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3ph Balancing Loads in Building - Best Practices/Understanding?


Joseph Tese

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I read the following thread, which was helpful to a degree. Perhaps an in-depth degree.

But I still have some questions, as it seemed to just briefly touch on it.

Why is it important to balance the load when plugged into a three-phase building? In my case, we have a company switch tied into a three-pole 100Amp breaker. Does everyone agree I should keep it within 20% difference between phases? Prior to this question, I thought this applied only to gennies, but I thought differently after seeing this video

Granted, it's in 240v land - but will the same affect result here if unbalanced to that degree? I've never heard of that happening to a cable. What do you do if you have to run just a 5k on one hot, and the rest of the lights are LEDs.... Obviously imbalanced, so do I have to source and burn higher-wattage ghosts on the other phases to be safe and proper? If a shoot just lasts a day or two - Doable? Is this detrimental to the cable, distro, and anything within the building power? Also, concerning the neutral taking the difference between hots: Does this apply only to single phase situations, or three phase as well? Eg, if I run 50AMPS on one hot, and nothing on the other two, can I expect 50AMPs on my neutral? This may tie into my previous question, but in situations where my neutral is carrying a load, how/why is this bad? or is it not?

On a sort of similar note - Here's a pic of a two-fer I had made up.

 

It's so I can use a 100AMP lunchboxes on a 50 AMP CS 6364 (125/250v) connection. It splits each hot to get dedicated 120v. Obviously, I'm limited by 50AMPS on each line, and not 100AMPS. 1) Are there any concerns about this? To my understanding, it's the exact same practice as a consumer Generator Y-Splitter from homedepot  2) When using this two-fer in a 3-phase building, does it treat unbalanced loads differently if used in a single phase?

 

TwoFer.jpg

Edited by Joseph Tese
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"Eg, if I run 50AMPS on one hot, and nothing on the other two, can I expect 50AMPs on my neutral? "

Yes but the neutral is designed to carry the load.

the problem with your 2-fer splitter is that there is the possibility that someone can overload a 50amp circuit since you used 100amp connectors. And since you mentioned 100amp lunch boxes someone on set might just look at available receptacles on the lunchbox and starting plugging stuff into it.

It doesn't really matter if it is 3-phase or Single phase- you want to balance the load as best you can.



 

 

 


 

Edited by Ed Conley
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9 hours ago, JD Hartman said:

A 100A lunchbox has a 100A male bates input, how are you going to connect that to your 60A feeder.   You need to get 60A lunchboxes which have only 3 circuits.

Hey JD, there’s no 60amp feeder. Where do you see that? In this scenario, it could only be powered by two phases, and by a 50amp (125/250) plug. Each hot is split and a 100amp Bates connector is put on. The connector accepts the 50amp gauge safely. This works fine, but just needs to be clear and heavily labeled so no one gets the wrong idea, as Ed pointed out.. it’s 50amps per line not 100. To me, this set up just felt a little sloppy, and is my last resort whenever I have to pull power from a 50amp twistlock, and wanted to hears other’s thoughts.

So in scenarios I can’t do much where I just need one high-wattage light, and the rest are leds (which happens often) are there actual negatives to pulling from one leg?

Edited by Joseph Tese
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It is not the worst thing in the world.

the neutral is sized to handle an unbalanced load.

 

( 3-phase motors will run mo' efficiently when the load is balanced)

Even when using a construction style spider box connected to that style 50amp CSA connector there is no way to make sure a crew is balancing the loads.

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On 10/30/2019 at 12:33 AM, Joseph Tese said:

On a sort of similar note - Here's a pic of a two-fer I had made up.

It's so I can use a 100AMP lunchboxes on a 50 AMP CS 6364 (125/250v) connection. It splits each hot to get dedicated 120v. Obviously, I'm limited by 50AMPS on each line, and not 100AMPS. 1) Are there any concerns about this? To my understanding, it's the exact same practice as a consumer Generator Y-Splitter from homedepot  ....

TwoFer.jpg

My concern with your two-fer is that it would not pass a spot electrical inspection.  I once had an electrical inspector pop into one of my sets simply because he drove by it on his way home from work.  An electrical inspector is not required by Code (Section 90.7) to re-evaluate  the safety of equipment that is "listed" and "labeled".  However, if equipment has been modified from its original condition or is installed or used under conditions not stipulated in its' listing, the inspector will likely require it be taken out of service.  An inspector will likel reject this two-fer  for its misuse of listed equipment.

iRobot_Comp_1.jpgiRobot_Comp_2.jpg

A better approach is to use a small step-down transformer to convert the two 50A legs of this 240V receptacle into a single 100A circuit at 120V.  That way you can use a 100A lunchbox without reservation or caveat. A transformer/distro like that pictured here offers a number of other benefits as well.  For details see a white paper I wrote on the use of portable generators in motion picture production, of which Harry Box said "Following the prescriptions contained in this article enables the operation of bigger lights, or more maller lights, on portable  generators than has ever been possible before." The white paper can be found at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting rental and sales in Boston

