Alternating shop lights

[arewethereyet]

I was hoping someone could check my sanity.

I have a 48 x 42 Pole barn with 16 foot eaves.

The pole barn has four 12 foot wide bays.

For lighting, I'm planning on 32 twin bulb T8 fixtures. This will be ~2000 watts.

I am thinking of installing two breakers in the panel that Feed two double gang boxes. One box will feed the left two bays and the other box will feed the right two bays.

In addition to separating the lighting between sides of the shop, I wanted to alternate the fixtures by running a four wire from the double gang box to the light fixtures. That way I can alternate between the red wire and the black wire for the hot.

By doing it this way, I end up with four separate eight fixture circuits at approximately 500 W each. Each circuit will have its own switch and all four circuits will only take two breakers in the panel.

Does this seem like a good plan?



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[LX-Markham]

I sure hope you can, because that's the way my electrician wired up the lighting in my garage. Two lights, two switches, one 4-wire cable.

 

[srmofo]

correct me if Im misunderstanding you, but those 4 wire romex runs are essentially a 220v run. Every 220 I have seen takes up 2 spots in the box. One for each leg.

Wont each leg of your run require its own breaker?

Still sounds like a good plan I just think you will still be using 4 spots in the box

 

[CJCar]

correct me if Im misunderstanding you, but those 4 wire romex runs are essentially a 220v run. Every 220 I have seen takes up 2 spots in the box. One for each leg.

Wont each leg of your run require its own breaker?

Still sounds like a good plan I just think you will still be using 4 spots in the box

The 4 wire (more properly 14/3) is from the switches to the fixtures. 14/2 from the panel to the switches.

arewethereyet, your plan is good.

 

[srmofo]

The 4 wire (more properly 14/3) is from the switches to the fixtures. 14/2 from the panel to the switches.

arewethereyet, your plan is good.

Yup skipped over that part.

 

[2ManyProjects]

I was hoping someone could check my sanity.

I have a 48 x 42 Pole barn with 16 foot eaves.

The pole barn has four 12 foot wide bays.

For lighting, I'm planning on 32 twin bulb T8 fixtures. This will be ~2000 watts.

How are you planning to lay out those lights? In the long run, that will be at least as important as how many lights.

Also, how will the interior of this space be finished? That too will have a major impact on lighting effectiveness.

And finally, particularly if you're planning to mount the lights up at (or near) that 16-foot level (and don't get me wrong; that's a good thing from a light-distribution standpoint), you might consider upping the tube count somewhat. As it stands, you're looking at only about 89 source lumens/ft.^2 -- i.e., BEFORE we deduct for the losses at working height and/or tube aging.

I am thinking of installing two breakers in the panel that Feed two double gang boxes. One box will feed the left two bays and the other box will feed the right two bays.

I completely agree that the lighting load should be split between (at least) two breakers, and NOT solely because it's going to comprise a significant load when all is said and done.

But from there on out, I'd very probably take a somewhat different approach.

In addition to separating the lighting between sides of the shop, I wanted to alternate the fixtures by running a four wire from the double gang box to the light fixtures. That way I can alternate between the red wire and the black wire for the hot.

So basically, what you're saying is, you want to do both zoning and brightness control. Again, I agree in principle; but I'd implement it differently.

By doing it this way, I end up with four separate eight fixture circuits at approximately 500 W each. Each circuit will have its own switch and all four circuits will only take two breakers in the panel.

Does this seem like a good plan?

It seems minimally workable, but with plenty of room for improvement.

First, ditch the so-called "four wire" (a.k.a. **/3 NM-B) cable. Use separate runs of 14/2 (or, preferably, 12/2) NM-B. This will not only simplify the actual wiring chores (consider that the two "Hot" conductors in that 14/3 need to go to different places, and the Neutral needs to go to MORE places than either Hot wire does); it will also make future troubleshooting easier. And finally, while IN THEORY, 120V MWBCs can be built using 3-conductor NM-B, in real life the loads are virtually NEVER so precisely balanced that this represents a good idea. This remains true in your case: Fluorescent lamps (and their ballasts) are sufficiently reactive that the Neutral WILL wind up carrying excessive currents, at least sometimes.

