(yet another!) 277V lighting question 2

[bentov]

A greenhouse customer has ordered 277V lighting fixtures (46 x 660W for each of 3 houses), needs to run on a generator for short grows, then roll everything up, take to other sites on a rotating annual basis (so everything is temporary/portable). My task is to provide wiring, etc.

One of my schemes uses 5 conductor 600V SO cable, 4 strings per house, 1st half of each string 8/5, second half 12/5, each on a 3 pole 30a breaker (with whatever contactors/timers end up being needed, with 16/3 pigtails to individual fixtures from fiberglass J-boxes, everything overhead). Reading around here, I'm thinking I may need to up-size for the neutrals depending on harmonics of the fixtures (which the supplier does not specify), but also I've seen suggestions that 1 pole breakers are required somehow. I would think the 3 poles would be safer? Also something about a ground fault wiping out whole banks of ballasts?

I would appreciate any feedback - don't much like the idea of a bunch of 480V cords running around but don't see another way . . .

 

[jraef]

On 277V, you would be using Phase to Neutral from a 480Y277V source. If you use 3 pole breakers, you will lose 3 banks of 277V fixtures if it trips. There is nothing inherently wrong with doing it that way so long as everyone understands that. You cannot use the 16ga whips however if the only breaker ahead of them is 30A, you will need a 10A OCPD for the 16ga.

You WILL have harmonics, you do NOT want to use a reduced size neutral conductor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bentov]

Thanks for catching that OCPD thing, I'll probably put 1p breakers or fuses in the J-boxes. About the neutral, the 8/5 is good for 28a, a segment will actually draw that much . . . will the #8 neutral in the bundle see more than 28?

 

[waross]

I worked on an expansion to a plant where the designer specified single pole breakers and two conductor cable for the 347 Vvolt lighting. (Canada, 347/600 Volts.)
The contractor knew a better way. He saved a lot of labour and cable by running 4 wire cable with shared neutrals.
The probem is that if someone is working on a circuit and opens a shared neutral he may get a 277 Volt backfeed from one of the other circuits sharing the neutral.
He was allowed to install 3 pole switches to cut the power to all the circuits on a shared neutral.
Now, if a circuit had to be de-energized, two other circuits were also de-energize.
A whole floor of the plant would go dark.
Use a three pole breaker for shared neutrals.
Use a 2 conductor cable per circuit with no shared neutral for single pole switching.
It doesn't take long to die from a 277 Volt backfeed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bentov]

The fixtures they've ordered come with a whip and plug, plus they've ordered the pigtails to go with them (cord with a female connector body). I guess I'll presume anyone working on a single fixture will be smart enough to unplug it?

These will be staggered over long open houses, so a tripped 3p feeder will just mean 25% less light, not a blackout.

I was reading just now, found this: "The NEC applies only to premises wiring, not to the supply cords of listed appliances or luminaires or extension cords." I'm thinking that's an out for the whips. Also, if I put OCP in J-boxes it will be kind of confusing plus maybe provide a false sense of security (they'll feel safe without unplugging, maybe get nailed by the neutral backfeed - unless I put 2p common trip stuff, break the neutral too, seems like overkill with a plug right there).

About the neutral ampacity though - if my 8/5 is rated 28a, I use it for branch feed of 3 fixture strings at 28a, relying on the magic of the neutral to carry it all (from 6 current carrying conductors to 4, hey presto!) - will that neutral overheat because of harmonics? Kind of takes the fun out of it if we have to upsize them all since we're using standard SO cable . . .

 

[waross]

A suggestion.
660 Watts at 277 Volts = 2.38 Amps.
Under the Canadian code, #12 SO is rated to carry 20 Amps.
De-rated to 80% for a continuous load equals 16 Amps.
6 X 2.38 Amps = 14.28 Amps.
Make up 8 identical fixture drop cords with 3c #12 Cab-Tire and put 6 pig=tails on each cord.
Possibly use PVC "T" or "TB" Condulets with strain relief connectors to connect the pig-tails to the drop cords.
Make up #12 AWG extension cords to connect the fixture drop cords to the panel.

In the panel install 8 x 20 Amp breakers feeding 8 20 Amp receptacles.
Advantages:
20 Amp cord ends are probably much cheaper than 30 Amp or 40 Amp hardware.
All of the drop cords are interchangeable.
Install the fixtures in groups of six complete with drop cords. Then use extension cords to connect to the panel.
The 3c #12 cords will be much easier to install and to remove.
With two wire circuits there is no danger of open neutral back-feeds.
With 2 wire circuits there is no issue with triplen harmonic currents adding on the neutral.
Condulets will be much smaller and better looking than JBs.
Putting fuses or breakers in JBs is not a good idea and finding suitable breakers or fuse holders approved for mounting in JBs may be difficult.
After doing your voltage drop calculations you may have to up-size the cord sizes.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bentov]

Hey Bill, your suggestion is perfect, proposed exactly that configuration the next day after making the list and pricing it out (but haven't heard back - I bet they weren't expecting the cost, 1.5 miles cable, 480V panelboard, big pile of breakers, plugs, connectors, strain reliefs, contactors, timers, etc. - takes a lot of stuff to supply that many watts).

In my scribbles (adding strands)I kind of stopped at 4 per house after considering 6, not seeming to gain advantage (and couldn't let go of the 4 conductor 3 phase neutral thing). The next step you saw, 8 per in 2 conductor with identical cords, made everything fall into place, right down to the PVC condulets - truly an elegant solution, whether they go for it or not.

Thank you! If you get to my neighborhood (Central California) dinner's on me (you too Jeff, both you guys have helped me out several times now). This forum is very cool . . .

 

[waross]

Thanks for the kind words.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bentov]

How this turned out: guys I quoted own the greenhouses,(hemp)guys with the lighting are renters. So the renters did their own thing -(2) strands of 6-4 SO cord per house, stuck into un-glued PVC conduit clamped to trusses, spliced in PVC conduit bodies to pigtails; owner said the generator rental guy hooked them up (stuffed in the lugs downstream of the 600a breaker). That generator had a problem, guy who brought the replacement didn't know how to connect things so the owners called me (how I got to see it). I told them where the conductors would need to end up so they could connect at their own risk, explained a little about what circuit breakers are for. Reminded me for some reason of Bogart, saying "if you guys are the electricians, how come you don't have any breakers?" "Breakers? We don't need no stinkin' breakers!"

The hemp guys haven't been around lately, or paid the rent. And, if the owners don't get the blackout tarps up and the lighting back on line for the correct artificial day cycle (manually switched each night by the live-in caretaker), the THC content goes above 3% - then it's not hemp anymore, a new kind of problem . . .