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[Chris Jensen]

I have always been an end user using provided or simple power distribution with proper manufactured products.  This has led me to a lack of some basic knowledge due to the fact that things are plug and play for the systems I deal with.  I am purchasing a new sound system for a facility and going through the stage of predicting where exactly it is going to be used and how to get power to it.  In looking at my infrastructure it's pretty easy to deal with as I have a 120/208V 100A pin and sleeve service or a 120/208V 400A cam panel to use. 

The question that I have arose when looking at voltage drop for feeder runs to see what kind of distance I can get from those locations to keep in the back of my mind for future use.  I'm looking at buying 200' of 2/0 cable with the system so I used one of the online calculators to see what my range with 2/0 would be.  What I ran into is what the 120/208V 100 amps really mean. 

When working with single phase 240 you use 240 as there are 2 legs of 120 right.  When working with 3 phase is the actual voltage being used 208 for these types of calculations?  I know the 208 is phase to phase but the voltage of any one phase is 120.  Is 120V the actual voltage rating that should be used for this type of calculation?  It seems like it should be.  208 is just describing that it is 3ph and not single.

The second part is what does the 100 amps refer to?  Is this a total that each phase can provide?  300 amps of 120V with no more than 100 amps per phase?  I understand that you can't load phase A to 200 amps and then pull 50 on B and C. 

It would seem that I have the potential to use at least 300', probably more, of 2/0 cable for a 208/120V 100amp service.

This is more for learning than actual use for me.  Using the calculator to see what was actually happening got me thinking what these numbers that many of us throw around every day mean.

Thanks,
Chris

[Stephen Swaffer]

 
When working with single phase 240 you use 240 as there are 2 legs of 120 right.  When working with 3 phase is the actual voltage being used 208 for these types of calculations?  I know the 208 is phase to phase but the voltage of any one phase is 120.  Is 120V the actual voltage rating that should be used for this type of calculation?  It seems like it should be.  208 is just describing that it is 3ph and not single.

The second part is what does the 100 amps refer to?  Is this a total that each phase can provide?  300 amps of 120V with no more than 100 amps per phase?  I understand that you can't load phase A to 200 amps and then pull 50 on B and C. 

It would seem that I have the potential to use at least 300', probably more, of 2/0 cable for a 208/120V 100amp service.

There are a lot of "ifs" "ands" and "buts" here.  Not to beat you up, but in the interests of learning here are some thoughts.

The first question I ask-especially in larger cable and long distances is are we dealing with aluminum or copper  (and I usually install some of each every week).  Since you did not specify, I assume you were using copper.

As to voltages, yes 120/240 V is typical single phase, 120/208 is a typical 4 wire "Y" connected service.  Probably won't run in to them much in a venue catering to events using sound, but there is a 240 V 3 phase system that does not have 120 V available.

You would use the voltage of the equipment in question to calculate voltage drop-so if it is a 120 volt amp, use 120 volts.  But you also need to consider the whole system.  The NEC recommends 3% voltage drop on either a branch or feeder, but no more 5% total.  So if you are running the new cable from a subpanel, you technically should consider the wire feeding that subpanel as well.

Yes, you can pull 100 amps on each phase-unless the load is continuous (audio loads are not) then you can only pull 80% or 80 amps per phase-IF the loads are linear/resistive.  If you have switching power supplies, etc. that distort the sine wave, you need to be very careful about overloading the neutral-especially if you are loading each phase to near capacity.  Often neutrals are undersized on the assumption that large loads will be phase to phase not phase to neutral.

Finally, the voltage drop is usually calculated for feet of wire and you have a wire running to the load and then back to the source, so if your load is 200' from the source, you have 400' of wire in the circuit.  For a 3% drop I came up with approximately 370' of wire, or 185' conduit run from panel to load.

