Trying to understand electrical distro

London E

New Member
Hi,
Trying to figure out calculations for understanding purposes. We have three-phase company power and it says 100 amps. It doesn't seem to specify if that is 100a total or each phase that I can tell. We hook it up to a dimmer rack of 24 120v 20a dimmers. My understanding is that if I was to max out all the dimmers it would have 57,600 watts available. How do I calculate what the max from the company power is? (we have another one that says it's 60 amps - again no idea if it's per leg or total).
Thanks,
Bonus points if it can be explained in a way that I can actually understand (I'm a little slow on the math side of things)
 
100A per leg. 3 x 100 = 300A total available. Dimmers are single phase devices, so 24 x 20a = 480A total needed / 3 phase = 160A needed/ phase. That is 60A/phase under powered. If you are able to move some dimmers to the other 60A distro, you could get away with it. It is good to have capacity head room because any non-linear loads, etc can put harmonics on the neutral, causing it to be undersized. A 200A distro would be the ideal way to go....if you could wish for anything.
 
Elsewhere you'll find similar logic. Dryer, stove, welding, truck outlets are usually 240V, 2 phase or bi-phase, 2 hot legs of opposite waves with a neutral. When you look at the breakers they are joined so one tripping takes the whole circuit down. Therefore the size of a hot leg is far more important than the grand total, which relies on perfectly balancing the legs.

"Load Diversity" is an important concept here too. It's the idea that you'll never fill those dimmers and run them all up to max. Again the idea is common. Standard duplex outlets have 2 15A rated receptacles and there are often many of them on a single 15 or 20A breaker. Don't use too many power tools at once!

@brucek mentioned headroom which is also critical to power calculations. 80% of the rated supply is a common safety factor. Before going over that you should know a lot more than the nominal load.
 
Elsewhere you'll find similar logic. Dryer, stove, welding, truck outlets are usually 240V, 2 phase or bi-phase, 2 hot legs of opposite waves with a neutral. When you look at the breakers they are joined so one tripping takes the whole circuit down. Therefore the size of a hot leg is far more important than the grand total, which relies on perfectly balancing the legs.

"Load Diversity" is an important concept here too. It's the idea that you'll never fill those dimmers and run them all up to max. Again the idea is common. Standard duplex outlets have 2 15A rated receptacles and there are often many of them on a single 15 or 20A breaker. Don't use too many power tools at once!

@brucek mentioned headroom which is also critical to power calculations. 80% of the rated supply is a common safety factor. Before going over that you should know a lot more than the nominal load.
Bi-phase? Two Phase? There was such a thing as 2 phase power (there was also DC service on Broadway) but I'm pretty sure there are only a couple of plants to produce that service.

In small commercial and residential occupancies we have "240/120 Volt split-phase" service, The neutral is taken from the center tap of the transformer's 240V secondary (and bonded to grounding electrode system). The neutral carries the *imbalance* of load between the L1 and L2.

Three phase Wye service has the same neutral behavior - the imbalance of current between L1, L2, and L3 flows on the neutral. {optional story} As arena electrician 20+ years ago I was hooking up services for a holiday tour's LX. The show ME mentioned that the 100% moving light design was load balanced and we'd see only a couple of Amps on the neutral, caused by the load of FOH console and misc on L1. Bored and challenged, I put the clamp ammeter around the neutral and measured more than 2 amps (the exact number now escapes me, but ~8 amps comes to mind). This is both curious and alarming, because if the show loading is genuinely balanced, either a load is mis-patched or there is a compromised conductor/connection. I measure L1, L2, and L3, and L2 is pulling more current than the others. I go find the show ME and tell him what I've found. He's not calling or operating, so we follow the feeder from switch to dimmers/distro. We find a hot CamLok in the feeder run that's noticeably warmer than its black, blue, white and green friends. The ME tags it "reterminate" with white gaff tape. Had he not mentioned his balanced loading and my bit of boredom, it would likely have gone undetected until it became a really hot, smelly mess. {/optional story}
 
100A per leg. 3 x 100 = 300A total available. Dimmers are single phase devices, so 24 x 20a = 480A total needed / 3 phase = 160A needed/ phase. That is 60A/phase under powered. If you are able to move some dimmers to the other 60A distro, you could get away with it. It is good to have capacity head room because any non-linear loads, etc can put harmonics on the neutral, causing it to be undersized. A 200A distro would be the ideal way to go....if you could wish for anything.
It's pretty normal for dimming racks to be hooked up to services that aren't capable of powering all the dimmers at a full load. In most instances the odds of you wanting running all of your dimmers at full is pretty small.
 
