Commercial hood: Run dedicated neutral for shunt-trip breaker or use neutral in panel?

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Jon456

Jon456

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I suspect this is simply a matter of personal preference. But I'd like to hear opinions or recommendations.

There is only one distribution panel for this space. There is a commercial hood that's fed control power from a 120V/15A dedicated branch circuit (12AWG conductors). The hood control package includes a two-terminal connection (hot and neutral) for tripping a shunt trip breaker; this is to de-energize all electrical equipment under the hood in the event of a fire. The shunt trip neutral terminal in the hood control package is simply jumpered from the incoming control power branch circuit neutral.

I could run two 14AWG conductors from these shunt trip terminals (hot and neutral) back to the shunt trip breaker. Or I could just run a single 14AWG conductor (hot) back to the breaker and connect the neutral from the breaker's shunt trip coil to the neutral bar in the distribution panel. Either way would allow the system to work as intended.

I contacted the hood manufacturer and asked for their recommendation and they said they've seen both methods used in installations.

On one hand, adding the dedicated neutral conductor does create a physical manifestation of the shunt trip loop circuit and it may appear more "professional." On the other hand, the hood is ~100 ft from the distribution panel, so installing a second shunt trip conductor just to carry neutral back to the distribution panel seems like a waste of copper. Plus, that means the round trip for the current is ~400 ft (as opposed to ~200 ft for jumpering the neutral in the distribution panel), although voltage drop won't be an issue either way since the shunt trip coil only draws 0.6A.

Which way would you wire this?
 
Think about the requirements for grouping conductors in the same racewaraceway or cable.

If the switched hot being run back to the panel can go in the same raceway or cable as the supply, then just run the switched hot. What you have is a common switch loop.

But if you can't group the switched hot with the supply hot, then you need to run the neutral.

Jon
 
Is the shunt coil disconnected (internally to the breaker) when the shunt is tripped?

I know this setup has been used for decades, but the fallacy (IMO) is a failure of the control system does not disconnect power and won't trigger a service call.

As far as running the neutral back from the hood, I wouldn't be concerned about it. Although Jon is correct, it's not a load that exists for any significant length of time so no real field canceling concerns (IMO). It would still be best if the switch leg to the shunt of the breaker is in the same conduit as the circuit feeding the hood control box or switch.
 
Seeing that this is a case where the trip current is sent for just a moment only during testing or a fire, I would (and have) just pick up the trip-coil neutral in the panel.

Either way, most trip coils are not rated for continuous energization, you should make sure the trip power comes from a point that loses power after a system trip.
 
LarryFine said:
Seeing that this is a case where the trip current is sent for just a moment only during testing or a fire, I would (and have) just pick up the trip-coil neutral in the panel.

Either way, most trip coils are not rated for continuous energization, you should make sure the trip power comes from a point that loses power after a system trip.
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Having designed well over 100 kitchen suppression systems, I can tell you that we would always spec a breaker that opens the coil after the trip.
 
Most of the fire-suppression systems I have wired were in existing kitchens, so I wasn't involved in breaker selection.
 
There must be a certain manufacture that designs a shunt trip breaker that does not have clearing contacts, or a constant voltage coil? Not saying it’s not out there, but I have yet to run across one like that in all that I’ve hooked up. Larger breakers maybe, but the common cooking equipment size, I have not.
 
oldsparky52 said:
Do you have concerns that a shunt breaker system fails on and not off (if control power is lost)?
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Very much so, which is why I would typically use contactors that require power to pull in. The other option is to power the shut from the circuit it is controlling. That way, as long as the device is powered, so is the shunt trip. You still have a vulnerability in the field wiring, so if you get an open between the shunt and the suppression system control head you'll have a failure you won't be aware of.
 
To answer some of the posed questions/concerns:
  1. All wiring between the distribution panel and the hood are within a single metallic raceway.
  2. The shunt-trip breaker is a Square D QOB2201021. The shunt trip coil circuit is open when the breaker is off or tripped.
  3. The shunt-trip is powered by the hood control package power. If power is lost to the hood control, then there are significantly greater problems than the shunt not tripping.
  4. I realize that a contactor is a more fail-safe design than a shunt-trip breaker. However, a shunt-trip breaker is an industry-accepted design (and probably the most common design) for commercial hoods. The probability of a failure leading to the shunt-trip breaker not tripping in the event of a fire is acceptably low.
 