[TJ (Tom) Cornish]

I have always been an end user using provided or simple power distribution with proper manufactured products.  This has led me to a lack of some basic knowledge due to the fact that things are plug and play for the systems I deal with.  I am purchasing a new sound system for a facility and going through the stage of predicting where exactly it is going to be used and how to get power to it.  In looking at my infrastructure it's pretty easy to deal with as I have a 120/208V 100A pin and sleeve service or a 120/208V 400A cam panel to use. 

The question that I have arose when looking at voltage drop for feeder runs to see what kind of distance I can get from those locations to keep in the back of my mind for future use.  I'm looking at buying 200' of 2/0 cable with the system so I used one of the online calculators to see what my range with 2/0 would be.  What I ran into is what the 120/208V 100 amps really mean. 

When working with single phase 240 you use 240 as there are 2 legs of 120 right.  When working with 3 phase is the actual voltage being used 208 for these types of calculations?  I know the 208 is phase to phase but the voltage of any one phase is 120.  Is 120V the actual voltage rating that should be used for this type of calculation?  It seems like it should be.  208 is just describing that it is 3ph and not single.

The second part is what does the 100 amps refer to?  Is this a total that each phase can provide?  300 amps of 120V with no more than 100 amps per phase?  I understand that you can't load phase A to 200 amps and then pull 50 on B and C. 

It would seem that I have the potential to use at least 300', probably more, of 2/0 cable for a 208/120V 100amp service.

This is more for learning than actual use for me.  Using the calculator to see what was actually happening got me thinking what these numbers that many of us throw around every day mean.

Thanks,
Chris

Stephen gives good information. One thing is unclear to me - is this for portable use, or for an install?  If an install, there are rules about where and how you can use temporary cabling - you would normally hard-wire distribution rather than using your pin/sleeve or Cam service.

The cable type also affects ampacity. Bundled cable must be derated for multiple current-carrying conductors, but usually you hit your voltage drop limits before you hit your derating limits.

[Stephen Swaffer]

Something else to keep in mind when considering voltage drop is actual system voltage vs nominal voltage.  Some time back I redid the service for a customer with roughly 400' feeders from meter to panels.  During routine voltage tests I noticed that the actual voltage on this 120/240 system was actually 114-115 volts at the meter.  A 3% drop at 120 volts is 3.6 volts-in this case that would potentially leave 111.4 volts or less at the panel instead of 116.4 after the drop.  That is really pushing the generally accepted plus or minus 10%..  I mentioned this concern to the POCO and when I was out to that site last week, actual reading was 120 volts on the nose.  If you are a guest in a facility, probably not much can be done, if it is "your" facility and voltage usually runs low there are usually ways to adjust it and get it right.  In an case, if voltage is borderline you might want to consider larger cable to minimize voltage drop and problems, depending on how sensitive your gear is to low voltage.

[Chris Jensen]

Stephen gives good information. One thing is unclear to me - is this for portable use, or for an install?  If an install, there are rules about where and how you can use temporary cabling - you would normally hard-wire distribution rather than using your pin/sleeve or Cam service.

The cable type also affects ampacity. Bundled cable must be derated for multiple current-carrying conductors, but usually you hit your voltage drop limits before you hit your derating limits.

Thanks for the info guys.  Details aside, this is more about theoretical approach so that I can start thinking about this stuff and observing more when I am on a site at any point in my career. 

Now a few details, right now I have a day job and this system will be a portable system for a student union property on a university campus.  It is primarily used in 3 spots.  One of those spots will be receiving a full install in the next 6-9 months, so that spot is now considered temporary as far as the design for putting this system together and not of long term concern.  In the primary spot the system will be deployed, there is only a need for either a pin and sleeve to cam adapter of at least 6' or 6' of cam tails.  Knowing that I will have to pull power occasionally to the other side of the plaza 100' of feeder will be fine following the established cable runs.  The furthest spot that would need power at this point is the building set for an installed system that will run roughly 300' of feeder the long way around.  I can cut some corners and though buildings if I need shorter.  The problem I have run into is I have 250V 50A drops around the site but my distro is based on 208V cam in to L21-30 out.  After getting it and really playing with it, I can determine if it is worth it to change the 250V drops to 208V.  The site electrician said that when the 250V drop was installed he only ran 2 legs for spider box compatibility.  All he would need to do is pull the third leg and install the proper disconnect.  My only objection to that is how much the pin and sleeve interlock costs for less than occasional use. 

