I have a flammable dust collection system and we need to ground/bond the ductwork. This is the first time I have had to do this and I am not too sure how to go about it. I have read the pertinent sections of the NEC but I am not sure what size conductor to use. Also, what is the best way to actually attach the grounding/bonding conductor to the ductwork? The duct is galvanized steel. The longest continuous run is under 70' so how often should the conductor be bonded to the duct? Any help appreciated.
Grounding HVAC ductwork
- Thread starter zachatak
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Re: Grounding HVAC ductwork
It is required by NFPA 664 8.2.2.2.1.8 - but a method is not spelled out. Basically a duct containing high velocity particles will cause minor vibration of the duct and induce a current into it. We cannot have an difference in potential between the duct and any equipment connected to the duct. If we get a spark, there will be a nice explosion due to highly flammable dust in the duct. NEC 250 contains some info on bonding piping & other raceways so I was looking for a method to bond the ductwork. Whether or not it is required by NEC, its a serious safety issue and I am looking for some help on making this work - not someone to tell me its not required.
It is required by NFPA 664 8.2.2.2.1.8 - but a method is not spelled out. Basically a duct containing high velocity particles will cause minor vibration of the duct and induce a current into it. We cannot have an difference in potential between the duct and any equipment connected to the duct. If we get a spark, there will be a nice explosion due to highly flammable dust in the duct. NEC 250 contains some info on bonding piping & other raceways so I was looking for a method to bond the ductwork. Whether or not it is required by NEC, its a serious safety issue and I am looking for some help on making this work - not someone to tell me its not required.
Re: Grounding HVAC ductwork
I am the engineer. This is the first dust collector of this nature that my firm has done. I have no guidance as to wire size or number of bonds per length of duct. I am in North Carolina. I will probably end up using a #6 or #4 and bonding to each individual section of duct then running back to the main ground in the panel that feeds all of the equipment. This should put all points along the duct and equipment at the same potential. I just hoped that someone else had done this before and could shed some light on the subject.
I am the engineer. This is the first dust collector of this nature that my firm has done. I have no guidance as to wire size or number of bonds per length of duct. I am in North Carolina. I will probably end up using a #6 or #4 and bonding to each individual section of duct then running back to the main ground in the panel that feeds all of the equipment. This should put all points along the duct and equipment at the same potential. I just hoped that someone else had done this before and could shed some light on the subject.
petersonra
Senior Member
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- Northern illinois
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- engineer
Re: Grounding HVAC ductwork
I have seen this done several different ways. I don't know that there is an "approved" way.
Sometimes they seem to be really overkill. I saw one made of galvanized sheet metal where they had used sheet metal screws to connect the pieces together. Around each connection they had installed a braided wire bonding wire using a nut and bolt that went through the flange. I guess the couple dozen sheet metal screws holding the thing togther was deemed inadequate and so was the nut and bolt used to connect the bonding braid to across the two flanges. I can sort of understand not relying on the sheet metal screws, but why they needed a braid was beyond me.
Another one I saw once had a large bare copper wire, maybe a #6, strung along the length of the duct work. periodically it would be wrapped around the duct, twisted back around itself, and than continue on. I seem to recall some kind of fastener was used at each of these points. Don't recall quite what they did. Very strange, but probably effective. It looked like something someone had gone to a lot of trouble to come up with, but seemed like an expensive thing to do and way overkill. This was a fly ash system using heavy SS pipe, each section bolted together to the next section with many SS bolts, so why it even needed the bond wire in the first place was a mystery to me.
I have seen this done several different ways. I don't know that there is an "approved" way.
Sometimes they seem to be really overkill. I saw one made of galvanized sheet metal where they had used sheet metal screws to connect the pieces together. Around each connection they had installed a braided wire bonding wire using a nut and bolt that went through the flange. I guess the couple dozen sheet metal screws holding the thing togther was deemed inadequate and so was the nut and bolt used to connect the bonding braid to across the two flanges. I can sort of understand not relying on the sheet metal screws, but why they needed a braid was beyond me.
Another one I saw once had a large bare copper wire, maybe a #6, strung along the length of the duct work. periodically it would be wrapped around the duct, twisted back around itself, and than continue on. I seem to recall some kind of fastener was used at each of these points. Don't recall quite what they did. Very strange, but probably effective. It looked like something someone had gone to a lot of trouble to come up with, but seemed like an expensive thing to do and way overkill. This was a fly ash system using heavy SS pipe, each section bolted together to the next section with many SS bolts, so why it even needed the bond wire in the first place was a mystery to me.
james wuebker
Senior Member
- Location
- Iowa
Re: Grounding HVAC ductwork
zachatak
I saw something like this and they had to put a grounding lug on each section. Bolted the lug with a bolt and nut(not a sheet metal screw). Ran a #6 through all lugs without a break in it. Normaly this type of dust collection system will have some type of information about this. Hope this will help.