 

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On 11/1/2019 at 3:01 PM, Guy Holt said:

My concern with your two-fer is that it would not pass a spot electrical inspection.  I once had an electrical inspector pop into one of my sets simply because he drove by it on his way home from work.  An electrical inspector is not required by Code (Section 90.7) to re-evaluate  the safety of equipment that is "listed" and "labeled".  However, if equipment has been modified from its original condition or is installed or used under conditions not stipulated in its' listing, the inspector will likely require it be taken out of service.  An inspector will likel reject this two-fer  for its misuse of listed equipment.

iRobot_Comp_1.jpgiRobot_Comp_2.jpg

A better approach is to use a small step-down transformer to convert the two 50A legs of this 240V receptacle into a single 100A circuit at 120V.  That way you can use a 100A lunchbox without reservation or caveat. A transformer/distro like that pictured here offers a number of other benefits as well.  For details see a white paper I wrote on the use of portable generators in motion picture production, of which Harry Box said "Following the prescriptions contained in this article enables the operation of bigger lights, or more maller lights, on portable  generators than has ever been possible before." The white paper can be found at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting rental and sales in Boston

 

Wouldn’t a lunchbox like this be easier and probably more affordable than working with transformers? Though I’m eliminating use of the 100amp boxes, the same end goal is still achieved (which would be 120v Edison access). I assume an inspector wouldn’t have a problem with this.

prod_lunchbox_FB-F5-PT-03_03.jpg

Edited by Joseph Tese
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On 11/2/2019 at 8:45 PM, Joseph Tese said:

Wouldn’t a lunchbox like this be easier and probably more affordable than working with transformers? ... I assume an inspector wouldn’t have a problem with this.

prod_lunchbox_FB-F5-PT-03_03.jpg

This style of lunchbox has very limited application. In most situations an Inspector would have an issue with its use.

You have to be really careful when splitting 240V circuits. If the 240V circuit is a four-wire system, one can use a "splitter box" like this as long as it does not bond ground and neutral.  Where you run into trouble is that almost all residential and industrial 240V receptacles use a three-wire system (the receptacle has three slots: one for ground, and two for hots, and no neutral.) Most household and industrial 240V receptacles use a three-wire system because they were wired for the sole purpose of powering single phase motors, compressors, or heating elements that draw a perfectly balanced load and return no current. A perfectly balanced load doesn't require a neutral because the single phase service legs are 180 degrees out of phase and cancel each other out - hence there is no return that would require a separate neutral.

You run into trouble with this kind of circuit when you start to pull an unbalanced load  on your splitter box.  And, where under most production situations you can never perfectly balance your lighting load, the two 120V circuits that make up this 240V circuit will not have 100% phase cancellation and the extra current  of the high leg  will need a return path. By necessity with a three-wire system you then have to bond the ground and the neutral together in the 4-pin to 3-pin adapter you use to plug it in, so that the extra current returns on the ground (after all what else can you do with the neutral when plugging into a three-wire 240V circuit.)

There are some people that will argue that it is not such a big deal to carry current on the ground wire. I would argue that it is both unsafe and unwise to carry current on the ground wire. It is unsafe because the ground wire connects all the non-current carrying  metal parts of your system (metal light housings, metal distro boxes, generator frames, etc.)  The ground wire is only intended to carry current in the event of an electrical fault to open the breaker.  It is unwise because bonding ground and neutral after the service side of  the main service head (which is what you have to do with the ground and neutral of a splitter box when plugging into a three-wire 240V circuit) is a violation of NEC Sections 250-23(a) & 250-24(a)(5).  If someone were to fall off a ladder because they took a non-lethal shock because the equipment they were handling  was energized your liability insurance would be null and void because you were using equipment  that did not meet code.

Transformer-Distro_Sam2.jpg

4k & 1.2k HMIs powered from a 30A/240V dryer outlet through one of our  step-down transformer/distros in a sixth floor loft for a Bose shoot.

The only safe way to pull power from three-wire 240V circuits that meets the requirements of the National Electrical Code is to run your lighting load through a small 240V-to-120V step-down transformer.  A transformer can make a 60A/120V circuit out of a 30A/240V dryer circuit that is capable of powering bigger lights, like a 5k Tungsten or 4k HMI, or more smaller lights than you could otherwise.  What makes it safe to use a step-down transformer is that the transformer automatically splits the load of whatever you plug into it evenly over the two legs of the 240V circuit.  Where there is no high leg, the loads on each leg of the 240V circuit cancel out and there is no return that would require a ground/neutral bond.