Stephen, good point on the feeder feeding the 100amp and 400amp switches.  They come from a panel about 30' away, not sure about actual cable distance though.  As far as I know they are copper runs.  I have yet to meter the 400amp power to see what it actually is reading as I have not used it yet.  The 100amp is good though.  The calculator that I used took into consideration round trip, although I know most don't think about that when they string out the 50' 18ga home depot extension cord and do some work around the house.  As far as the 80% de-rating, I have seen that before for projector installations.  So this is due to being a constant load device?

The system itself is 6 Arcs Wide over 4 SB28 power by 2 LA4 and 1 LA8.  The distro is a Motion Labs preconfigured 100A rack.  The distro will send an L21-30 to a RacPac in the amp rack.  The other L21-30s will be for stage and monitor power.  This will all be driven by a GLD.  I'm not sure how much voltage drop would be acceptable for this equipment, rather how picky this equipment lineup is.

The long and short of it is that I won't require super long runs of feeder cable, and if I do can rent larger cable if I every approach that 370' or more territory.  IF the need arises for programming at other locations in the facility I am going to be talking to the electrician about him getting power where I need it, not running feeder.

Thanks for the info guys!
Chris

[TJ (Tom) Cornish]

  The problem I have run into is I have 250V 50A drops around the site but my distro is based on 208V cam in to L21-30 out.  After getting it and really playing with it, I can determine if it is worth it to change the 250V drops to 208V.  The site electrician said that when the 250V drop was installed he only ran 2 legs for spider box compatibility.  All he would need to do is pull the third leg and install the proper disconnect. 

There is really no such thing as 250V wiring.  That number shows up on plugs as it represents the "maximum" voltage rating the plug can handle - slightly above the nominal 240/208.  For that matter, 240/208 are really the same thing as well (from the load side - it is derived differently), unless you're running a motor. 

Based on what your electrician said, you actually are already running on 2 legs of "208v" power from a 3-phase system.  You likely couldn't get 240V if you wanted to, since your campus is likely all 208/120v.  The decision for you to change plugs isn't about voltage, but about two-phase distribution - 14-series devices, and three-phase distribution - 21-series devices.

[Chris Jensen]

There is really no such thing as 250V wiring.  That number shows up on plugs as it represents the "maximum" voltage rating the plug can handle - slightly above the nominal 240/208.  For that matter, 240/208 are really the same thing as well (from the load side - it is derived differently), unless you're running a motor. 

Based on what your electrician said, you actually are already running on 2 legs of "208v" power from a 3-phase system.  You likely couldn't get 240V if you wanted to, since your campus is likely all 208/120v.  The decision for you to change plugs isn't about voltage, but about two-phase distribution - 14-series devices, and three-phase distribution - 21-series devices.

That is the way I took it, they all source from 208 panels with only 2 legs to the CS plugs, the 3rd leg goes nowhere.  The best thing then is to get the 3rd leg pulled and replace the CS connectors with the pin and sleeve interlocks.  I already have an adapter that takes the pin and sleeve to CS.  I take it all that is doing is taking two legs of 120 and the 3rd never passes out of the interlock and now I have a 240 service.

This all make complete sense.

[TJ (Tom) Cornish]

That is the way I took it, they all source from 208 panels with only 2 legs to the CS plugs, the 3rd leg goes nowhere.  The best thing then is to get the 3rd leg pulled and replace the CS connectors with the pin and sleeve interlocks.  I already have an adapter that takes the pin and sleeve to CS.  I take it all that is doing is taking two legs of 120 and the 3rd never passes out of the interlock and now I have a 240 service.

