Design rules, limits and cable counts

The previous lessons set out the parts of a structured cabling system and the rules for each. Design is the work of putting them together for a real building: deciding how many outlets go where, where the telecommunications rooms must be, how much cable each room serves, how big the pathways must be and, finally, how much material to order. On Cobalt Line's projects this is Sanjay's work, but every technician should understand it, because the field is where a design meets the building, and a technician who knows why a design looks the way it does can spot the problems it did not anticipate. This lesson gathers the design rules into one place and walks through the calculations, ending with the cable takeoff and bill of materials that turn a drawing into an order.

The rules in one place

Most design checks come back to a short list of numbers. Sanjay keeps them on one page at the front of every design file.

From requirements to outlets

Design starts with the owner's requirements and the architect's floor plans. Sanjay goes through the plans room by room and places outlets: the work area outlets from the owner's standard or the planning figure, the device outlets for every access point, camera, card reader, display and controller shown on the other trades' drawings, and the outlets the owner asks for in particular rooms. He then adds a grid of wireless access point locations, roughly every 60 ft in open areas and adjusted to the walls and ceiling heights, because wireless coverage is now the most demanding requirement in most buildings. Each outlet is a horizontal cable, and on current projects device outlets are a growing share of the total, in some buildings a third or more.

Every outlet location is then checked against the telecommunications rooms. Sanjay draws, on the floor plan, the area that each room can reach within 90 m of cable route. Because routes follow corridors and ceilings, that area is not a circle but an irregular shape, shrinking around obstacles. Any outlet outside every room's area needs a new room or enclosure, a different route or, for a device far away, a fiber-fed enclosure near it.

With the outlets placed, the counts follow: cables per room, panels and ports per room, switch ports, backbone strands, and then pathway sizes for each corridor and branch, using the fill rules of Lesson 9. Spare capacity is planned at each level. Sanjay typically leaves about 20 percent spare ports on patch panels, rack space for growth as in Lesson 3, and room in pathways for future cable, as the owner's standard requires.

Power over Ethernet and cable bundles

When a cable carries Power over Ethernet, some of the power is lost as heat in its conductors. A single cable warms only slightly, but in a large bundle the inner cables cannot shed their heat, and the bundle's temperature rises. Higher temperature raises insertion loss, as Lesson 5 showed, and can approach the temperature rating of the cable jacket. TIA's guidance, TSB-184-A, addresses this for PoE currents of up to about 1,000 mA per pair, which covers the 90 W level of IEEE 802.3bt Type 4. It recommends Category 6A or better cable for new PoE installations, avoiding bundling where possible and limiting bundles to 24 cables where they cannot be avoided, and it notes that larger conductors and shielded cables run cooler.

The heat in one cable can be estimated from Ohm's law. With power on all four pairs, current flows out on two pairs and back on the other two, with the two conductors of each pair sharing the current. The resistance of each conductor and the current through it give the power lost.

# Heat produced in one PoE cable (illustrative; current per powered pair set)
r_per = 2.0               # one 23 AWG conductor, ohm per 100 ft at 20 C
length = 295              # ft
per_pair_ma = {"802.3af (15.4 W)": 350, "802.3at (30 W)": 600,
               "802.3bt Type 3 (60 W)": 600, "802.3bt Type 4 (90 W)": 960}
r_cond = r_per * length / 100
for name, ma in per_pair_ma.items():
    i_cond = ma / 1000 / 2            # each conductor carries half the pair current
    pairs = 2 if "af" in name or "at" in name else 4
    conductors = pairs * 2
    loss = conductors * i_cond ** 2 * r_cond
    print(f"{name:24} {pairs} pairs powered, {ma} mA per pair set: {loss:5.2f} W lost in the cable")

