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[TJ (Tom) Cornish]

Jonathan, I see that we are in agreement that the pictured device is a NEMA 6-50.  However, we each suggest a different wiring of the poles. (One hot at 250v to ground, vs two hots at 120v to ground and 240v leg to leg.)  I thought that NEMA 10-50 had the two hots configuration with 3P,3W. Am I missing something, and if so, what? 

Edit:  I'm not a licensed electrician, so I always try to question what I think I know (and not screw up).

Mark, in the US, there isn't a standard voltage of 250V to ground.  The NEMA 6-50 is hot, hot, ground, missing the neutral wire.  Each hot is 120V to ground, and 240v between the two hots.

The NEMA 10 series is hot, hot, neutral, missing the ground wire.  Same voltage of the hot legs as the NEMA 6 series - 240v between hots, 120v hot to neutral.  This is an obsolete plug series.  https://en.wikipedia.org/wiki/NEMA_connector#NEMA_10

[Guy Holt]

A small portable transformer will weigh less than the printed version of your whitepaper.  I suggest again that if you want to be helpful, take some time to break up your information into a form where people will actually read it, and then provide some sources or specifications where people might have a chance to purchase and assemble the right components.  Better yet, get Multiquip or someone to package appropriate transformers into a listed product so they can actually be setup and used by non-electricians.

I can see why it would be hard to start in the middle of my white paper on the use of portable generators in motion picture production, but it is not meant to be a quick easy read. The purpose is to provide an in-depth technical understanding of the dynamics of power generation and power quality. It starts with the basics of power generation and then drills down to an electrical engineer’s understanding of power quality.  Based upon that understanding, it then tries to offer a better approach to small portable power generation and distribution for non-electricians.

I try to summarize pertinent information in my posts, but I don’t always have time and it is just quicker to provide links to the pertinent section of the white paper. I also write technical articles for trade journals that are more summary in nature. For those interested, there is one coming out in Protocol on paralleling Honda EU6500s and EU7000s.  If you are not familiar with Protocol, it is the quarterly publication of PLASA – an international organization working to raise standards, improve skills and strengthen the events, entertainment and installation industries. If you can’t find the print edition of the magazine, it will be available online on or about Oct. 9th at http://na.plasa.org/publications/protocol.html.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

[Stephen Swaffer]

A small portable 12kVA transformer will step down the 240V to 120V single circuit and create the neutral required for your 120V loads (use this link for details: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorSingle%20Large%2060A/120V%20Circuit.)

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

However, a more direct answer to the OP's question would be to use a 12kW 240 volt primary to 120/240 volt secondary which could be correctly wired (by bonding the neutral on the secondary to the ground) which can then be connected directly to a 14-50R or female CS-6364 and used with the existing distro without modification.

Given the "installed base" of 120/240 volt wire distro's in the field, IMO in many cases creating a single large 120 V circuit creates more headaches than it solves.  You can't simply tie the 2 hots together on a 4 wire system, because then you overload the neutral.  Most readily available panels are designed for 120/240 v split phase, etc.  Unless there is a driving technical need to do otherwise, keep life simple.

[Jonathan Johnson]

Mark, in the US, there isn't a standard voltage of 250V to ground.  The NEMA 6-50 is hot, hot, ground, missing the neutral wire.  Each hot is 120V to ground, and 240v between the two hots.

The NEMA 10 series is hot, hot, neutral, missing the ground wire.  Same voltage of the hot legs as the NEMA 6 series - 240v between hots, 120v hot to neutral.  This is an obsolete plug series.  https://en.wikipedia.org/wiki/NEMA_connector#NEMA_10

To add/clarify a bit, in the US, single/split phase 120/240V is a center tapped secondary on the transformer, with that center tap referenced to ground. That gives 120V (nominally) from each "hot" leg (each side of the transformer) to ground, and 240V (nominally) from leg-to-leg.

Note that utilities aim to supply a voltage of 120/240V, but NEMA classifications rate voltages at 125/250V.

[Frank DeWitt]

Agreed. No safe way to hard wire adapt that one.

Guy Holt took this from no safe way to do this to insert this transformer and you have good safe grounded power. That SEEMS like a good thing.

[TJ (Tom) Cornish]

Guy Holt took this from no safe way to do this to insert this transformer and you have good safe grounded power. That SEEMS like a good thing.

To Guy's credit, most of us wouldn't normally consider using a transformer to solve this problem.  Assuming the 6-50 receptacle is wired correctly, the right transformer would indeed make the receptacle safe to use, however I'm with Steve in that I'd rather have a 4-wire 240/120 center-tapped transformer rather than a 120v only stepdown.

The practical challenges to using this approach are the cost and weight of the transformer, as well as sizing the transformer to the supply and/or the size of the distro, and the resulting over-current protection issues- i.e. do you use one large transformer that matches your distro size that may be too big some of the time, or use smaller transformers that may require additional over-current protection devices, etc.  All of these can be overcome, but depending on your equipment and the frequency of encountering a supply like this, it might be better to use the receptacle as-is for equipment that can be run at 240v, and use standard receptacles for the rest.  That would be my approach under the circumstances, as the vast majority of my equipment can run at 240v.

[Rob Spence]

I encountered one of these at a venue years ago when advancing a gig. They had 14-50 receptacles elsewhere on the property. I told him it wasn't safe to use. He got it changed out to the correct one before the gig (and, yes, I checked that they added the 4th wire). It was a cheap and correct fix.


Sent from my iPad using Tapatalk HD

[Stephen Swaffer]

To Guy's credit, most of us wouldn't normally consider using a transformer to solve this problem.

Grainger has a 15 kVA transformer for $2039 that weighs 203 lbs.  They have another for $1220 that weighs in at 286 lbs.  You might beat these prices and weights by a bit-but this explains why these are not normally considered for this application.

A good technical answer is not always a good practical answer.  In a given situation, 120 volt distribution might make sense.  OTOH, if I have a limited budget and do a mix of shore power and genny gigs and need to be able to minimize load in/out time and weight I need to transport then the equation changes.

Giving options and thinking outside the box is a good thing-but quite often the "typical" answer is there for a very good reason.

[Guy Holt]

Grainger has a 15 kVA transformer for $2039 that weighs 203 lbs.  They have another for $1220 that weighs in at 286 lbs.  You might beat these prices and weights by a bit-but this explains why these are not normally considered for this application.

A good technical answer is not always a good practical answer.  In a given situation, 120 volt distribution might make sense.  OTOH, if I have a limited budget and do a mix of shore power and genny gigs and need to be able to minimize load in/out time and weight I need to transport then the equation changes.

Giving options and thinking outside the box is a good thing-but quite often the "typical" answer is there for a very good reason.

Grainger is not the place to look for portable transformers. For a situation like this I would recommend a 10kVA transformer like the one we built for our rental inventory pictured below. 

As you can see it is relatively small and weighs 160lbs.  It is equipped with a True RMS Volt/Amp meter, voltage boost knob, and an 84A breaker which, while not all the power available from the 14-50 receptacle under discussion, is plenty for most applications. Easily picked up with a hand truck, it can be easily transported without a forklift. And, as you can see it can be used equally with shore power and generated power.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

[Stephen Swaffer]

What type of a distro do you recommend?  You can't legally run 5-15 or 5-20 receptacles on an 84 amp circuit so you still need a custom distro.  Could be the right solution for some situations-depending on budget and overall cost.

[ProSoundWeb Community]