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Power Tap Cable Type

Discussion in 'Lighting and Electrics' started by jmac, Mar 7, 2009.

  1. jmac

    jmac Active Member

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    I seem to be on a role tonight. One more:

    We are getting a small 100 amp, 3-phase circuit breaker box with 20 amp SOOW cable pigtails for connecting a half dozen or so dimmer packs.

    We will be connecting this with 60 amp wire to lugs inside a mini "company switch", basically a standard commercial duty enclosed 60 amp fused safety switch (not cam-lock type).

    My question- is it acceptable practice to use SOOW cable for this type of (non plug-in type) temporary connection, or do I need to use something like flexible metal conduit and regular building wire? I would prefer the former as it will be easier to work with.
     
  2. icewolf08

    icewolf08 CBMod CB Mods

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    I get the feeling, from the questions that you are asking that you are not qualified to be doing this tie-in. This is the kind of job that should be left to the qualified if not certified electricians. If you are qualified to do this then I do apologize, however I think that answering this question in the clear borders on if not crosses the line of CB's TOS regarding electrical how-tos and safety.
     
  3. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Can you restate the question, and perhaps add pictures? You've mentioned 100A, 20A, and 60A, and a half-dozen or so dimmer packs (quantity and ampacity per pack?), and pigtails, but non plug-in type.

    Speaking of "(non plug-in type) temporary connection," I suspect that's an oxymoron. If there are no connectors, it can't be considered temporary, even if connected to a "mini-company switch."

    BTW, The term "portable" is preferable over "temporary" when referring to theatrical applications.
     
  4. TimMiller

    TimMiller Well-Known Member

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    its best for theatrical use, to use SO cable, and proper strain reliefs. I like to mount the recepticals to the distro (using j-boxes and proper conduit fittings of course). So then i dont have to worry about any pig tails hanging out and getting damaged.
     
  5. JD

    JD Well-Known Member

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    Steve Terry, feel free to correct me on this, but without connectors it becomes a permanent installation regardless of the intent. As such, the codes applicable would be those that cover a permanent installation of such equipment.
     
  6. STEVETERRY

    STEVETERRY Well-Known Member

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    This application is for a 60A 3-phase feed to a small distro, as I understand it. At that power level, and with no assurance of qualified personnel, I suggest a 60A 5-pole connector (pin-and-sleeve, or CEE-form). There is generally no reason to perform a bare-end tie-in of the multiconductor SO feed (although it is code compliant per section 520.51 providing the type SO cord is not being used as an illegal substitute for required permanent wiring methods. Note that a power "outlet" in 520.51 can be a fused disconnect switch.).

    Sounds simple, right? NOT!

    That feed must be sized from table 400.5(B) , and the neutral must be considered a current-carrying conductor because the dimmer packs connected to the distro are non-linear loads. Therefore, the ampacity must be derated per table 400.5.

    So, for 60A, and four current-carrying conductors in the cable, table 400.5(B) tells us we need #6 AWG 5-conductor 75 degree C cable, since the derating is 80% of the 77A ampacity of that cable. Note that table 400.5(B) does not actually list type SO and derivatives, only other types like G, W, SC, SCE, etc. That's because the code assumes that "normal" type SO is rated at only 60 degrees C and must have its ampacity determined from table 400.5(A). If that is the cable you are using, you will need #2 AWG 5-conductor, which is not a practical size for a 60A connector.

    So, if you get 75 or 90 degree C type SO, which is now common, and use table 400.5(B), you still have another problem. The temperature rating of the terminations at both ends must be rated for the same temperature as the cable, in order to use the 75C ampacity from table 400.5(B). If either the connector or the point of termination at the distro is rated for 60C, then you will have to increase the cable size to #4 AWG 5-conductor.

    So, all this is to say that if you're not well versed in these issues, please get someone to help you who is!
     
  7. TimMiller

    TimMiller Well-Known Member

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    BLARG!!! ITS COMMERCIAL ELECTRICAL CLASS ALL OVER AGAIN!!! Actually I loved the class and its very good stuff to know. Now how some of the people met the prerequisits and didnt know how to wire a subpanel was beyond me.
     
  8. Esoteric

    Esoteric Well-Known Member

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    Yeah, get a professional in there. Anyone who has to ask that question should not be performing the actions that require said question.

    Mike
     
  9. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    So [user]jmac[/user], as you can see from Mr. Terry's response, the answer is not nearly as simple as one might think. Furthermore, "Is it acceptable practice...?" is not the best question to ask. The only person who can answer that is your local AHJ.
     
  10. jmac

    jmac Active Member

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    Ok- Where to start?

    First thank you all for your thoughts and interest. Bear with me, this might get long, perhaps record length. Extra credit if you make it to the end-

    Second a little more history of the situation-

    Twice a year, we rent a small HS theater space for small community theater shows. On stage is an old 120/208V, 3-phase, 4-wire dimmer board that I guess is about 50 years old. It is no longer used to control lights because of its age and condition, but is still useful as a power source. It is has a fairly new 100A feeder incoming, and the main bus bars are fine. Tapped off two main phase busses is a 50A/2P circuit breaker which in turn feeds an old 3-wire (no ground) 50A range receptacle, both mounted to side of board.