Second, seriously consider breaking the lighting array into more than four switch banks. For starters, you should have one switch bank which controls ONLY a very minimal number (in a space that size, I'd guess four or so) of fixtures, to act as your "walk through" lighting. Next, I'd target about 30-40% of the remaining lights (in each zone, if still doing zoning) for Switch Bank #2. Finally, the remaining 60-70% of the lights (again, in each zone, if still doing zoning) go on Switch Bank #3. This arrangement will give you the greatest degree of flexibility in terms of controlling the brightness levels (and power consumption) at any given moment.

 

[JoeFin]

First, ditch the so-called "four wire" (a.k.a. **/3 NM-B) cable. Use separate runs of 14/2 (or, preferably, 12/2) NM-B. This will not only simplify the actual wiring chores (consider that the two "Hot" conductors in that 14/3 need to go to different places, and the Neutral needs to go to MORE places than either Hot wire does); it will also make future troubleshooting easier. And finally, while IN THEORY, 120V MWBCs can be built using 3-conductor NM-B, in real life the loads are virtually NEVER so precisely balanced that this represents a good idea.

Sure don't mind spending Other peoples money - do ya

Running 12/3 is sound advice and practice on many levels the least of which is cost. The separate runs of 12/2 to the same light string or row of fluorescents increases the likelihood of the neutrals being swapped by mistake.

The lights will still work but now the Neutral Current and subsequent EMF will not be allowed to cancel out. Additionally the circuit will have increased impedance causing voltage drop and heating of conductors - Not good

Fluorescent lamps (and their ballasts) are sufficiently reactive that the Neutral WILL wind up carrying excessive currents, at least sometimes.

If your trying to describe Non-Sinusoidal currents - once again the cancelling effects of the opposing phases helps to mitigate this phenomena. Where the effects are Most prevalent and destructive are commercial 3 phase services where High Demand and lack of a 180 deg opposing phase contributes to the effect.

But I'm sure you know that

 

[ForceFed70]

I agree with the 14/2 wire runs for the actual fixtures. It's not much more expensive depending on what your wiring layout is. With 14/3 that cable has to go to every fixture, with 14/2 the cable only goes to 1/2 of the fixtures. At the end of the day we're talking MAYBE $10 in additional wiring costs and it's so much simpler to do.

My other comments:
- I split my overhead lighting into a bunch of zones - and rarely use the zones. It's either all on or all off. Turned out to be a bit of a wasted effort.
- I agree that this seems like you're on the low side for lighting. With 16' ceilings you could go with some HID highbay lighting. Perhaps put a row of highbay lights in and put them on their own circuit/switch.
- Make sure you use a breaker tie. It's code when feeding 2 phases/circuits into a single box. It needs to be wired such that if 1 pole trips the other pole is disconnected at the same time.

 

[JoeFin]

- Make sure you use a breaker tie. It's code when feeding 2 phases/circuits into a single box. It needs to be wired such that if 1 pole trips the other pole is disconnected at the same time.

You have a code article to cite for that

I may be getting old - but the only citation I can think of in that regard would be that which governs "Multiwire ckts" aka the 12/3 wiring the Op was discussing

 

[Will S.]

I am no authority on lighting, but I can tell you what has been working for me for the last 3 and a half years. In early 2010 I built a 36x48 PB, with 16' ceiling height. I installed 3 rows on T8, 110w H.O., 2 tube fixtures. I have a row of 5 fixtures (40' length) on the centerline. Two additional rows mounted on each side, about 4' from the sidewalls. Each row is on it's on switch, and I also have an entrance light on a 4th switch, right inside the door.

Often, I am using only the center row lighting. If I am working on something on just one side of the shop, or the workbench area, I use only that side. For max light, if I am detailing or otherwise all around the shop, I'll have them all on.

Also will mention that the entire building, including all lighting circuits, are 12ga wire, as is my house. No 14 ga wire whatsoever. In the old house, I got tired of lights dimming in one room, while my wife was ironing or running the hair dryer in another. Not a problem any more.