240/120V Multiwire Branch Acceptable?

JamesChain

Member
Location
Los Angeles
Occupation
Electrician
I have a job installing new luminaires on 17 light poles in a city park and installing new in-ground wiring to the poles. The new luminaires are LED with a listed current draw of 520 milliamps each, with universal power supplies listed at 120v-277v. The city also wants to place an approximately 8 amp, 120v gate operator at the end of the conduit line feeding the poles from the panel, using the same wiring as the light poles. The conduit run is approximately 1200' long. Their design shows the system wired with 5 #2 conductors: 2 hots, 2 neutrals, one ground. The design is for all luminaires to run at 120v, with 7 of the light poles and the 8 amp 120v load on one 20A single pole breaker, and the other 10 light poles on another 20A single pole breaker. The final piece of the puzzle is the light poles are pre-wired with #10 solid THHN/THWN and have in-line fuses at the base of the poles which then will tap into the main #2 lines in an in-ground hand hole next to each light pole. Seems reasonable. The problem is the existing conduit is old and only had 4 conductors originally (no ground). Pulling the old wire out was pretty difficult in a couple of the segments even with our 8000# tugger, so I would like to keep the diameter of the new wire bundle we pull in about the same as the old one if possible. Is there any reason this setup would not work as a 240/120v 4 wire multi wire branch circuit (3 wires plus ground)? The 17 luminaries would all run at 240, and the neutral would only be used for the 120v operator at the end of the line. Instead of 2 single pole breakers, there would be one 240v 2pole breaker. It seems like this should meet code, but I'm always confused as to why NEC 210.4.C says "line to neutral loads only" but then has exception #2, which seems to say the same thing as 210.4.B. Any help from you smarter people is greatly appreciated!
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I would check several things:
1) Are the lamp power supplies rated for L-L use, or must one of the circuit conductors be the grounded (neutral)?
2) Is there a 240V version of the gate opener available?
3) What is the maximum allowed pulling tension of the conductors you are using?

-Jon
 

JamesChain

Member
Location
Los Angeles
Occupation
Electrician
I would check several things:
1) Are the lamp power supplies rated for L-L use, or must one of the circuit conductors be the grounded (neutral)?
2) Is there a 240V version of the gate opener available?
3) What is the maximum allowed pulling tension of the conductors you are using?

-Jon
Hi Jon,

Thanks for your input! My responses:
1) Manufacturer tells me the power supplies are good for line to line supply with anything from 120-277v. They also told me the fixture in question draws 91 watts.
2) My understanding is the gate operator the owner plans to provide has a 120/230 nameplate rating, so the motor could run at 240, but some of the control components of the gate system (keypad, sensors, intercom) are 120v. Is there a code reason (or other reason) why we wouldn't want to have both 240 and 120v equipment on this circuit?
3) Figuring the max pulling tension is a good point, and I will do that. But at this point my main concern is determining whether or not I should propose the multiwire circuit option to the owner so that we can reduce the quantity of #2 wires in the existing conduit. My thinking is that we will be able to get the new wire in properly as long as the bundle size is similar to what we pulled out. The old wire had thicker insulation than the new THHN2, so I think if we are pulling in 4 conductors instead of 5 we will get the wire in properly.

James
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
2) My understanding is the gate operator the owner plans to provide has a 120/230 nameplate rating, so the motor could run at 240, but some of the control components of the gate system (keypad, sensors, intercom) are 120v. Is there a code reason (or other reason) why we wouldn't want to have both 240 and 120v equipment on this circuit?
If the nameplate is 120/230, then it might be the case that you must have 240V for the motor and 120V for the controls. In other words you might be required to do exactly what you propose. Off hand I can't think of a code reason not to supply 120/240 and run the motor at 240V.

3) Figuring the max pulling tension is a good point, and I will do that. But at this point my main concern is determining whether or not I should propose the multiwire circuit option to the owner so that we can reduce the quantity of #2 wires in the existing conduit. My thinking is that we will be able to get the new wire in properly as long as the bundle size is similar to what we pulled out. The old wire had thicker insulation than the new THHN2, so I think if we are pulling in 4 conductors instead of 5 we will get the wire in properly.
IMHO running either a single 240V circuit (2 circuit conductors plus EGC), or an MWBC to provide both 240 and 120V (3 circuit conductors plus EGC) is a better installation than running 2 separate circuits.

The downside that I can see is that if the circuit trips you will lose all of your lighting, whereas running 2 separate circuits you would most likely lose half of your lights. You might consider running this as two separate 240V circuits on smaller conductors to get the same cross section as your 4 #2

-Jon
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
One thing I'd be concerned about is the chance of depleting the voltage with a motor 1200 ft from the source which in turn can create the LED Flutter from a voltage just a tad under its rating. It would sure be nice to keep the sensitive lighting separate from a gate motor which will be opening and closing dozens of times, unless the gate only operates during the day. still It may work but I'd hate to have that design issue on my watch.
 
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