    All our lighting equipment is rented for each show. For past productions, the rental company has used the range receptacle for show power. This by plugging in a range cord, which in turn feeds, I think, 4 standard 20A duplex receptacles, two per phase, all tied together in a j-box, probably with big wire nuts, all nicely mounted to a piece of plywood. Yes, 20A recepts protected at 50A...

    Obviously not Kosher, but it has worked for years in making available four 120V circuits which in turn are used to connect four Leviton #D4-DMX dimmer packs, which are each good for 4 channels and 2400W max. total (typically 500W per channel). They plug into standard Edison recepts. Two or three other dimmer packs plug into various wall recepts, giving us 24 dimmer channels typically.

    After doing two shows with this scheme, and being the conservative engineer that I am, I decided I would like to improve the situation, because nobody else ever will. My goal was two-fold. Upgrade this scheme to something safer, and increase capacity. Going to a 60A/3P supply will approx. double capacity from that with the 50A/2P, and will allow up to nine 20A circuits, 3 per phase (or at least six for now, as I posed in my original post).

    As we don't own the facility, we are at the mercy of the School. I approached the School (starting with the A/V dept. and finally after several steps, getting to the director of facilities), hoping to replace the range receptacle and breaker with a 60A/3P breaker, tapped off the main busses, and a Hubbell 60A, 5-wire, 120/208V "Y" pin and sleeve receptacle (much as Mr. Terry I believe suggested). Into this, we would then hopefully plug in something like a Nutech #CV-60-Expo-2 portable power distribution unit, which would in turn serve the dimmer packs.

    (I'm trying to link to this item, but not quite sure how to)....If it works, here it is-

    Nutech Industries, Inc. Power Distribution Products

    Basically it plugs into the 60A P&S receptacle, and has nine 20A/1P breakers, 3 per phase, each feeding a standard 5-20R recept. I think you would all have to agree this is basically a workable solution, and a definite improvement, yes (?).

    Alas, the almighty dollar comes in to play. The School has said they can only provide the least expensive option, which is the 60A/3P fused safety switch, referenced in my original post. The school has an electrician who would make the hardwired connection to the safety switch lugs. Not quite as nice, but still better than current scheme. Further the Nutech PDU (and other comparable ones) go for about $1000, which is about our lighting rental budget for the show. So we might get good safe power, but no lights. Whatever I end up doing, will most likely come out of my own pocket.

    So, my next thought is to build my own quasi-PDU using a standard Square D or similar, 120/208V, 3-phase, 4-wire, 125A, 12-pole load center panel with at least 6, maybe 9 or 12, 20A/1P breakers, for about $150. This could feed SO cable tails hanging from bottom of panel, terminated with 5-20R (or stage pin) female connectors, as I think I mentioned in the OP, because it seems easiest. Alternatively, I could mount steel handy boxes to the side of panel (or put the whole thing on a piece of plywood) and use duplex receptacles. Certainly not as elegant, or as easily portable as the $1000 PDU, but $700-800 cheaper.

    It was about at this point, I made my OP here. What I was trying to ask was, if we hard connect my QPDU to the 60A fused switch, does that then make the installation a “permanent” installation, as opposed to what I incorrectly called a “temporary” installation, and as was duly pointed out, I should have referred to as a “portable” installation. I stand corrected.

    If portable, SO cable is ok. If permanent, need conduit and wire.

    Contrasting other’s responses, Mr. Terry cites NEC 520.51 as permitting this install for a portable system. I’m inclined to agree, although this paragraph falls under Part IV, “Portable Switchboards on Stage”, and I think we would be hard pressed to call the Nutech-like PDU or my QPDU a “switchboard”. The more applicable part of Article 520 appears to be Part V, “Portable Stage Equipment Other then Switchboards”, which in 520.62 includes rules for portable power distribution units, which seems more like what we have here. My QPDU seems to meet all requirements of 520.62.

    Also note 520.68(B) allows sizing cables by Table 520.44 (in lieu of Table 400.5) where cables are not in direct contact with heat producing equipment, which would be the case here.

    However, there is no mention in Part V of “supply” connections like there is in 520.51. So we have one section allowing what I want to do, but it appears to address a different class of equipment, and another that seems more to apply to my type equipment, but does not address the issue. This seems to me a legitimate “gray area” wherein the NEC may not anticipate every possible scenario.

    Is it not permitted because it’s not specifically called out in the seemingly more applicable Part V; or can we say the rule for a switchboard in Part IV could also reasonably apply to a small panel or PDU?? Fair minded people may disagree. Thus my OP, attempting to see what others may think, or what may typically be seen in the industry. I am an experienced engineer, but relatively new to theater life. I am more used to the world of “permanent” installations, and was seeking advice from those more experienced in the “portable” world. I’ll concede my use of “accepted practice” was a poor choice in words.

