code for an authoritative answer to questions about the "Why" behind code rules. I don't have access to my copy just now. Is there an equivalent handbook for the NEC?.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Bill,

Yes there are at least a couple. The one published by NFPA - the NEC Handbook is a good resource and also includes the entire text of the NEC.

 

[WhyDoYouAsk]

You can check it; measure open circuit on ductwork with respect to ground, then lay a live insulated conductor on the ductwork and repeat.

If there is any AC/DC voltage at all, load it with 1, 10 or 100 ohm resistors and figure out the Thevenin equiv. source impedance.

There might be 100 pF/foot of capacitance between Romex and the ductwork.

 

[Morrand]

I guess this has to do with NEC 110.26(F)(1)(a).

Our 2008 NEC Handbook explains, "This reserved space permits busways, conduits, raceways, and cables to enter the equipment," which confirms dpc's explanation.