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??
