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Author Topic: (How) Can I connect my single-phase distro to a three-phase tie in?  (Read 17691 times)

Jeffery Foster

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Let's say I arrive at a new venue five hours from home.  Since I did a fastidious advance of the venue, I know that they have a single-phase 100 amp cam lock tie in.  That's great, I stroll in with my single-phase distro with cam lock tails, but what's this? Five cams at the tie in? And one of them is BLUE? And it says 200A on it?! The owner says "My bad, I gave you incorrect information."  Ah nuts.

So, would I be able to connect only my two hot legs, the neutral, and the ground to the power? And if so, what kind of voltage/amperage should I expect?

Thank you for anyone who can provide insight to this very potential situation!
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TJ (Tom) Cornish

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #1 on: September 17, 2014, 02:20:58 pm »

Let's say I arrive at a new venue five hours from home.  Since I did a fastidious advance of the venue, I know that they have a single-phase 100 amp cam lock tie in.  That's great, I stroll in with my single-phase distro with cam lock tails, but what's this? Five cams at the tie in? And one of them is BLUE? And it says 200A on it?! The owner says "My bad, I gave you incorrect information."  Ah nuts.

So, would I be able to connect only my two hot legs, the neutral, and the ground to the power? And if so, what kind of voltage/amperage should I expect?

Thank you for anyone who can provide insight to this very potential situation!
It is safe to ignore the third phase, though the unused cam must be covered or taped to prevent a shock hazard. 

The question of it being safe to plug a 100A distro into a 200A company switch depends on the circumstances.  If your feeder cable can handle 200A and your distro has a 100A main breaker, then yes.  If your feeder is only rated for 100A or your distro doesn't have a main breaker, technically no, though it is commonly done.

The general rule of thumb is that the role of an OCPD is to protect the downstream wiring.  If your OCPD is 200A, in the letter of the law, all the wiring between this 200A OCPD and the next step down in distribution must be rated at 200A.  If not, this wire could potentially overheat, and the 200A OCPD will not prevent an unsafe condition.

Depending on the construction of your distro, if it doesn't have a main breaker, that makes it even more of a risk to plug in to an oversize supply, as now you could potentially overload your distro's internal distribution busses. 

If your distro has an appropriate main breaker, say 100A, then the distro itself is protected, and to a large degree your feeder is as well.  Though technically it is a code violation to plug a 100A main breaker distro with 100A rated tails into a 200A company switch, it is commonly done and reasonably safe.  The technically correct solution would be to carry a set of 200A-capable feeder either all the way to your distro, or a short distance to a portable 100A breaker, then use your 100A feeder to go the rest of the way to the distro.

As to power capacity, if you have what is commonly called a 100A distro - a 100A two pole breaker, then you will have two 100A legs available for use, no matter whether you are supplying it from a single phase or 3-phase service.
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Jeffery Foster

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #2 on: September 17, 2014, 02:26:42 pm »

Outstanding Tom, I appreciate the detailed answer.

Not that I would ever run into that situation....
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Jonathan Johnson

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #3 on: September 17, 2014, 03:59:45 pm »

As to power capacity, if you have what is commonly called a 100A distro - a 100A two pole breaker, then you will have two 100A legs available for use, no matter whether you are supplying it from a single phase or 3-phase service.

Be aware that if you are tying into a 3-phase Y service, you will NOT have 240V split-phase service available in your distro. You will have 120V single phase between each hot and neutral, and 208V between the two hots. (The neutral is tapped where the three transformer windings meet.)

If you are tying into 3-phase high-leg Delta service, the neutral is typically center-tapped on one winding of the transformer, so you will have 120V single phase between each hot and neutral and 240V single-phase between the two hots (AKA 240V split-phase). The high leg is opposite the neutral -- that is, the high leg is where the two other windings on the transformer meet. You will have 208V between either hot leg and the high leg.

Even though the 208V is using two 120V phases that are 120 degrees relative to each other, the waveforms add to produce a sine wave at 208V RMS so it appears as a single phase to the equipment. Equipment with universal power supplies that spec a single-phase range from 100V-250V (or so) will operate fine on 208V service.

Three phase service is really only useful for motors and other loads with a matched triplet of coils or elements. Pretty much all lighting and sound equipment is single phase, though distributing the single-phase loads among the three phases is done for balance. With switching power supplies and loads not power-factor corrected, there can be issues with overloading the neutral of three-phase service if it is undersized, as distorted current waveforms can be additive rather than cancelling.

Probably too much information for what you need. Always meter the service prior to connection. Never assume anything!
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Stop confusing the issue with facts and logic!

