Codes, bonding and firestopping review

Code questions are the ones candidates most often get wrong for a reason that has nothing to do with knowledge: they answer from habit instead of from the code. A crew that has always run riser cable in a ceiling space, because the inspector never checked, will answer a plenum question wrongly. This review covers LVT-330: how the electrical code applies to low-voltage work, cable ratings, spaces, separation, entrances, firestopping and bonding. It also covers the parts of the TIA-607 bonding standard that installer exams expect.

Exam note: BICSI's exams are international and refer mainly to the NEC and to TIA standards, with the principle that the local code governs. A Texas candidate works under the NEC as adopted and amended by the city, so the NEC article numbers in this review are the ones to know.

How the code applies

LVT-330 lessons 1 and 2 placed low-voltage work in the NEC. Chapter 7 covers special conditions, including Article 722 for cables for power-limited circuits, Article 725 for Class 2 and Class 3 circuits and Article 760 for fire alarm systems; Article 770 covers optical fiber cables; Chapter 8 covers communications systems, with Article 800 for general rules and Article 805 for communications circuits. The code is enforced by the authority having jurisdiction, in Austin the city's development services, under the NEC edition the state and city have adopted.

The standards, such as TIA-568 and TIA-607, are not law unless a code or a contract calls them up, but the specification almost always does. The building code governs fire separations and penetrations.

Cable ratings and substitution

LVT-330 lesson 3 covered the listings. For communications cable, CMP is for plenums, CMR for risers, CMG and CM for general use, and CMX for limited use such as one- and two-family dwellings. Optical fiber has parallel listings, with N for nonconductive and C for conductive: OFNP, OFCP, OFNR, OFCR, OFNG and OFCG. Power-limited cables have their own listings, such as CL2P and CL3P. A higher listing may replace a lower one: CMP may be used anywhere CMR is required. For optical fiber, nonconductive cable may replace conductive cable at the same or higher level, but not the reverse.

Exam trap: a cable's category and its fire rating are separate. A Category 6A cable can be plenum-rated or riser-rated; the category says nothing about where it may go. And the rating needed for a run is set by the strictest space it passes through, so a run that passes through one plenum ceiling needs plenum cable for its whole length unless it changes type at a proper transition.

Spaces, separation and support

A plenum is a space used for environmental air, such as the space above a ceiling used as a return air path; a riser is a vertical shaft or a penetration through floors; everything else is general. LVT-330 lessons 4 and 5 covered the rules:

• Communications and Class 2 conductors are kept out of raceways and boxes with power conductors unless a barrier or a listed divider separates them (805.133, 725.136).
• Exposed communications conductors are kept at least 2 in (50 mm) from power conductors unless raceway, metal sheath or a fixed nonconductor provides the separation.
• Cable is supported from the building structure, not from ceiling grid or its support wires (300.11), conduit or pipes; plenum ties and supports must be listed for the space.
• Elevator hoistways are never used as cable pathways.
• Accessible portions of abandoned cable are removed (800.25), and cable kept for future use is tagged.

Entrances and protection

LVT-330 lesson 6 covered entrances. Copper communications conductors that enter a building from outside and could be exposed to lightning or power contact get a listed primary protector, installed as close as practicable to the point of entrance (805.90). The protector is bonded to the building's intersystem bonding termination or grounding electrode system with a short, straight bonding conductor of at least 14 AWG copper (800.100). Unlisted outside plant cable may enter no more than 50 ft (15 m) before it ends at a transition to listed cable or is enclosed in raceway (800.48).

Exam trap: long, looping bonding conductors defeat the purpose. A surge rises quickly, and every meter of conductor and every bend adds impedance, so the voltage between the protector and ground during a surge rises. Short and straight is the rule.

Fire separations and firestopping

LVT-330 lessons 8 and 9 covered penetrations. A fire-rated wall or floor has an hourly rating, and every penetration through it must be sealed with a tested firestop system that restores that rating. A tested system is a specific combination of penetrant, opening size, annular space, fill and materials that has passed a fire test and is listed by a testing organization, such as UL or ULC. Two ratings describe it: the F rating, the time the system prevents flame passage, which must be at least the assembly's rating, and the T rating, the time before the unexposed side gets too hot, which the building code requires in some locations.

The installer chooses the system from the listing that matches the actual condition, installs it exactly as listed, and labels it with the system number, the date and the installer's name. Changing any variable, such as using a larger sleeve, more cables than the listed fill or a different sealant, takes the installation outside the listing. Reusable firestop devices and pathways, which let cables be added without breaking the seal, are the usual answer where cables change often.

