NEC and Grounding Requirements for a mobile broadcast vehicle

[Dick Perin]

My background is I am a EE (electronics as opposed to electrical) and have worked in engineering with 53' Mobile TV trucks for the last 11 years and I am helping a college that is putting together a "bread truck" sized unit for sports use and have questions on power distribution and grounding.

If you are curious, for large mobile units, there are two sets of 5 Cam-Loc connectors. One set is for HVAC (typically two 10 ton systems) and the other set is for video equipment. A transfer switch allows both HVAC and equipment to be powered by one set of connectors. Incoming power is 3-Phase Wye (208V). If HVAC and equipment are combined, each leg uses a little under 200A. If separate, about 95A for equipment and about 85A for HVAC (depending on outdoor temperature). Besides the 3 phases of power, there is a neutral and an earth ground. Earth ground goes to the mobile unit's chassis. If anyone is curious, I can describe what happens downstream.

For the truck I am helping the college with, they want to be able to plug the truck in at different venues. Eventually they want to tow a generator behind and have the ability to be completely stand-alone for power. The challenge is the university has standardized on a Hubbell connector for all of their venues (for other needs). It is a four pin connector, with each phase rated at 60A. It is also a low Wye system with a neutral - no earth ground. So the question becomes, do they need a Hubbell connector and a Cam-Loc connector, where the Cam-Loc connects the truck body to earth ground? If that isn't needed for code requirements, I assume the neutral should be bonded to the truck body? The load consists of two 120VAC air conditioners (roof mounted) that draw about 13A each. I would put them on one leg. The other two legs would power the equipment (120VAC) and all of the equipment together uses about 30A - 40A. The work will be done by the university electricians, but I am suspicious of their knowledge of the NEC. I attempted to research this on my own and every 'answer' led to a new question. Sure glad I don't have to deal with the NEC everyday. ; - ) Thanks.

 

[K8MHZ]

My opinion, not an NEC covered installation.

See 90.2 (B) (1)

Also, the NEC is not intended as a design manual. 90.1 (C)

The electricians you think don't know the NEC probably are aware of at least the above.

 

[petersonra]

This is not much different than anything else that one would plug into a supply of power.

I will tell you this though - connecting neutral to the frame that is connected to an EGC is a bad idea while it is connected to an external source of power. That bond is supposed to be made at only one point and that is generally somewhere upstream.

If you are powered from a generator, that bond probably should be at the generator.

It is also a low Wye system with a neutral - no earth ground. So the question becomes, do they need a Hubbell connector and a Cam-Loc connector,

I don't know what a low wye system is.

Generally speaking code requires that the equipment grounding conductor be run with the rest of the branch circuit conductors so running it separately is not a code compliant way.

In any case, from what you are saying, you do not seem to have a good understanding of the difference between earth ground, equipment grounding conductor, and neutral. I would suggest that perhaps it would be best if you hired someone competent to handle this design for you.

It seems unlikely to me that a ten ton A/C unit would only draw 13 amps at 120V. Ten tons is 120,000 BTU/hr. My guess is it would draw more like 130 Amps and that would be a huge single phase motor, and highly unlikely. I can't imagine any ten ton A/C running on 120V shingle phase.

 

[K8MHZ]

I would suggest that perhaps it would be best if you hired someone competent to handle this design for you.

Thanks for saying that for me. I was going to and decided not to.

 

[ron]

Sounds like a unique situation.

Do not bond the neutral and ground onto the chassis, because then the chassis might carry current.

The four pin connector could likely be used for (2) ungrounded conductors, a grounded conductor (neutral) and an equipment grounding conductor (you call earth ground). I would suggest you use it that way, or change the connector to have (5) pins. You want the EGC to be there separated from neutral in the event of a fault.

Install a small panelboard in the van to distribute this 60A worth of power so the individual components are protected at their rating.

