Single Conductor Penetration Question

[Talmadge]

I have never done this, however I have seen other electricians do this and it cause harmonic issues, voltage imbalance and fires.

Example: Installing all of A phase conductors in one conduit, B phase conductors in a second conduit and C phase conductors in a third for a parallel circuit. When I install any parallel circuit (generally a service entrance) I install all phase conductors and the neutral in each individual riser conduit. I don't think the power company would hook it up otherwise.

Another example: A friend installed a panel in his garage. It was a 100amp sub panel fed from the "mobile home" panel located on his meter pole. when he penetrated the panel he ran each phase conductor and the neutral and ground through four separate knockouts in the panel. This caused a voltage imbalance and it began heating up the 100 amp enclosure. He had unintentionally set up an induction heater. I corrected the issue by running them all through a single 2" knockout and all was fine. I have worked house fires where the service caught fire for the same situation (according to the cause and origin investigator).

If we run a ground from a service in EMT to the ground rod, we are required to install a bonding bushing on the EMT fitting in the panel for the same reason.

I used to be an electrician/mechanic in an industrial facility, we used bearing heaters to heat bearings to get them to slip onto a shaft easier, it's same principal except the conductor is a series of wraps that close around the bearing.

So this was my real life situation last week. Installing a temporary power generator, 400 amp distribution panel with 4 100 amp 480 volt breakers feeding four large temporary air conditioners. The equipment was designed to install female cam-lock tails on the breakers and run them all through one large opening that was shielded from water intrusion by a rubber flap (you see this on large trailer mounted generators). The city electrical inspector wouldn't allow the large hole to serve as a proper penetration and required us to install cord grip fittings as a strain relief. The wire was single conductor 1/0 SOOW (like welding leads). I raised the question about running a single conductor through one hole in a metal enclosure. We ended up doing what the inspector required, however I cut a large Teflon plate to fit the large opening and drilled out the Teflon plate to accommodate the fittings. I was still worried about the fittings heating up, but they didn't. I spray painted the Teflon grey to match the enclosure.

The result was that we passed the inspection, but the city inspector didn't really examine the plate closely. If he knew that it was Teflon, he might have raised a question or failed it.

Upon further examination of the generators, I noticed that the individual phase, cam-lock receptacles (factory installed) on the generator are mounted in a metal panel that is bonded to the frame and ultimately grounded to a ground rod. These don't heat up, cause a voltage imbalance or harmonic issues. Here lies my question, what am I missing? Why doesn't this cause problems when I have seen the same principal cause issues in other situations?

I've been an electrician for 32 years and a Master for 20, I don't know it all and I have to seek advice/answers when I run into an odd situation.

 

[ron]

The code requires the phase conductors to be bundled together along with neutral and EGC where applicable for magnetic field cancelation (except very specific applications like underground in non-metallic conduit. If they are not bundled and there is ferrous materials adjacent, they will inductively heat the metal.

 

[synchro]

Upon further examination of the generators, I noticed that the individual phase, cam-lock receptacles (factory installed) on the generator are mounted in a metal panel that is bonded to the frame and ultimately grounded to a ground rod. These don't heat up, cause a voltage imbalance or harmonic issues. Here lies my question, what am I missing? Why doesn't this cause problems when I have seen the same principal cause issues in other situations?

As Ron indicated, having an individual phase pass through ferrous material is problematic due to heating from hysteresis losses. Perhaps the metal panel in the generator you saw was aluminum. Some currents will be induced in aluminum by the magnetic field surrounding an isolated phase conductor, but aluminum is a very good electrical conductor and so there are minimal resistive losses that could produce heating.

 

[infinity]

If the holes were in ferrous metal you could cut a slot between all of the holes.

 

[BarryO]

I spray painted the Teflon grey to match the enclosure.

I didn't think painting teflon would work.

