QOTD: Leaving a phase out

Isn't this why God invented Crutch tips?

And if they want three wire 220 they simply are getting it out of a 3 phase 208 feed..... unless they also provide a roto-phase.
 
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Ok, now that we're past a week, let me further clarify my point from before. If an event comes in with a dimmer rack that's set up for split-phase 120/240V, it (in 99%) of cases will run perfectly normal on 2 legs of a 3-phase 120/208V system. They same would apply to a split-phase audio distro that doesn't have any phase to phase loads. The event electrician should know whether their system has any phase to phase loads. If they don't have any phase to phase loads, then 2 legs of a 3-phase 120/208V system is just as good a split-phase 120/240V system. As a matter of fact, most modern loads that require more than a 120V circuit will run on 208V or 240V. Obviously, this would have to be verified on a case by case basis. A lot of amps with switchmode poser supplies and moving lights with electronic ballasts are perfectly happy switching between 208V and 240V.
 
Assuming that the equipment is happy being fed with 120/208, is it acceptable to load only two legs of the three phase service? Is it OK to have an unbalanced load?
 
Generally, when is it not?

When you're operating near the capacity of the electrical service. Circuit protection operates on a per phase basis, but it trips all phases. So, if you overload one phase, you'll trip the entire feeder. You have to be aware of how much capacity you have available when you're in one of these less than optimal situations. For example, a 3 phase 120/208V, 400A company switch is rated for 400A per phase, 1200A total. If you're only hooking up 2 of the 3 phases, you only get 800A. Of course, this a simplified example, but hopefully that paints the picture.
 
Generally, when is it not?
When using a generator. Ever had your washing machine go nuts and make that thump-thump-thump noise due to an undistributed load? Same concept.

epimetheus' concept of the issue becoming more important the closer one gets to capacity holds true for generators as well. A 100kVA unit won't care or even notice a 20A imbalance, but may well balk at 200A.
There is no doubt that aiming for approximate phase balance is desirable. However, as I previously stated, if the system and its feed from utility power is properly designed, there should be no ill effects from imbalance. For a portable generator where the dimming system is a high percentage of the generator capacity, phase balance does indeed become more critical. ...
...And, if the transformer, switch gear, and dimmer rack are Listed, by NRTL and installation is done to Code [in the US, the NEC], the system as a whole can readily tolerate up to 100% phase imbalance. ...
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Isn't this why God invented Crutch tips? ...
Please advise where one can locate listed, by NRTL crutch tips for the purpose intended. While a clever idea and likely very safe and certainly better than an exposed contact, I don't think we should advocate such a practice, especially when the proper item exists and is readily available.
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CL Female protective cap w/ 20" lanyard (black) | Marinco
 
On a 3 phase 120/208v service, the voltage will only ever be 208 between phases - A-B, A-C, B-C. Dropping a leg doesn't get you 220.

Isn't that what I said ? We hold these truths to be self evident: you can't 220 out of 208.
 
This may be the third thread I've mentioned this on, but we have a 220/127V 3ph system in our building. No, really, P-G is 127V. And let's not forget the two-phase 5-wire we just RE-installed to power an old blower. Or the motor-generator providing DC power to the 1928 elevator, with mechanical relay controls. Our building excels at strange electrics.
 
if the distro is rated to run on 120/240 split phase, which the cam colors seem to indicate, you should be able to safely hook up the two hot cams to the first two phases on the company switch. This will cause an unbalanced load, however, but if the power supply was designed correctly, this should not matter. You may need to double the neutral cable capacity because of the unbalanced load. Since the distro needs basically 2x 120 single phase feeds this should work. If I was in this situation, I would request a distro that matched the company switch, and if it was unprocurable, I would tie the red and black cams on the distro to the first two phases on the switch, and cover the third phase.
 
Not sure if this topic is still open or being monitored by folks here, but have a question. I am faced with this exact situation in a couple of months. I have a suitcase style single phase distro that has a mix of single 15A and 20A breakers and outlets. The venue I am setting up in has 3 phase power, so my plan, as some suggested here, was to tie in my black and red, and cap or tape the blue leg. The good news in my case (I think) is that my feeder is #2 which should be good for 195A.