 

[Dick Perin]

Clarification... and Thank You

Clarification... and Thank You

Thank you all who replied. Especially Mr. Petersonra and Ron. For Mr. Petersonra, a point of clarification... the two 10 Ten Units were used with the 53' professional broadcast trucks. Yes - 3 phase motors, each drew about 85 Amps per leg. The bakery truck I was trying to help out with uses two ceiling mounted A/C units, probably about 8,000 BTU each. They draw 13 Amps per unit. Sorry, my bad... I was talking about two different things. Also, it could be a regional thing, or something one of the electricians here invented, but I have heard of "low wye" as referring to 3-Phase 208 V and "high wye" as 3-Phase 480 V.

I wouldn't connect a neutral and a ground together, I explained that very poorly. I realize they are two different things. Although I will show my ignorance... I thought "bonding" just meant "connecting securely". I will continue reading other posts in the forum as I like learning and I'm obviously in the company of some very smart people. Getting back to the original question, please see the enclosed PDF - those were the two options I was considering. Either the 4-wire on the top of the page or the 5-wire on the bottom of the page. If I had to work on a TV Production truck, I would take the bottom over the top scenario any day. I feel confident that the bottom 5-wire solution meets code. I didn't know that a ground had to be in the same cable as the mains. Although that makes perfect sense for the reasons you gave. So obviously, adding a CamLoc connector to provide a ground isn't code. So it would appear the solutions are either the top drawing or, something else (such as suggesting the university put in additional facilities). If the top drawing doesn't meet code, I would be willing to pay one of you gentlemen to consult, if anyone is interested.

Once again, thank you all for your wisdom and advice. I appreciate it.

 

[tom baker]

In any case, from what you are saying, you do not seem to have a good understanding of the difference between earth ground, equipment grounding conductor, and neutral.

This is the area your university electricians understand. Failure to get this correct can result in a shock or fire, especially when used with a generator.
And in my state, Washington, this type of vehicle is regulated as a Factory Assembled Structure, and wired to certain standards, typically NEC Article 550. If wired out of state and purchased in state, the rules still apply. Either case requires inspections.

 

[ron]

The five wire is the way it would need to be. Why are there transformers? I thought the supply was 208/120V three phase, which means no transformers, just a panelboard to protect the circuits and equipment at their ampacity.

 

[Dick Perin]

Mobile Broadcasst Vehicle

Mobile Broadcasst Vehicle

Hi Ron,

The transformers are there to protect the equipment. In the olden days, each piece of equipment had a power transformer with a nice thick laminated core. Great for swallowing spikes. Nowadays, all of the equipment has switching supplies with just a little toroidal transformer. (Switching is around 100 kHz on a lot of the equipment). The toroidal transformers offer no protection. Even with spike protection on the input to the switching supplies, equipment reliability over the long term goes up remarkably with the additional (1:1) transformers. Thanks for your reply. I appreciate it.

Dick

 

[zbang]

In no particular order-

Is the "standard" Hubbell connector 4 isolated pins and a shell for EGC? Check the connector's data sheet for that. Sounds like the univ. wires them for 120/208v 3-phase 4-wire, but you still need that EGC; however the shell may be rated for that use. Let us know the exact connector and we can give better advice. (Some "Hubbellock" devices are rated as 4-pole 5-wire grounding, the 3-pole 4-wire ones aren't spec'd for carrying a neutral.) If the univ. isn't wiring them with an EGC, they're doing it wrong everywhere; don't be a party to that.

Forget the top drawing.

Forget the transformers unless you have extra cash. Broadcast mobile trucks may want them for isolation but for a collage truck you don't really need them. I do mobile productions 4-5 times a year and all we use is a UPS for some of the digital equipment (switcher, servers, etc), not for monitors/lights/etc. For the paranoid, add a TVSS.

 

[petersonra]

Are the transformers on the truck or something supplied externally?

If they are supplied externally I think you would need to have a grounding electrode system (GES) connected downstream of the transformers and 5 wires coming from the transformers to the truck - 3 phases, N, and EGC. The EGC would be connected to the truck frame. There would a single point downstream of the xfmrs where the GES, EGC,and N are bonded together. It has to be a permanent connection so cannot be made through temporary cables.

