The NEC, stray voltage, and ductwork
The NEC, stray voltage, and ductwork
(OP)
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
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






RE: The NEC, stray voltage, and ductwork
RE: The NEC, stray voltage, and ductwork
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
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"Why not the best?"
Jimmy Carter
RE: The NEC, stray voltage, and ductwork
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.
RE: The NEC, stray voltage, and ductwork
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.
RE: The NEC, stray voltage, and ductwork
Our 2008 NEC Handbook explains, "This reserved space permits busways, conduits, raceways, and cables to enter the equipment," which confirms dpc's explanation.