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Warehouse Lighting Wiring Size Question

Total distance from panel to end of the longest line is 310 feet. Each fixture is 42 watts, and there will be up to 30 fixtures on each circuit. The first fixture on the longest line will be at the 150 foot from panel point. All lighting fixtures will be run in parallel (pigtailed at each fixture, a.k.a., plug-in fixtures).

Each circuit is 20 AMP, 120V, single phase AC

What size wiring is needed for these longest runs?

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    Will your circuits provision at least 0.25 watts per square foot? How are you switching these lights? Do you have 120/240V split-phase, 208V 3-phase, or 277V 3-phase wye available? Hauling power these long distances is better done at higher voltages. Even if you can't do that, tell me if they are multi-voltage 120-277 lamps (because those draw more current when the voltage sags, and that must be allowed for.) – Harper Jul 2 '17 at 19:31
  • There will be (12) 20A circuits for lights, in a 24,000 square foot warehouse. We are considering a change from the current 'using breakers as switches', to a nicely laid out and labeled and diagramed warehouse switch grid / array of 12 switches. – dougabc123 Jul 2 '17 at 22:01
  • Service is 600A 3-phase with a bitch leg. – dougabc123 Jul 2 '17 at 22:14
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Thirty 42-watt fixtures are nominally 1260 watts, which is 10.5 amps at 120V.

Your longest run has the lights evenly spaced between 150' and 310' and the halfway point between is 230'.

Hit a voltage drop calculator

Set it for 5% voltage drop, as your lamps can handle that... and ask it for at least 3 points:

  • 10.5 amps (max) at the 150' (start) distance
  • 8 amps (3/4) at the 190' (1/4 way) distance
  • 5.25 amps (half) at the 230' (halfway) distance

TLDR: I get 12AWG for each of those calcs.

For each test, the calculator will recommend a wire size and give you the expected voltage drop to be found there. Its recommendation is applicable from the panel to the point you asked about.

They say to use 3% voltage drop, but that is silly, 5-6% will work fine. But don't push it to 10% (unless you really want to) because if you upgrade to multivoltage lamps, those will answer voltage drop by drawing more current in proportion, throwing calculations off.

Is voltage drop OK?

It's not unsafe to have high voltage drop. The 20A breaker definitively protects your 12 AWG wire if your wiring methods are legal. (an example of "illegal" would be more than 9 conductors in a single conduit without derate). Of course the voltage drop is heating the building. Electric heat is a fairly expensive heating method, especially if you're trying to air-condition the place! So get out the sharp pencil and figure out the voltage drop, multiply by amps to get watts, figure the cost of the power and additional A/C load, and see if you can cost-justify thicker wires.

The voltage drop calc will keep you legal -- however it doesn't know about your 20A breaker. 14AWG wire is never legal downstream of a 20A breaker, 12 is your minimum.

Did you catch the part about "9 conductors per conduit"?

That's kind of an issue. If you want to cram 12 circuits in a single conduit, that is 24 conductors. That requires a 45% conduit derate, forcing you to 8 AWG wire. Don't do that, unless you would anyway for other reasons.

It might help to re-label your circuits as 15A circuits and fit 15A breakers. 10-20 wires per conduit would be legal with 12AWG wire on a 15A circuit.** You can only have 12A of continuous loads on a 15A circuit, but you are under 12A.

How do big warehouses work around this?

  • Multi-wire branch circuits. In an MWBC, the neutral does not count as a conductor since it only carries differential current. With 120/240 split-phase, you can get 4 split-phase MWBCs (8 hots) in a single conduit without derate.
  • Three-phase "wye" MWBCs such as 120/208 "wye". This adds a third "hot" conductor. With 3-phase, you can fit three MWBCs (9 hots) before hitting the derate limit.
  • 277V voltage. With 277/480 3-phase, you can get more than double the power in the same conduit.

Combine all these tricks, and a single 1/2" conduit with twelve 12AWG wires can carry 39,888 watts** - enough power to light up an entire CostCo. (that being 9 circuits x 277V x 20A x 80%, the latter the continuous derate).


** why is this? The 50% derate comes off the 90 degree C column in the table, so 12AWG is 30A@90C. 10-20 wires calls for a 50% derate, or 15A. (and just so you know, 7-9 wires does require a 70% derate, but 70% of 30A is 21A... and 12AWG is limited to 20A for other reasons.)

  • Is running 10AWG for the first 150', and then 12AWG the rest of the way help reduce voltage drop? – dougabc123 Jul 2 '17 at 21:53
  • Yes, that will help a lot. 10 AWG will also satisfy the derate requirement for up to 20 (not 24, sorry) conductors in the same conduit. – Harper Jul 2 '17 at 22:52
  • On the first 150 feet, when using 10AWG, is the return also required to be 10AWG, or can the return be down graded to 12AWG? – dougabc123 Jul 3 '17 at 1:00
  • Depends. Your minimum is 12AWG owing to the 20A breaker. Upsizing of any conductor is completely optional on any wire, unless it is made mandatory by conduit derate rules (>9 conductors). If you expect an MWBC to be evenly loaded, don't bother upsizing the neutral any more than required. It only carries imbalance current. – Harper Jul 3 '17 at 8:38

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