This appears to me to be a separately distributed system (SDS) and the rules for such things would need to be followed.

I reiterate my suggestion to get someone competent to design this. There are just way too many things that can go wrong. You can probably find an experienced PE to design something for you for next to nothing compared to the cost of the trucks. My guess is you are going to need sealed drawings anyway to get a building permit if the xfmrs are external.

 

[iwire]

The challenge is the university has standardized on a Hubbell connector for all of their venues (for other needs). It is a four pin connector, with each phase rated at 60A. It is also a low Wye system with a neutral - no earth ground.

OK first off this needs to be cleared up.

There is no code compliant use of a receptacle with. 3 hots, 1 neutral no equipment ground.

So it has to be one of the following

• 3 hots and an equipment ground
• 2 hots, 1 neutral, and an equipment ground
• A code violation

Before you can begin to design this job you must find out what is up with those receptacles.

 

[Dick Perin]

Thanks... Enclosed are the spec sheets on the male and female Hubbell connectors the university uses. As you can see, a shell ground isn't an option since there is no metal other than the pins (and strain relief). And I was told by the university electrical staff that it is 208V Wye with a neutral. Did they mean 208V with a ground? Maybe. But if you were using a 4-pin connector, seems like Delta with a ground would be a better use of the connector (smile). Although then the truck would need a Wye to Delta transformer. Oh... the PDF datasheets are too big for the forum. But the connectors are HBL460P9W and HBL460R9W. They are a waterproof and the body of them is made of "Zytel 801 Nylon".

 

[iwire]

And I was told by the university electrical staff that it is 208V Wye with a neutral. Did they mean 208V with a ground? Maybe. But if you were using a 4-pin connector, seems like Delta with a ground would be a better use of the connector (smile). Although then the truck would need a Wye to Delta transformer. Oh... the PDF datasheets are too big for the forum. But the connectors are HBL460P9W and HBL460R9W. They are a waterproof and the body of them is made of "Zytel 801 Nylon".

In my opinion that plug / receptacle is designed for 3 hots and a equipment ground.

If they are using it as 3 hots, neutral and no ground that is a violation, it is dangerous and there is no way in the world I would have any part in designing a system to connect to it.

 

[jaggedben]

And I was told by the university electrical staff that it is 208V Wye with a neutral. Did they mean 208V with a ground? Maybe. But if you were using a 4-pin connector, seems like Delta with a ground would be a better use of the connector (smile).

If it's wye, there's a neutral. If it 3-phase with no neutral, it's a delta. A 4-pin connector would be appropriate for delta or for single-phase 120/240 with 2 hots, neutral, and ground. The connector is probably intended primarily for the latter. It's not appropriate for use with a wye. I suspect we have a code violation here.

 

[iwire]

I suspect we have a code violation here.

I am going with the guys at the University don't have a clue and may even be using the ground as a neutral.

 

[ron]

It is possible that the plug and receptacle is a 18-60P and 18-60R.

The NEMA table uses messed up language to describe this, which is 3 pole, 4 wire grounding or I've also seen it called 4 pole - 4 wire.

This is a 60A connector, but as mentioned, used for 3 "hots" and a ground, or 2 "hots" a neutral and a ground.

You application requiring 60A and three phases a neutral and a ground is not a standard NEMA connector.

 

[iwire]

It is possible that the plug and receptacle is a 18-60P and 18-60R.

He has this pin and sleeve receptacle / plug combo http://www.hubbellcatalog.com/wiring/section-e-datasheet.asp?FAM=PS&PN=HBL460P9W

I cannot think of any NEC compliant application of it without one pin being used as the EGC.

 

[zbang]

I cannot think of any NEC compliant application of it without one pin being used as the EGC.

And if you dig into the catalog, it's spec'd for "3Ø 250V" not "3ØY 120/208V" (which would be a HBL560P9W), or a HBL460P12W (120/240V).

No matter how you slice it, there's a problem here, and I also suspect CDD (Clue Deficit Disorder) on the part of the univ. electricians.