Stephen Swaffer

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #4 on: September 17, 2014, 04:45:33 pm »


The question of it being safe to plug a 100A distro into a 200A company switch depends on the circumstances.  If your feeder cable can handle 200A and your distro has a 100A main breaker, then yes.  If your feeder is only rated for 100A or your distro doesn't have a main breaker, technically no, though it is commonly done.

The general rule of thumb is that the role of an OCPD is to protect the downstream wiring.  If your OCPD is 200A, in the letter of the law, all the wiring between this 200A OCPD and the next step down in distribution must be rated at 200A.  If not, this wire could potentially overheat, and the 200A OCPD will not prevent an unsafe condition.

Code allows undersize wired after a OPCD if it runs to a single breaker/set of fuses that limits current to the wire ampacity-under the tap rules within certain limitations.  Though, I am guessing that since portable cord-usually on the ground-is susceptible to damage causing a short that it is probably not allowed in that case? (even if allowed, likely not a good idea!)
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Steve Swaffer

TJ (Tom) Cornish

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #5 on: September 17, 2014, 04:55:40 pm »

Code allows undersize wired after a OPCD if it runs to a single breaker/set of fuses that limits current to the wire ampacity-under the tap rules within certain limitations.  Though, I am guessing that since portable cord-usually on the ground-is susceptible to damage causing a short that it is probably not allowed in that case? (even if allowed, likely not a good idea!)
Don't quote me on this, but I believe that is only allowed for a length of something like 10'.  I don't have a code reference off the top of my head.
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Jay Barracato

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #6 on: September 17, 2014, 08:33:13 pm »

I was remembering the same thing with the same short length of the feeder

Sent from my DROID RAZR HD using Tapatalk

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Jay Barracato

Neil Sakaitis

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #7 on: January 21, 2016, 04:21:47 pm »

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 is 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, which matches nicely with their 100A panel (200A total).

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
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TJ (Tom) Cornish

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #8 on: January 21, 2016, 04:48:08 pm »

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 is 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, which matches nicely with their 100A panel (200A total).

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
You really should have a main breaker in your distro.

The upstream breaker is designed to protect the downstream wiring.  In your specific case, if you have #2 SC cable, you need to have an upstream breaker at <195 amps.  If you have some other kind of #2 cable such as multi-conductor SOOW cable, this number is different and almost certainly significantly lower than 195 amps, and there may be other problems with your setup; e.g. it is not allowable to break out SOOW cable into camlok connectors.

If you are drawing power from a 100A main breaker, you will have 2 legs of up to 100A/120V available each, or 200A/120v total.  If you are using 208/240v loads, you will have 208/240v/100A available, or whatever combination is in between.

The 400A breaker is definitely not acceptable for your situation.  Even if you aren't intentionally pulling 400A, one or more faults could exceed the rating of some component(s) in your system and you would be in big trouble.

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Neil Sakaitis

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Re: (How) Can I connect my single-phase distro to a three-phase tie in?
« Reply #9 on: January 21, 2016, 05:30:19 pm »

Thanks Tom for the info !!! Yes I am using SC cable (#2) which is good for 190A. I hear you on the main breaker thing, but most of the companies who make custom distro panels usually don't have or sell a main breaker for the distro, including my distro, not really sure why. I guess they don't want to limit the distro as it all depends on which wire and application the distro will be applied.

In my case, I have 8 X 15A mini breakers and 8 X 20A SqD breakers all tied to Edison outlets on panels, and underneath the panels, all wiring from all breakers tie straight into the CAMS. My suitcase style case doesn't even have room for another panel to house a main breaker.

But I hear you loud and clear, the main breaker protects the total capacity of the feeder, in my case #2 / 190A, when tying into a higher panel like a 3-phase 100A or more.

Do you know of any companies that sell a separate CAM box (thru) with main breaker?? This would allow me to tie it into my distro without having to figure out how to squeeze it in where I have no space now.

Thanks
Neil

You really should have a main breaker in your distro.

The upstream breaker is designed to protect the downstream wiring.  In your specific case, if you have #2 SC cable, you need to have an upstream breaker at <195 amps.  If you have some other kind of #2 cable such as multi-conductor SOOW cable, this number is different and almost certainly significantly lower than 195 amps, and there may be other problems with your setup; e.g. it is not allowable to break out SOOW cable into camlok connectors.

If you are drawing power from a 100A main breaker, you will have 2 legs of up to 100A/120V available each, or 200A/120v total.  If you are using 208/240v loads, you will have 208/240v/100A available, or whatever combination is in between.

The 400A breaker is definitely not acceptable for your situation.  Even if you aren't intentionally pulling 400A, one or more faults could exceed the rating of some component(s) in your system and you would be in big trouble.
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