 

[Talmadge]

The code requires the phase conductors to be bundled together along with neutral and EGC where applicable for magnetic field cancelation (except very specific applications like underground in non-metallic conduit. If they are not bundled and there is ferrous materials adjacent, they will inductively heat the metal.

Yes, I am aware that the NEC (National Electric Code) requires phase conductors to be bundled with the neutral and EGC (Equipment Grounding Conductor). However, this was #1) mandated by the city electrical inspector, #2) The installation is for temporary power for 14 days and #3) I was working from NEC Article 525 (Carnivals, Fairs & Special Events). This installation was for a major movie production company's 10,000sqft tent set up for a promotional event that the public is charged admission to enter. There were a lot of other things that the city electrical inspector required that the head or Key Grip told me has never been required before in other jurisdictions across the United States. He has been working these events for many years and has set up in nearly every state and a lot of local jurisdictions. I was told that the reason the city inspector was so hard was that this was the first time an event or installation had ever come to this particular area and it was all new territory for this inspection department. Therefore they waned to make sure all of the bases were covered and the installation was required to meet every code that applied. My argument was that not all permanent codes applied to this application and there are articles in the NEC that allow for temporary power installations. This installation was in an suburb city to Houston (Where I normally work). In Houston nothing we did would have been required. In Houston a fence is required to be placed around a generator and all umbilicals have to be protected from damage via traffic (foot or otherwise). Cord ramps are required where umbilicals are ran across sidewalks and schedule 80 PVC is minimum across non-driving areas whereas rigid conduit is required across driveways and parking lots. This inspector required us to cut the cam-locks off of the ends of the umbilicals that connect to the factory cam-lock receptacles on the generator and place the stripped ends on the mechanical lugs also factory installed on the generator. The generator rental company supplied the umbilicals, they were bundled in sets of 5 conductors (Brown, Orange, Yellow, Grey and Green for 480v and Black, Red, Blue, White and Green for 120/208v), the inspector required us to unbundle these sets as they wouldn't lay flat, he wanted them to be laid flat and not cross each other when on the pavement. All this was behind a fence and not a public traffic area. He required that all of the cam-lock connections be elevated and not laid on the pavement in case it rained, even though cam-lock connections are rated to be waterproof or at least weather resistant. I was also required to correctly identify all phases with phasing tape from the generator to the distrubution panels and at every cam-lock junction. He required two 3/8" ground rods placed 6' apart and bridged with 4/0 then another 4/0 ran to the ground lug on the generator and a jumper to the frame of the generator (lugs had to be installed on the frame for this). Each Generator (we had two 150K, one tapped to 480v and the other to 208v) had to have it's own sererate 4/0 ground all connected to the ground rods with a Bundy ground clamp (yes, he spcified only a Bundy clamp could be used) he wouldn't let me attach an aluminium lug to a brass ground clamp as the aluminium would corrode. He also required the tent frame to be bonded to the ground rods. At first he said he wanted all of the existing exit/egress lights to be hard piped with EMT and mounted on proper 4/0 junction boxes. I pointed out to him that Article 525 allowed for SOOW to be ran to fixtures and devices for special events and and only then he allowed us to wire them with 14/3 SOOW (connected to the fixture with a strain relief cord connector) with a plug end and plugged into 12/3 SOOW that we ran from the distrubution panels to create a network that powered just the exit/egress lights, but only 10 lights were allowed on each circuit. Originally the plastic exit/egress lights were screwed to the plywood scene panels and wired with two conductor 16 gage lamp cord with a simple polarized two prong plug end, no ground because the fixtures were plastic and had no lug to terminate the ground. He required us to mount a ground lug in the plastic fixture with a #8/32 machine screw and nut and terminate the ground from the SOOW on that lug. Keep in mind that this is only a two week event, not a permanent installation.

I have a great rapport with most of the inspectors in the areas I work, I rarely have a problem, but this inspector was a real tough nut. In speaking with the managing director of the event, I told him that if the inspection didn't pass after doing all the inspector required, he needed to threaten the city with a torturous interference suit. But we did pass inspection and I have to return in two weeks and dismantle the whole shebang.