It's pretty normal for dimming racks to be hooked up to services that aren't capable of powering all the dimmers at a full load. In most instances the odds of you wanting running all of your dimmers at full is pretty small.
In theatre, perhaps.

In music concert work it is/was common to have a 'full bump on' of every 1kW PAR can at some point in the show and the smart LDs would try that at sound check, if not before. I got to witness a show nearly 'not happen' because the 400 Amp 3 phase service would trip the breaker with an instantaneous 340 Amp load. Generator was delivered, patched, and tested with full load just minutes before doors.

I think we are seeing greater circuit utilization than in the past and what we used to think of as having a bit of headroom will cease to be the case.
 
In theatre, perhaps.

In music concert work it is/was common to have a 'full bump on' of every 1kW PAR can at some point in the show and the smart LDs would try that at sound check, if not before. I got to witness a show nearly 'not happen' because the 400 Amp 3 phase service would trip the breaker with an instantaneous 340 Amp load. Generator was delivered, patched, and tested with full load just minutes before doors.

I think we are seeing greater circuit utilization than in the past and what we used to think of as having a bit of headroom will cease to be the case.
ETC did a big survey sometime before mass adoption of LEDs and found that the vast majority of installed 20A dimmers usually have a single 575W or 750W load because of the desire for individual control of fixtures in many/most cases. Places that have since converted to LEDs with existing power infrastructure may be more heavily loading individual circuits, but the total load for the rack has only gone down. Newer installs are more likely to be sized closer to the expected load to save money, and of course for a tour you would try to only bring exactly what you need.

So, yes: it's a good idea to make sure that your feed is adequate to power all of your fixtures at full at the same time, but that doesn't necessarily mean matching the capacity of the rack.

Bi-phase? Two Phase? There was such a thing as 2 phase power (there was also DC service on Broadway) but I'm pretty sure there are only a couple of plants to produce that service.
Some people (myself included) have adopted "bi-phase" as a term to help distinguish between true 120V single-phase (one hot) and 120/240 (two hots 180 degrees out of phase)--which is often also referred to as single-phase. It makes more sense than calling that single-phase, but since it isn't in universal use yet, you usually still end up needing to explain it much of the time.
 
Some people (myself included) have adopted "bi-phase" as a term to help distinguish between true 120V single-phase (one hot) and 120/240 (two hots 180 degrees out of phase)--which is often also referred to as single-phase. It makes more sense than calling that single-phase, but since it isn't in universal use yet, you usually still end up needing to explain it much of the time.
As Lenny Bruce said, "words are my business."

Split phase is, I believe, the correct term for this 240/120V service. It IS single phase, as the center tapped neutral does not (cannot) alter the time relationship between L1 and L2 of the split phase service relative to each other, not does it alter the time relationship with the other rotational phases of the primary service.

Phase is about time, not polarity.

Edit ps: In full agreement about so many 2.4k dimmers with single 575-750W loads in the desire for granular control of luminaries. What I'm observing is the venue MEs who do not power LED or similar fixtures from parked dimmers are running out of non-dim capacity. Many are putting in requisitions for relay packs or similar offerings, but they're running out of fixed voltage circuits at the moment.

I think we're already seeing consultants spec fewer dimmers and more relays and it will be fairly soon that conventional dimmers will largely be obsolete in most theatre applications.
 
As Lenny Bruce said, "words are my business."

Split phase is, I believe, the correct term for this 240/120V service. It IS single phase, as the center tapped neutral does not (cannot) alter the time relationship between L1 and L2 of the split phase service relative to each other, not does it alter the time relationship with the other rotational phases of the primary service.