This all make complete sense.

You are correct except your terminology - you will not ever have a "240 service" in your building.  Whether you have two phases or three at your destination, if they are sourced from a 3-phase service, they will always be 208 volts leg to leg, whether you have two legs or three.  It is better to say "two-phase", "two-leg", or use the plug designation - L14-30, rather than say "240 volts". 

Some devices such as motors, UPS devices, and a few older moving light fixtures that use two hot legs do care about the voltage and have a wiring adjustment for whether they are powered with 208 volts fed from a 3-phase service, or 240 volts fed from a single-phase service.  If you tell a visiting LD bringing a bunch of old Martin fixtures that you have 240 volts and he sets his fixtures accordingly, his lights will be voltage-starved, and may not operate correctly. 

This is an increasingly rare issue in modern production, as most devices now have a universal power supply that can accept anywhere between 100 and 250 volts, but it is still an important distinction.

[Chris Jensen]

You are correct except your terminology - you will not ever have a "240 service" in your building.  Whether you have two phases or three at your destination, if they are sourced from a 3-phase service, they will always be 208 volts leg to leg, whether you have two legs or three.  It is better to say "two-phase", "two-leg", or use the plug designation - L14-30, rather than say "240 volts". 

Some devices such as motors, UPS devices, and a few older moving light fixtures that use two hot legs do care about the voltage and have a wiring adjustment for whether they are powered with 208 volts fed from a 3-phase service, or 240 volts fed from a single-phase service.  If you tell a visiting LD bringing a bunch of old Martin fixtures that you have 240 volts and he sets his fixtures accordingly, his lights will be voltage-starved, and may not operate correctly. 

This is an increasingly rare issue in modern production, as most devices now have a universal power supply that can accept anywhere between 100 and 250 volts, but it is still an important distinction.

This is exactly what I am looking for.  Aside from my specific application mentioned above, I am looking for basic info, not job site talk that makes sense in industry speak.  I want to be able to talk to an electrician and not get funny looks.  I find it very frustrating to work in an industry with standards and then talk to an electrician who doesn't get anything about those standards that our gear is using.  I come rolling in with a sensor rack and say I need 100amps at 208V and they look at me like I'm crazy and tell me there is no way that equipment needs to run that way.  My only answer is, well they all do it this way.  As it is with my stated example my electrician thinks I'm crazy, because back on the job site in the day 240V was enough for them.  The meeting of worlds!

Thanks for the terminology info, I will think in those terms instead from now on.  It does make more sense that way.

[Stephen Swaffer]

 As far as the 80% de-rating, I have seen that before for projector installations.  So this is due to being a constant load device?

Yes. A 1000 watt projector lamp will pull 1000 watts, likely for longer than 4 hours (the NECs definition of continuous).  A 1000 waytt audio amp might pull 1000 watts for a fraction of a second rather infrequently-a much lower average load-and the average load is what determines the heating effect on wire, breakers, etc.

As for the 3rd leg not being used, if the facility is wired correctly, some of the "2 phase" receptacles will be connected to phases A-B, some to B-C and some to A-C in hopes of keeping the load more or less balanced.  It is a pain not to use one of the 3 phases in a 3 phase panel, so it unlikely that you have an utilized phase.

I questioned being concerned about low voltage for just the reason Tom mentioned.  I am seeing a lot of light fixtures designed for 100 to 277 volts, since 277 is a common lighting voltage in larger facilities.

FWIW my day job is an electrical contractor, I spent 13 years as an industrial electrician.  Practically speaking, 97% of the time I think in terms of 3 voltages-110 V, 220 V, 460 V.  We had equipment with all kinds of nameplate voltage ratings, but they all "translated" to one of these three. If you ask me for 208, 220, 0r 240 without something telling me I need to be picky, you will get whatever I have available and won't think twice about it. 

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