Exam trap: a fire-rated wall needs the firestop at every penetration, including the ones hidden above the ceiling. Inspectors look above ceiling tiles first, because that is where the shortcuts are.

Bonding and TIA-607

LVT-330 lessons 10 and 11 covered bonding. Grounding connects a system to earth; bonding connects metal parts together so that they stay at the same potential. All the systems in a building, power, telecommunications, cable television and others, bond to the one grounding electrode system, so that a surge or fault cannot drive large voltages between them.

TIA-607 sets up a telecommunications bonding infrastructure. Its current names, from the D edition onward, are:

Bonding conductors are copper and at least 6 AWG. The TBB is sized by its length, with larger conductors for longer runs, up to a maximum size, and is routed as straight as practicable. Racks, cable tray, metal pathways and equipment in each room are bonded to the busbar in that room. A common acceptance check is a low resistance between busbars; many specifications set a limit such as 100 milliohms between the PBB and any SBB.

# Three code and bonding checks: cable rating for a space, TBB size by length, busbar resistance
LADDER = ["CMP", "CMR", "CM"]                        # highest first
NEED = {"plenum": LADDER[0], "riser": LADDER[1], "general": LADDER[2]}

def rating_ok(cable, spaces):
    need = max((NEED[s] for s in spaces), key=lambda r: -LADDER.index(r))
    return cable, need, LADDER.index(cable) <= LADDER.index(need)

for cable, route in [(LADDER[1], ["riser", "plenum"]), (LADDER[0], ["general", "riser"]), (LADDER[2], ["general"])]:
    c, need, ok = rating_ok(cable, route)
    print(f"{c} through {'+'.join(route):15s} needs {need}: {'OK' if ok else 'NOT ACCEPTABLE'}")

TBB_TABLE = [(13, '6 AWG'), (20, '4 AWG'), (26, '3 AWG'), (33, '2 AWG'), (41, '1 AWG'), (52, '1/0 AWG'), (66, '2/0 AWG'), (9999, '3/0 AWG')]                          # (up to this length, conductor size)
def tbb_size(length):
    for limit, size in TBB_TABLE:
        if length <= limit:
            return size
    return TBB_TABLE[-1][1]

for floor, length in [(2, 12.5), (6, 52.0), (11, 110.0)]:
    print(f"TBB to floor {floor:2d}: {length:5.1f} ft -> {tbb_size(length)}")

LIMIT_MOHM = 100
for busbar, mohm in [("SBB-2", 18), ("SBB-6", 41), ("SBB-11", 137)]:
    print(f"PBB to {busbar}: {mohm} milliohm", "OK" if mohm <= LIMIT_MOHM else "OVER LIMIT, check connections")

CMR through riser+plenum    needs CMP: NOT ACCEPTABLE
CMP through general+riser   needs CMR: OK
CM through general         needs CM: OK
TBB to floor  2:  12.5 ft -> 6 AWG
TBB to floor  6:  52.0 ft -> 1/0 AWG
TBB to floor 11: 110.0 ft -> 3/0 AWG
PBB to SBB-2: 18 milliohm OK
PBB to SBB-6: 41 milliohm OK
PBB to SBB-11: 137 milliohm OVER LIMIT, check connections

The code collects three checks from this area: whether a cable's rating is acceptable for a space, the TBB size for each riser length using a table like the one in LVT-330 lesson 11, and a resistance check of busbar test results. The size table in the code is the one Cobalt Line uses on its projects, following the TIA approach; on the exam the table, if needed, is given in the question.

Inspections and corrections

Code questions sometimes take the form of an inspection scenario: a list of conditions found on a walk, and a question about which one is a code violation, or which must be corrected before cover-up. LVT-330 lesson 12 followed a full pre-inspection, and its findings make a good revision list: riser cable in a plenum ceiling; cables lying on ceiling tiles; data cable in the power side of a floor box; a missing firestop above a ceiling in a rated corridor wall; a firestop with more cables than its listing allows and no label; a looping protector bond; abandoned cable left in a ceiling; a rack in the working space in front of a panelboard; an unbonded cable tray in a telecommunications room; and a TBB that was spliced partway up the riser with an unlisted connector.

Each finding has a reason, and the exam rewards knowing the reason rather than the list. Cable ratings and firestops protect people from fire and smoke; separation and protectors protect people and equipment from power faults and surges; support rules keep cables from falling or loading other systems; working space keeps electricians safe; and bonding keeps every system at one potential. A candidate who knows the reason can answer a question about a condition she has never seen.

One more habit helps on code questions: notice which code the question is written to. A question that names NEC articles expects NEC answers, even for a Canadian candidate, and a question about local practice expects the candidate to say that the authority having jurisdiction decides.