 

[Talmadge]

As Ron indicated, having an individual phase pass through ferrous material is problematic due to heating from hysteresis losses. Perhaps the metal panel in the generator you saw was aluminum. Some currents will be induced in aluminum by the magnetic field surrounding an isolated phase conductor, but aluminum is a very good electrical conductor and so there are minimal resistive losses that could produce heating.

I though that the panel my have been aluminum also, however I saw where the paint was chipped on the edge of the door panel and it was rusted indicating that it was indeed ferrous.

 

[Talmadge]

If the holes were in ferrous metal you could cut a slot between all of the holes.

I'm sure that would have worked to eliminate the threat of it melting down, bt the inspector wanted the gland fittings because of water intrusion if it rained. He also mentioned that "critters" could get in the distrubution panels if we left the hole as we originally installed them. I can't see a racoon, opposum or other rodent crossing a huge parking lot at the mall to get inside a temporary electrical distrubution panel.

 

[Talmadge]

I didn't think painting teflon would work.

The Teflon took the pain well, it was textured and I just used Home Depot's cheap Stone Grey primer/paint. I don't think the inspector realized that it wasn't a metal plate. When he made the official inspection he wasn't onsite 10 minutes and never went inside the tent to inspect the lights, SOOW or the bond to the tent poles. I told the event managing director, who was frustrated that he didn't look at all the work "just let it go, the longer he stays the more things he'll require us to do".

 

[LarryFine]

Paragraphs, please. If I may:


Yes, I am aware that the NEC (National Electric Code) requires phase conductors to be bundled with the neutral and EGC (Equipment Grounding Conductor). However, this was #1) mandated by the city electrical inspector, #2) The installation is for temporary power for 14 days and #3) I was working from NEC Article 525 (Carnivals, Fairs & Special Events).

This installation was for a major movie production company's 10,000sqft tent set up for a promotional event that the public is charged admission to enter. There were a lot of other things that the city electrical inspector required that the head or Key Grip told me has never been required before in other jurisdictions across the United States. He has been working these events for many years and has set up in nearly every state and a lot of local jurisdictions.

I was told that the reason the city inspector was so hard was that this was the first time an event or installation had ever come to this particular area and it was all new territory for this inspection department. Therefore they waned to make sure all of the bases were covered and the installation was required to meet every code that applied. My argument was that not all permanent codes applied to this application and there are articles in the NEC that allow for temporary power installations.

This installation was in an suburb city to Houston (Where I normally work). In Houston nothing we did would have been required. In Houston a fence is required to be placed around a generator and all umbilicals have to be protected from damage via traffic (foot or otherwise). Cord ramps are required where umbilicals are ran across sidewalks and schedule 80 PVC is minimum across non-driving areas whereas rigid conduit is required across driveways and parking lots.

This inspector required us to cut the cam-locks off of the ends of the umbilicals that connect to the factory cam-lock receptacles on the generator and place the stripped ends on the mechanical lugs also factory installed on the generator.

The generator rental company supplied the umbilicals, they were bundled in sets of 5 conductors (Brown, Orange, Yellow, Grey and Green for 480v and Black, Red, Blue, White and Green for 120/208v), the inspector required us to unbundle these sets as they wouldn't lay flat, he wanted them to be laid flat and not cross each other when on the pavement.

All this was behind a fence and not a public traffic area. He required that all of the cam-lock connections be elevated and not laid on the pavement in case it rained, even though cam-lock connections are rated to be waterproof or at least weather resistant. I was also required to correctly identify all phases with phasing tape from the generator to the distrubution panels and at every cam-lock junction.

He required two 3/8" ground rods placed 6' apart and bridged with 4/0 then another 4/0 ran to the ground lug on the generator and a jumper to the frame of the generator (lugs had to be installed on the frame for this).