Here is my question, the venue has 2 CAM panels, one is rated at 100A the other at 400A. I think I read somewhere that this means each leg has that amperage rating, and not the entire feed, is this correct ?? So in the case of the 100A panel (3-phase), if I only use 2 legs I get 2 X 100A, correct ??

Even though I won't be pulling more than 100A total in gear, I am thinking that since my feeder is #2, and I don't have a main breaker in my distro, it is probably a better (not good) idea to cable into the 100A panel and not the 400A one, but again, I won't be pulling more than 100A in gear, so my feeder should be fine.

Thanks
Neil
 
Not sure if this topic is still open or being monitored by folks here, but have a question. I am faced with this exact situation in a couple of months. I have a suitcase style single phase distro that has a mix of single 15A and 20A breakers and outlets. The venue I am setting up in has 3 phase power, so my plan, as some suggested here, was to tie in my black and red, and cap or tape the blue leg. The good news in my case (I think) is that my feeder is #2 which should be good for 195A.

Here is my question, the venue has 2 CAM panels, one is rated at 100A the other at 400A. I think I read somewhere that this means each leg has that amperage rating, and not the entire feed, is this correct ?? So in the case of the 100A panel (3-phase), if I only use 2 legs I get 2 X 100A, correct ??

Even though I won't be pulling more than 100A total in gear, I am thinking that since my feeder is #2, and I don't have a main breaker in my distro, it is probably a better (not good) idea to cable into the 100A panel and not the 400A one, but again, I won't be pulling more than 100A in gear, so my feeder should be fine.

Thanks
Neil

The best place to ask electrical questions here is in the Lighting section of the forum.
Now here is an instance where I have to do my due diligence and say the following very loudly...

IF YOU HAVE TO ASK THIS QUESTION YOU ARE NOT QUALIFIED TO TIE IN CAM-TAILS.

Okay so, most of what you have asked has been already answered in several threads in the lighting area.

A 100A 3-phase, disconnect intended for typical cam-tail tie-in applications has 3 "hot" conductors, a "neutral" conductor and a "bond" or ground conductor hook-up. Each of the 3 "hot" legs is fused at 100A. With the TYPICAL setup you can hook up two of the three (along with the neutral and bond) without a problem, however this MAY not be the case (hey I haven't seen it so I can't say for sure how it is hooked up or what it is hooked up TO). Delta, Wye? etc?

You also MUST have a main breaker in your distribution panel in many cases.

Your profile says you are in Montreal, and I am not directly familiar with any local electrical code requirements for Quebec or Montreal (I am in Ontario), but the CEC parts I and II certainly still apply.

Here AHJ requires cam-lock tie-ins to be done by a "qualified" person, and to have an inspection or at least have a permit. Not that everyone follows that.

You should talk to the venue specifically regarding this. IF you have any further questions feel free to send me a message.
 
My statement was ambiguous, I was referring to cable to be used with common disconnects.

100, 200, and 400 amp disconnects are the most common, so you choose a cable with a rating that exceeds the maximum draw.

In the U.S. Though without reading the entire link about Canada, it appears to be a document about permanent residential wiring; rules up north might be different.
 
My statement was ambiguous, I was referring to cable to be used with common disconnects.

100, 200, and 400 amp disconnects are the most common, so you choose a cable with a rating that exceeds the maximum draw.

In the U.S. Though without reading the entire link about Canada, it appears to be a document about permanent residential wiring; rules up north might be different.
It is a list of the CEC code changes that were made active in 2012, part of which is a change in rated capacity of conductors Changing Tables 1-4.

I think my earlier statement stands however. If you need to ask this or be told this stuff (ie how to select a conductor size) you are not qualified to hook-up cams to the disconnect. Not sure about the US, but the poster who asked is in Quebec and you are supposed to be licensed to do such connections here (not that it is always followed).
 

Just to be clear, the ampacities listed in Table 2 of the link refer to cables or conduits with not more than three current carrying conductors. For single conductor portable feeder cables, both the CEC and the NEC use the free-air ratings, which are much higher than Table 2.

While we're at it, how about some Canadian CB'ers getting on the case to propose updates to Section 44 of the CEC covering theatres? It is woefully out of date--it looks like the NEC did in 1980!

ST
 

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