Beside creating a larger 120V circuit capable of powering larger lights, or more smaller lights, than you could otherwise, transformers offer other benefits over splitter boxes.  For instance, since you typically have to run cable some distance to find a 240V receptacle, we tap our transformer/distros to boost the voltage slightly in order to compensate for the voltage drop over the long cable run from the receptacle to set.  Using transformers with the Honda EU6500 and EU7000 generators bring them into OSHA compliance (they don't meet OSHA requirements for use on job sites otherwise.) For more details about these benefits, as well as others, see my white paper on the use of portable generators in motion picture production.

Guy Holt, ScreenLight & Grip, Lighting Rentals and Sales in Boston 

Edited by Guy Holt
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On 11/4/2019 at 1:54 PM, Ed Conley said:

Same issue as other "splitter box" when adapting to the more prevalent three-wire circuit.  This box has the added problem of having the inexpensive GFCIs  that are prone to tripping with motion picture lighting loads on each circuit.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

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21 hours ago, Guy Holt said:

Same issue as other "splitter box" when adapting to the more prevalent three-wire circuit.  This box has the added problem of having the inexpensive GFCIs  that are prone to tripping with motion picture lighting loads on each circuit.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

Don't adapt it- Anything that requires adapting always poses a possible problem.

 

GFI tripping I can see as quite annoying.

 

Edited by Ed Conley
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Guy, Appreciate the write up. But as you probably know, the connection pictured is a CS6364, and is indeed a 4-wire setup, so the neutral is there to carry the unbalanced load, which is what most of your write up is about.

If I may sum up your post..I gathered the following:

1) Don't split 3-wire 240v receptacles

2) an inspector might not like that style lunchbox, but I don't know why. 

3) A transformer provides double the amperage while maintaining 120v. It also helps with long runs and compensates for voltage drop

Entertain my considerations.. If I wanted to utilize higher-wattage lights, I can just run straight 240? I suppose an adapter from the 30amp twistlock or the 50amp plug, to a 60AMP or 100AMP bates respectively will work..If utilizing a 4-wire receptacle, the neutral would tie into the ground in the bates plug in this case (Or left capped off?) -  but not a problem because everything is staying 240, and the phases are not being split. The only disadvantage would be..well, not having 120v!?

Don't know too well how transformers work, but trying to understand how we're able to keep the amp rating you'd expect from 240 after it's stepped-down to 120v..

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18 hours ago, Joseph Tese said:

Don't know too well how transformers work, but trying to understand how we're able to keep the amp rating you'd expect from 240 after it's stepped-down to 120v..

Nevermind - My brain approached it from the wrong angle. ? Anyway, appreciate all the good thoughts here.

-Joe

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On 11/11/2019 at 1:46 PM, Joseph Tese said:

Entertain my considerations.. If I wanted to utilize higher-wattage lights, I can just run straight 240? I suppose an adapter from the 30amp twistlock or the 50amp plug, to a 60AMP or 100AMP bates respectively will work..If utilizing a 4-wire receptacle, the neutral would tie into the ground in the bates plug in this case (Or left capped off?) -  but not a problem because everything is staying 240, and the phases are not being split. The only disadvantage would be..well, not having 120v!?

The number of higher wattage lights that can run 240V are limited.  While 240V 5k & 10K bulbs do exist, most rental houses lamp their fixtures with 120V bulbs.  4k HMIs with electronic ballasts will operate either 240 or 120, but operating them at 240 is a not very efficient use of power.  A 4kw HIM with Power Factor corrected ballast will draw about 19A per leg operating at 240V. Which means you are tying up the remaining 11A/leg on a 30A dryer plug or 31A/leg on a 50A range plug just to power that one 4k.  A more efficient way to operate the 4k is at 120V through a step-down transformer. That way you still have access to the 22A on a 30A dryer plug or 62A  on a 50A range plug to power other 120V lights (you can power quite a few more lights with that 22A or 62A.)

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental and Sales in Boston

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On 11/11/2019 at 1:46 PM, Joseph Tese said:

... I suppose an adapter from the 30amp twistlock or the 50amp plug, to a 60AMP or 100AMP bates respectively will work..If utilizing a 4-wire receptacle, the neutral would tie into the ground in the bates plug in this case (Or left capped off?) -  but not a problem because everything is staying 240, and the phases are not being split. The only disadvantage would be..well, not having 120v!?

While 60 and 100A Bates females are listed for 240V use, they are customarily used for 120V and so will cause confusion on set leading to the possibility that someone might plug a 120V light into 240V and fry it.  If you want to run a large light at 240V off a dryer or range plug be sure to use the yellow coded 220 100A Bates female. On a four-wire system cap off the neutral. Again, bonding ground and neutral downstream of the main service head, as you suggest here, is against the code.

Yes, the disadvantage is that you don’t have access to 120V and it is an inefficient use of the power available from 240V receptacles for the reason given above.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental and Sales in Boston

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