Each Generator (we had two 150K, one tapped to 480v and the other to 208v) had to have it's own sererate 4/0 ground all connected to the ground rods with a Bundy ground clamp (yes, he spcified only a Bundy clamp could be used) he wouldn't let me attach an aluminium lug to a brass ground clamp as the aluminium would corrode. He also required the tent frame to be bonded to the ground rods.

At first he said he wanted all of the existing exit/egress lights to be hard piped with EMT and mounted on proper 4/0 junction boxes. I pointed out to him that Article 525 allowed for SOOW to be ran to fixtures and devices for special events and and only then he allowed us to wire them with 14/3 SOOW (connected to the fixture with a strain relief cord connector) with a plug end and plugged into 12/3 SOOW that we ran from the distrubution panels to create a network that powered just the exit/egress lights, but only 10 lights were allowed on each circuit.

Originally the plastic exit/egress lights were screwed to the plywood scene panels and wired with two conductor 16 gage lamp cord with a simple polarized two prong plug end, no ground because the fixtures were plastic and had no lug to terminate the ground. He required us to mount a ground lug in the plastic fixture with a #8/32 machine screw and nut and terminate the ground from the SOOW on that lug. Keep in mind that this is only a two week event, not a permanent installation.

 

[herding_cats]

We call this "single wire transformer rule"

If you use a non-conductive PVC conduit, the issue goes away.

 

[winnie]

Using a plastic panel for isolated penetrations (say for individual cam-lock phases) is a good solution to the induction problem.

Teflon is a horrible plastic to use for this; it is very expensive and not strong. I'd probably use fiberglass GPO board or something similar.

Cutting slots between the holes is another accepted solution to this problem, essentially putting all the cam-locks through a single weirdly shaped hole.

Isolated phase installations have a bunch of benefits in terms of easy conductor routing, ensuring all parallel conductors are the same length, etc. But the installer has to be _very_ careful to avoid all of the induction issues described. Years ago one of the members here (iwire) had pictures of a service which burned up because of improper isolated phase installation (in steel conduit) as well as pictures of a beautifully clean proper isolated phase installation in underground PVC.

-Jonathan

 

[ggunn]

I once ran across a service where there were three sets of service conductors in two conduits. One conduit had two sets and the other had a single set.

 

[roger]

I have never done this, however I have seen other electricians do this and it cause harmonic issues, voltage imbalance and fires.

Example: Installing all of A phase conductors in one conduit, B phase conductors in a second conduit and C phase conductors in a third for a parallel circuit. When I install any parallel circuit (generally a service entrance) I install all phase conductors and the neutral in each individual riser conduit. I don't think the power company would hook it up otherwise.

I don't see how it contributes to harmonic issues but it can have inductive heating issue if not done properly.

If you were to use PVC it can be a better installation.

 

[LarryFine]

Cutting slots between the holes is another accepted solution to this problem, essentially putting all the cam-locks through a single weirdly shaped hole.

Wouldn't anything metallic, such as lock-nuts or flanges, bridge the gaps?

 

[infinity]

Wouldn't anything metallic, such as lock-nuts or flanges, bridge the gaps?

Non-ferrous locknuts as in Mike's graphic.

 

[xformer]

Non-ferrous locknuts as in Mike's graphic.

What about a duct bank with ferrous rebar?

 

[winnie]

If you have penetrations in a ferrous panel, with slots to create a single weirdly shaped hole, then I'd expect problems with _any_ conductive rings around the conductors. I would only do the 'slots in a ferrous panel' trick with insulating penetrations.

The cam-lock connectors are insulated and mount with screws at the 4 corners...but now I wonder if the screws are joined by a metal ring. Something to check!

-Jonathan

 

[winnie]

What about a duct bank with ferrous rebar?

That could be a problem if the rebar goes between the pipes.

-Jon

 

[infinity]

What about a duct bank with ferrous rebar?

Technically an issue but would it really make much of an impact?