Bonding of Conduit for Fire Alarm with Metal Conduit and Plastic Boxes

[DM2-Inc]

I'm looking for some opinions here...
The equipment being installed is European which has US approvals. The smoke detectors are supplied by the manufacture as a plastic box and fall under the fire alarm device listing. Like all modern day fire alarm systems I'm familiar with, the system is less than 50 volts. I have to be honest, I don't think I'll ever have a good understanding on what defines the "Class" of power supply, but for now I'm assuming it's a Class 1 power supply (maybe someone can provide a link to educate me on that point).

My references are to the 2011 edition of NFPA 70.

1. NEC 250.112(I) does not require conduit to be grounded when the circuit is less than 50 volts and is supplied from a Class 1 power supply (i.e. the fire alarm step down transformer).
2. NEC 250.86, which addresses “Conductor Enclosures and Raceways” requires conduits to be grounded with the exception of fire alarm systems, due to it’s reference to 250.112(I).
3. NEC 314.3 permits the use of “nonmetallic” boxes but when used with metal raceways, requires a “…internal bonding means…between all entries…”. This section doesn’t say either the box or the conduit has to be grounded. That requirement would fall under 250.86, which permits their exclusion based on 250.112(I).
4. NEC 314.4 does not require metal boxes to be grounded due to it’s reference to 250.12(I).

I'm concerned about 314.3 which requires the plastic box to have an internal means of bonding the conduit entries, which this box does not have. Article 314.3 or 4 don't address bonding or grounding requirements as to me this would be the function of 250.86. I have to ask...should 314.3 have a similar reference to 250.112(I), that 314.4 has?

Is my conclusion that the use of the plastic "Manufacture Supplied" box is acceptable for use with the metal conduit, even without the internal bonding method?

 

[fmtjfw]

some questions

some questions

1. Do the instructions indicate the current and voltage of external circuits? If so look at ch 9 tables 12() for parameters for power limited.
1.5: If it has a US listing, contact the listing organization if you can't find the power classification in the documents. The fire alarm systems I have installed always have a roughly 100 page manual that contains this information.
2. Are you required by specification to use metallic raceway? If not, consider non-metallic raceway or fire alarm cable.
3: Are you installing circuits in locations that require metallic raceway? If so ... I don't know what you should do.
3.5: See if the AHJ will allow you to use metallic conduit as a protective sleeve rather than as a conduit system.

good luck

 

[jaggedben]

1. NEC 314.3 permits the use of “nonmetallic” boxes but when used with metal raceways, requires a “…internal bonding means…between all entries…”. This section doesn’t say either the box or the conduit has to be grounded. That requirement would fall under 250.86, which permits their exclusion based on 250.112(I)

I'm concerned about 314.3 which requires the plastic box to have an internal means of bonding the conduit entries, which this box does not have. Article 314.3 or 4 don't address bonding or grounding requirements as to me this would be the function of 250.86. I have to ask...should 314.3 have a similar reference to 250.112(I), that 314.4 has?

Is my conclusion that the use of the plastic "Manufacture Supplied" box is acceptable for use with the metal conduit, even without the internal bonding method?

Is there more than one raceway entry to the box? Does the equipment have a terminal for an equipment ground (EGC)?

The purpose of 314.3 is to ensure that the electrical continuity of metal raceways is not broken where a plastic junction box is used. Internal bonding means must be used to ensure that metal raceways on the 'far side' of the box are bonded to ground just as those on the 'near side', if that makes sense.

If you only have one entry then no 'internal bonding means' are required 'between all entries' because there is no 'between' to speak of. (However if there is an EGC connection on the equipment, you'll need to connect something to that.)

If there are multiple entries, then you probably need to install bonding bushings on each entry and run your EGC or a bonding jumper between them, sized to 250.122. If there is an EGC connection in the equipment then the EGC or jumper should hit that, too.

 

[DM2-Inc]

fmtjfw

Do the instructions indicate the current and voltage of external circuits?

Yes, 24 VDC, 2 Amps

...look at ch 9 tables 12()...

I'm not too sure how to use this table, but I'll give it a shot. Table 12(A) applies to AC power source, and 12 (B) applies to DC power source. The system uses a step down transformer and full wave bridge rectifier thereby supplying 24 VDC, 4 Amps to the main board of the fire panel. I'm guessing I need to focus on Table 12(B). As the V_max is 24 it falls under the "Over 20 and through 30" column. The I_max is 4 Amps. Based on note 1 for the table, I'm assuming the circuit is "Inherently Limited Power Source (Overcurrent Protection Not Required)". Is that correct?

Are you required by specification to use metallic raceway?

Yes

Are you installing circuits in locations that require metallic raceway?