Jim
zachatak
I saw something like this and they had to put a grounding lug on each section. Bolted the lug with a bolt and nut(not a sheet metal screw). Ran a #6 through all lugs without a break in it. Normaly this type of dust collection system will have some type of information about this. Hope this will help.
Jim
Re: Grounding HVAC ductwork
Zachatak, I do a lot of this type of installation in telephone offices for different reasons, but I think the objective is the same, to prevent any sparks or flashover.
I specify each section of HVAC duct shall be bonded with a short piece of stranded tin coated # 6 AWG conductor terminated with 2-hole compression connectors that have #10 bolt holes spaced ? inch on center. The compression lug shall be terminated using a light coat of NO-OX applied to the skinned conductor and compressed using a UL approved crimper specified by the terminal manufacture. I even go as far as to specify the part number and tool model number with dies.
The terminal shall bonded on each end across the HVAC junction by lightly sanding the area with scotch-brite to remove any debris and a light coat of NO-OX applied to the junction area. 2 number 10, ?? self tapping screws, and external tooth lock washers shall be used to bond the finished terminal to the HVAC duct.
As for bonding the duct to a known good ground I use the same technique just using a longer 6 AWG to what we call an integrated collection bus. I assume you will run to either building steel, GES, or a panel EGC buss somewhere.
Zachatak, I do a lot of this type of installation in telephone offices for different reasons, but I think the objective is the same, to prevent any sparks or flashover.
I specify each section of HVAC duct shall be bonded with a short piece of stranded tin coated # 6 AWG conductor terminated with 2-hole compression connectors that have #10 bolt holes spaced ? inch on center. The compression lug shall be terminated using a light coat of NO-OX applied to the skinned conductor and compressed using a UL approved crimper specified by the terminal manufacture. I even go as far as to specify the part number and tool model number with dies.
The terminal shall bonded on each end across the HVAC junction by lightly sanding the area with scotch-brite to remove any debris and a light coat of NO-OX applied to the junction area. 2 number 10, ?? self tapping screws, and external tooth lock washers shall be used to bond the finished terminal to the HVAC duct.
As for bonding the duct to a known good ground I use the same technique just using a longer 6 AWG to what we call an integrated collection bus. I assume you will run to either building steel, GES, or a panel EGC buss somewhere.
Re: Grounding HVAC ductwork
Thanks Dereck. Any idea where I could find anything about this in a codebook somewhere? Different NFPA codes mention bonding and grounding duct but nowhere do I see any approved or recommended methods. I am sure I will make something work but I would like to have some code to stand on when my contractor doesn't want to do the work. This job has already been bid/constructed but the contractor overlooked our specifications where we required the bonding/grounding but we caught it in the field during an inspection. The contractor then asked us how they were supposed to achieve this. Our specs did not call out any certain method (just said 'ground/bond per code' - something to that effect) so they are looking for a cheap/easy way out and it would be great to point to a specific code reference.
Thanks Dereck. Any idea where I could find anything about this in a codebook somewhere? Different NFPA codes mention bonding and grounding duct but nowhere do I see any approved or recommended methods. I am sure I will make something work but I would like to have some code to stand on when my contractor doesn't want to do the work. This job has already been bid/constructed but the contractor overlooked our specifications where we required the bonding/grounding but we caught it in the field during an inspection. The contractor then asked us how they were supposed to achieve this. Our specs did not call out any certain method (just said 'ground/bond per code' - something to that effect) so they are looking for a cheap/easy way out and it would be great to point to a specific code reference.
Re: Grounding HVAC ductwork
zach, you are welcome. Unfortunately you are not going to like what I have to say, but I shoot from the hip, so here goes. I do not think you are going to find any minimum requirements or at least details in NEC covering your situation. In my industry we use our own standards that exceed NEC and have specific details. Like you, I was in that boat before once and left out the details like this, and had to write the dreaded change order on my nickle. Obviously what you want is beyond and above any NEC requirement that I know of. Afraid you are just going to have to eat some humble pie and fork over some Change Order cash.
zach, you are welcome. Unfortunately you are not going to like what I have to say, but I shoot from the hip, so here goes. I do not think you are going to find any minimum requirements or at least details in NEC covering your situation. In my industry we use our own standards that exceed NEC and have specific details. Like you, I was in that boat before once and left out the details like this, and had to write the dreaded change order on my nickle. Obviously what you want is beyond and above any NEC requirement that I know of. Afraid you are just going to have to eat some humble pie and fork over some Change Order cash.
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