    In an ideal world, every design would be stamped by an experienced licensed engineer, installed by an experienced licensed electrician, and inspected by an AHJ who knew more than that GFI’s are required in bathrooms, but this is just often not the case.

    So who is qualified to be involved with this tie-in? I am not a licensed electrician, although I have bent a fair amount of conduit and pulled a few feet of wire, and wired more than one house, and replaced panelboards, etc

    Who is a professional? You of course had no way of knowing, but I have been a licensed professional electrical engineer for over 25 years, and have designed the electrical systems for a few hundred buildings, including industrial plants, laboratories, hospitals, college buildings, and yes, even a few theaters. I am well versed in the NEC, architectural lighting, and power systems up to 15KV.

    Alas, I am tiring of it all, and am embarking on a new career as a theatrical lighting designer….. (wishful thinking, with three daughters in college…) Thus my membership in this esteemed group. I mean that sincerely. I am amazed at the wealth of knowledge and experience represented here, and very much appreciate the time and effort you all spend helping out all comers including me, and the generally thoughtful and helpful responses generated seemingly every hour of every day.

    Let me give you one example to make a point. Last summer I was connecting rental DR cam-lock tails to our outdoor theater 400A company switch lugs (a bigger example of the current situation). Yes, in an ideal world this should have been done by an electrician the town should have hired… I noticed the equipment ground lug was loose. When I went to tighten it, I discovered it wouldn’t tighten because it was fastened to the enclosure by a lag screw sunk only into the sheetrock behind it… I knew enough to take care of it. Not the first time I’ve seen ground wires not properly connected to the enclosure.

    Who would you rather have making connections to your company switch- me, or the licensed electrician who installed the switch, or the several others who have connected to it over the past few years?? …

    In this case, not an issue, because the School has an electrician who will do the tie-in.

    Semi/mini-rant over…

    -----------------

    So, where am I now? Well, a P&S Nutech system would be nice, but is just way too expensive. I think my original QPDU idea is probably ok, and certainly a big improvement over status quo. I’m now thinking handy box recepts is probably a better idea than 24” SO pigtails with connectors (a bit more work, but likely cheaper, and less tangle of cords; also reduces possibility of problem, if it turns out someone says I have to follow “permanent” rules). That just leaves the matter of the main feed to the 60A switch; about where I was with my OP, a week ago...

    1- SO cable for “portable”?

    -Or-

    2- Mount panel to wall and connect with flex conduit and THHN wire for a 2-week “permanent”??

    So to re-phrase my question and cut to the chase, which would you do??
     
  11. TimMiller

    TimMiller Well-Known Member

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    Theatrical powerdistros are nothing more an a glorified panel board, or circuit breaker box, or similar OCPD enclosure providing properly protected branch circuits. I have several distros that are nothing more than a circuit breaker box, with plugs on the side, and cam connections at the top. I like the mounted boxes, because it prevents them from getting snagged and pulled on. Gear goes through hell when out on the road. There is nothing wrong with doing pig tales of SO cable. I like wiring up to recepticals also because (at least when you are using edison (U-ground) recepticals you get to have 2 plugs per circuit. Lots of times you arent completely loading up a circuit, and need to plug something little in such as a cell phone charger, or data splitter. (i end up plugging my cell phone into my power distro quite often). They just came out with some new conduit fittings that are designed much better than anything else for attaching handy boxes to panel boards. I usually mount the handy boxes up using several of these emt fittings to keep the boxes from moving at all, because lots of times thats what is grabbed when picking up and moving around the distro. I wouldnt mount the panel to the wall, i would just use a SO cable to run to the buildings plug. I think somewhere in the NEC it states that if you are using a plug, then you have to use a flexible cord.

    So in a nut shell, i would mount plugs to handy boxes, and use an SO cord for the mains plug.
     
  12. icewolf08

    icewolf08 CBMod CB Mods

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    If you wanted to be really nifty (though a little more expensive), you could mount a set of Cam-loks on your board and get a set of cam tails for the tie in. This will give you the most flexibility, especially if you ever go to a different venue that may have a cam-lok company switch.
     
  13. STEVETERRY

    STEVETERRY Well-Known Member

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    If you permanently install the parts you suggested, using a licensed electrician, you are code compliant. If you take those parts and construct a home-made distribution and connect it with a portable cord, you are not code compliant and you have liability. Any portable power distro that you install should have a UL1640 listing, like the Nu-Tech unit has.

    Given your budget, I would do the permanent install and have it there for next year.

    ST
     
  14. mrb

    mrb Active Member

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    if the school's electrican can install the fused disco switch for you, why not have them just install a small 3ph square D QO loadcenter with a bunch of 20a circuits for you? Wouldnt cost that much more in material than the fused switch....just sayin....
     
  15. jmac

    jmac Active Member

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    Sounds like a good plan.

    That's a good idea too, but too late now...

    Thanks to all.
     

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