No

See if the AHJ will allow you to use metallic conduit as a protective sleeve rather than as a conduit system

The cable is a CI cable so essentially it's more than a "...protective sleeve..."

jaggedben

The purpose of 314.3 is to ensure that the electrical continuity of metal raceways is not broken where a plastic junction box is used

I understand that, but in this case why? I don't believe that bonding the conduit has a benefit. Understand that the power supply is a floating power supply. If the insulation of the cable were to come in contact with the conduit, the breaker supplying power to the transformer isn't going to trip. Consider that 314.4 doesn't require a metal box to be grounded based on the reference to 250.112(I). This seems to imply that there isn't a concern with the bonding of a metal box if the box falls under 250.112(I), so why the concern for a plastic box under the same conditions?

...'internal bonding means' are required...

I would agree that internal bonding means are required as part of the box, but bonding of any kind isn't required by 250.86 because the circuit is less than 50 volts and 250.86 precludes the need for bonding based on the reference to 250.112(I).

 

[fmtjfw]

If the system is 4 wire (two wires into each device, two wires out of each device, and a terminating resistor at the each loop) then:

In a standard 4 wire fire alarm system, if a conductor is grounded, the system should produce a trouble alarm. The condition of a single ground does not stop the system from functioning as a fire alarm system. The trouble alarm notifies that the system needs to be repaired, so that the second grounding (if on the detector side does not produce a false alarm, or on the alarm side does not disable alarm devices).

Nothing short (no pun intended) of a short in the power supply should trip the supply breaker or fuse. Further, almost all fire alarm systems have a backup battery that supplies the system in the case of commercial power loss.

If it's a system that uses addressable devices, then I know little about it.

 

[DM2-Inc]

fmtjfw,
This is an addressable system,

I'm guessing that an a change should be submitted to section 314.3 to modify the language by adding and exception based on 250.112(I), similar to that of 314.4.

 

[gadfly56]

I'm looking for some opinions here...
The equipment being installed is European which has US approvals. The smoke detectors are supplied by the manufacture as a plastic box and fall under the fire alarm device listing. Like all modern day fire alarm systems I'm familiar with, the system is less than 50 volts. I have to be honest, I don't think I'll ever have a good understanding on what defines the "Class" of power supply, but for now I'm assuming it's a Class 1 power supply (maybe someone can provide a link to educate me on that point).

My references are to the 2011 edition of NFPA 70.

1. NEC 250.112(I) does not require conduit to be grounded when the circuit is less than 50 volts and is supplied from a Class 1 power supply (i.e. the fire alarm step down transformer).
2. NEC 250.86, which addresses ?Conductor Enclosures and Raceways? requires conduits to be grounded with the exception of fire alarm systems, due to it?s reference to 250.112(I).
3. NEC 314.3 permits the use of ?nonmetallic? boxes but when used with metal raceways, requires a ??internal bonding means?between all entries??. This section doesn?t say either the box or the conduit has to be grounded. That requirement would fall under 250.86, which permits their exclusion based on 250.112(I).
4. NEC 314.4 does not require metal boxes to be grounded due to it?s reference to 250.12(I).

I'm concerned about 314.3 which requires the plastic box to have an internal means of bonding the conduit entries, which this box does not have. Article 314.3 or 4 don't address bonding or grounding requirements as to me this would be the function of 250.86. I have to ask...should 314.3 have a similar reference to 250.112(I), that 314.4 has?

Is my conclusion that the use of the plastic "Manufacture Supplied" box is acceptable for use with the metal conduit, even without the internal bonding method?

This is very unlikely. All the fire alarm panels I've ever seen have Class 2 or 3 power supplies, at least for the SLC and NAC's. The "big boy" panels from Siemens, etc will have separate power limited and non-power limited outputs available. You're looking at article 760 for wiring requirements, and unless a 760 sub-article refers to Chapter 3 specifically (for power limited) then Chapter 3 wiring requirements do not apply.

 

[DM2-Inc]

When installing a system as "Non-Power Limited" the reference section applies, regardless of the class of power supply and regardless if the circuit could be installed a "Power Limited". An installer is afforded the opportunity to install a "Power Limited" circuit of a fire alarm system in accordance with "Non-Power Limited" rules. Such is very often the case with industrial applications in plants. As a result all of the referenced sections apply.

But lets move beyond whether this is a fire alarm system or some other system, and focus on question, which has to do with section 314.4 which doesn't require metal junction boxes to have a ground terminal if the circuit complies with 250.112(I) and section 314.3 which doesn't have the similar exception.

Does the omission in 314.3 to section 250.112(I) make sense when in reality is the circuit which should determine of the junction box needs a ground terminal?