I was sent to a new job Monday. It's a school renovation thats just starting.
Part of the reason I was sent there was to get the office trailers connected. The guy that is going to run the job laid this out and is working with me.
There are four small office trailers across the driveway from the school that take 60A 120/240 (208 in this case) single phase feeds. Here's the plan;
Run 4/0 Quad-plex 40' from a 3-pole 150A breaker in the MDP, through a utility room zip tied to a run of 1/2" EMT where it is connected via clear-taps to 2/0 THHN. (The bare conductor is being used as the neutral)
The 2/0 goes out through a block wall to the outside of the building through a 2" PVC sleeve. (The sleeve only goes through the wall then stops)
The 2/0 then, within 12", enters a run of 2" PVC (6' above grade to this point) which takes it up to the roof of the school.(2 stories)
On the roof, the 2/0 exits the 2" PVC via a weatherhead where it is connected to another run of 4/0 quad-plex via clear-taps which is zip tied to the lightning protection conductor appx. 100' along the roof.
The quad-plex then spans over the driveway Appx. 100' to a 16' pole made of scabbed together 2"X4"'s.
The quad-plex runs down the pole, unspliced and without conduit, into a 200A 3-Phase MLO N3R panel which is mounted on the 2" X 4" post.
Now, this is as far as it is right now. But, I know that the trailers are going to be fed with tri-plex zip tied to the trailers. Going from trailer to trailer.
Tell me what you guys think so far and I will tell you more about the trailer feeds tomorrow.
I,ve done a few schools, old and new. This seems kind of hazzard to me. We have generally had a temp service within proximaty of the trailer and ran direct burial cable to the trailer. Trailer would sit there till the school was almost done. The wiring would simply be buried and forgotton. Temp service would be removed, then the trailer personal would find accomodations in the school. On some rehab or renovations we ran 4/0-3 SER and zip tied it to the structure members. Had to consider the distance to obtain the proper size wire due to voltage drop. You know those guys in the trailer, they have A/C and heat, refridgerators, puters, fax's, lites etc.. Sounds like this guy your working with can't place a temp on site. So he's making one up with a bunch of scrap wire and 2"x4"s.
Re: Temp. Power for Office Trailers#18589 12/11/0208:01 PM12/11/0208:01 PM
They are small trailers with a panel intended to be fed with a 4 wire single phase feeder.
Unless some "illegal" modifications are made, the frame of the trailers as well as all grounded receptacles will be "floating" as it will not be bonded back to the MDP via a grounding conductor.
In other words, in a worst case, the frame of the trailer could become energized at up to 120V to ground.
This doesen't even take into account the places that the bare "neutral" conductor comes into contact with the lightning protection conductor as well as the 1/2" EMT or anything else that it may inadvertantly touch providing more than one path to ground for the neutral current etc...
Re: Temp. Power for Office Trailers#18593 12/11/0209:42 PM12/11/0209:42 PM
They just built a highschool in my area, there are 6 portables (basiclly permentant trailers), they have a meterbase and weatherhead going up the sides (of couse the metersocket is jumpered with a cover on it), originally they ran 2 conductors and a neutral from trailer to trailer. The school opened, next thing i saw was a green conductor being strung from trailer to trailer with the feed. Beside all of these feeds was the phone/network/anonuncement/fire alarm wiring. I guess someone planned 3 conductor and it wasn't any good when mr. inspector came by ;P
Re: Temp. Power for Office Trailers#18595 12/11/0210:06 PM12/11/0210:06 PM
There was one school where I was working as 2nd man, seems like a situation close to yours. He had a 200 amp cable 4/0-3 feeding a sub panel in the school. From that panel he had 4 sets of 6/3 ser branching out to 4 more sub panels that had 100 amp c.b backing them up because the 60s kept tripping. What a fricken mess. With all the subs running heavy equipment power tools thing kept burning up, Finally the main temp C.B. 200 amp burned out. Power was out for the whole project. The foreman tied directly into the lugs, no protection 200" to the 1st temp panel indoors. They had so many down times happening there do to under sizing and improperly sizing the temps. Just all the temp lite alone was close to 200 amps, it was 130k sq. ft. bldg. The GC and all the other subs complained about it constently. That made things really hard for us, they thought less of us, they didn't work well withg us. We had guys joking around pretending they were blind with stick. Think about all the time wasted fixing temp lites and equipment. I was just as shocked as you are that they would let something like this happen. The only thing I can think of is they bid low to get in, possibly for several different reasons. Maybe their behind the 8 ball, in debt, so they take a job that will take them out of the red a little. All I know is when I was there, there was constant complaint of inadequate temp lite and power.
Re: Temp. Power for Office Trailers#18596 12/12/0208:45 PM12/12/0208:45 PM
The 3Ø MLO panel on the 2x4 structure would be under 250.32(B). If there is no parallel paths with the grounded (bare neutral) of the quad, that part of 250.32(B)(2) would be legal, but are there telephones or even cable TV in those trailers?
Appears that 250.32(B)(1) [ a 5 plex ] is the only proper way to get across the roadway and grounding per 250.32(A) and then on to the trailers , assuming 1Ø 3W 208/120v volts to each trailer, would require 4 insulated conductors to meet 550.33.. Those 550 sections do not mention 208/120V. They do mention 120/240V. Now what ?