Phase is about time, not polarity.
Exactamundo.

ST
 
As Lenny Bruce said, "words are my business."

Split phase is, I believe, the correct term for this 240/120V service. It IS single phase, as the center tapped neutral does not (cannot) alter the time relationship between L1 and L2 of the split phase service relative to each other, not does it alter the time relationship with the other rotational phases of the primary service.

Phase is about time, not polarity.

Edit ps: In full agreement about so many 2.4k dimmers with single 575-750W loads in the desire for granular control of luminaries. What I'm observing is the venue MEs who do not power LED or similar fixtures from parked dimmers are running out of non-dim capacity. Many are putting in requisitions for relay packs or similar offerings, but they're running out of fixed voltage circuits at the moment.

I think we're already seeing consultants spec fewer dimmers and more relays and it will be fairly soon that conventional dimmers will largely be obsolete in most theatre applications.

In new pro theatre builds, would dimmers still make sense at this point?
 
I'd be surprised if some dimmers didn't make sense, even in a new build, but I think the days of rack upon rack of a couple of hundred dimmers in a basement somewhere may rapidly be numbered.
 
It is important to note that removing the racks upon racks of dimmer will allow the power service to be smaller (150A vs 200A vs 400A, etc) which can result in a lifetime reduction of electrical charges that power companies charge for service capacity. If you put in 400A service, you will be paying for 400A service until the building is torn down...even if you are only pulling 60A for your LED fixtures. Bigger is not better.
 
In new pro theatre builds, would dimmers still make sense at this point?
Potentially. Regional theaters that are going to buy new swaths of LED fixtures and throw their 20 year old fixtures? Maybe not. Regional theaters that have a decent, functional inventory of tungsten fixtures yet or roadhouses? More than likely yes. Though that may be in the form of something like ETC's ThruPower modules that can dim or power LED/mover loads, or for a smaller number of dedicated dimming circuits to be interlaced in the distribution scheme with relay-controlled circuits, which would be cheaper than ThruPower

Personally -- in a new build, unless something like ThruPower is necessary or appropriate, I prefer to avoid it if it's going to be driving LED's/movers/etc 98% of the time. I'd rather project budget go toward more LED fixtures than toward the expense of more copper/feeder/distribution/ThruPower. Saves a fair chunk of change only putting in a 200A feed to a relay panel compared to 800A for a dimming system. If tungsten fixtures are going to be an edge-case for a particular venue, a handful of dimming circuits can be mixed into the circuit plot that's otherwise relay circuits -- or a portable Sensor rack can be found on-the-cheap and tied into a company switch.

All depends on the client, their use case, likelihood of having to meet riders or be compatible with outside equipment someone else may bring in, so on, such forth. Pro venues come in all shapes, sizes, and expectations, so really this answer is a big fat "It Depends."
 
In new pro theatre builds, would dimmers still make sense at this point?
A former coworker has been planning installs for school and house of worship systems. They have not specced a dimmer rack for greenfield builds in almost a decade. The current solution is to have a few single dimmer packs and shoebox dimmers for set practicals that can set up when needed. Otherwise not worth the overhead to install.
 
In new pro theatre builds, would dimmers still make sense at this point?
I'm with the "some" crowd. I think there will be a market for some dimming and the tungsten loads for the next 20 years or so but the days of 200+ 2.4kW dimmers in new construction are over.
 
As a former LD at a large road house, 300 dimmers, 300 incandescent S4's, we contemplated upgrading to "some" S4 LED fixtures, likely replacing about 1 in 3 to LED. We recognized that many touring companies, especially if they had cues pre-written on ETC compliant USB, were not going to have the time to convert a show to an LED based plot. We were aware of methods to make LED's function as an incan. with a color filter, but were were worried about the time required if the entire plot were LED. We had as of last year a lot pf experience with LED as we had essentially converted over our Dept. of Theater large space with near 300 Lustres, Desires, etc.... so were pretty aware of what was involved in terms of power and data distribution.
 

Users who are viewing this thread

Back