Jon456 said:
To answer some of the posed questions/concerns:
  1. All wiring between the distribution panel and the hood are within a single metallic raceway.
  2. The shunt-trip breaker is a Square D QOB2201021. The shunt trip coil circuit is open when the breaker is off or tripped.
  3. The shunt-trip is powered by the hood control package power. If power is lost to the hood control, then there are significantly greater problems than the shunt not tripping.
  4. I realize that a contactor is a more fail-safe design than a shunt-trip breaker. However, a shunt-trip breaker is an industry-accepted design (and probably the most common design) for commercial hoods. The probability of a failure leading to the shunt-trip breaker not tripping in the event of a fire is acceptably low.
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The problem is that if you don't monitor the power to the shunt trip breaker, you aren't in compliance with NFPA 17A. Having also been a tech and serviced these systems, I can say that my personal experience is that exactly "0" systems with shunt trip breakers were supervised. If you use a contactor that drops out on power loss, you're OK:

5.3.3 Where fixed automatic fire-extinguishing systems are
interconnected or interlocked with the cooking equipment
power sources so that if the fire-extinguishing system becomes
inoperable due to power failure, all sources of fuel and heat to
all cooking appliances serviced by that hood shall automatically
shut off, and electrical power monitoring shall not be required.
 
The fryer under the hood is gas-fired, not electrical; the electrical power under the hood is to operate the fryer's gas controls (thermostat, ignitor, etc.).

The hood's automatic fire suppression system is mostly mechanical (including the gas shut-off valve), although the hood exhaust fan is designed to operate in the event of a fire to draw out combustion products.

If there is a total power failure, the shunt won't have to trip because there won't be any power under the hood anyway. The larger problem will be the lack of the exhaust fan to evacuate smoke. But the automatic fire suppression system will still operate just fine.

This is a new CaptiveAire hood. CaptiveAire is a recognized industry leader in commercial hoods. I am confident their system and design are fully compliant with applicable codes and regulations.

I am not concerned about installing a shunt-trip breaker in accordance with CaptiveAire's engineering.
 
Jon456 said:
The fryer under the hood is gas-fired, not electrical; the electrical power under the hood is to operate the fryer's gas controls (thermostat, ignitor, etc.).

The hood's automatic fire suppression system is mostly mechanical (including the gas shut-off valve), although the hood exhaust fan is designed to operate in the event of a fire to draw out combustion products.

If there is a total power failure, the shunt won't have to trip because there won't be any power under the hood anyway. The larger problem will be the lack of the exhaust fan to evacuate smoke. But the automatic fire suppression system will still operate just fine.

This is a new CaptiveAire hood. CaptiveAire is a recognized industry leader in commercial hoods. I am confident their system and design are fully compliant with applicable codes and regulations.

I am not concerned about installing a shunt-trip breaker in accordance with CaptiveAire's engineering.
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Well, news for you, CaptiveAire doesn't design their suppression systems. Ansul does. Or they have an engineer on staff using the Ansul manual. The Ansul AUTOMAN regulated release assembly doesn't have the capacity to monitor power to the shunt trip breaker, if you have one. Their designs also don't extend past the hood envelope, so they aren't addressing supervisory issues directly, although they may mention them in their drawing note. Whether they do or not doesn't relieve the installer from complying with the relevant portions of 17A. And while it's certainly true that in a total power failure circumstance there's no issue, it's the pathway from the shunt trip breaker to the releasing assembly that's the point of concern.

And come to think of it, if the fryer(s) are gas-fueled, the standard doesn't require that the electrical supply to the fryer be turned off if all it's doing is providing power for the controller.

17A snip.PNG

The fact that CaptivAire provided a micro switch for use with a shunt trip circuit doesn't mean you have to use it. Unless it's part of the signed and sealed design and not just a generic drawing submittal.
 
Jon456 said:
  1. All wiring between the distribution panel and the hood are within a single metallic raceway.
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That's all that matters. The supply to the controller and the wire to the shunt-trip are in the same conduit, just like a switch loop.

Thus, there is no need for an additional grounded conductor to return to the panel for the breaker. Pick it up in the panel itself.
 
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