802.3af (15.4 W)         2 pairs powered, 350 mA per pair set:  0.72 W lost in the cable
802.3at (30 W)           2 pairs powered, 600 mA per pair set:  2.12 W lost in the cable
802.3bt Type 3 (60 W)    4 pairs powered, 600 mA per pair set:  4.25 W lost in the cable
802.3bt Type 4 (90 W)    4 pairs powered, 960 mA per pair set: 10.87 W lost in the cable

The figures run from under a watt per cable at the lowest PoE level to about ten watts at the highest. That sounds small, but a bundle of 48 cables at the Type 4 level puts several hundred watts, the output of a small space heater, into a bundle a few centimeters across, spread along its whole length. That is why designs that serve many high-power devices, such as an office lit by PoE luminaires or a wall of PoE displays, spread their cables across more pathways, use smaller bundles in open tray rather than tight ones, and avoid routing them through hot spaces.

The cable takeoff

A takeoff is the measurement of material from the drawings. For horizontal cable, the usual method is to measure the route to every outlet from its telecommunications room, add the allowances of Lesson 5 for the rack, the wall drop and the service loops, and sum. On a large job with hundreds of outlets, estimators often use the average-length method instead: measure the routes to the nearest and farthest outlets served by a room, take their average, add the allowances, and multiply by the number of cables. The method works well when outlets are spread evenly across the area a room serves, and Sanjay checks it against a few measured routes before he trusts it.

Horizontal cable is sold in boxes or reels, most commonly 1,000 ft. A box cannot be used to its last meter, because each pull uses whole cable lengths, and the end of a box is usually shorter than the next cable. The number of full cables that can be pulled from one box is the box length divided by the cable length, rounded down, and the boxes needed is the number of cables divided by that, rounded up. Pulling several cables at once, as is normal, means pulling from several boxes at once, so a crew pulling a bundle of six cables needs six boxes open.

# Cable takeoff and bill of materials for one telecommunications room
import math
shortest, longest = 46, 270          # measured routes, ft
allowance = 30                       # rack, wall drop and service loops, ft
cables = 212                           # horizontal cables in this room
box = 1000                             # ft per box
waste = 0.10

average = (shortest + longest) / 2 + allowance
per_box = math.floor(box / average)
boxes = math.ceil(cables / per_box)
total = cables * average * (1 + waste)
print(f"average cable length {average:.1f} ft, {per_box} cables per box, {boxes} boxes")
print(f"cable for budget with {waste:.0%} waste: {total:,.0f} ft")
bom = {"Category 6A outlet jacks": cables,          # panel ports are built into the panels
       "48-port patch panels": math.ceil(cables / 48 * 1.2),
       "faceplates (average 2.5 ports)": math.ceil(cables / 2.5),
       "labels (two per cable, faceplate and port)": cables * 4,
       "equipment cords": cables}
for item, n in bom.items():
    print(f"  {item:42} {n:5d}")

average cable length 188.0 ft, 5 cables per box, 43 boxes
cable for budget with 10% waste: 43,842 ft
  Category 6A outlet jacks                     212
  48-port patch panels                           6
  faceplates (average 2.5 ports)                85
  labels (two per cable, faceplate and port)   848
  equipment cords                              212

Each item in the list is checked against the design. The panel count includes the 20 percent spare ports. The faceplate count depends on the mix of single and multiple outlets, and Sanjay counts faceplates from the outlet schedule rather than from an average when the schedule is ready. Equipment cords are often supplied by the owner, and the specification says who provides them.

Checking a design in the field

Designs are drawn on plans that do not always match the building. Walls move, ducts take the space reserved for tray, ceilings are lower than drawn and outlets are relocated by the tenant. Technicians are the first to see these changes, and Cobalt Line asks them to report anything that affects a design rule: a route that will now exceed 90 m, a pathway that will be overfilled, a room that has lost space, an access point location that has moved behind a new wall. Dev's practice is simple: measure, compare with the design, and raise anything that crosses a rule with Sanjay before the cable is pulled, not after it fails its test.

Designing for the next twenty years

The last design question is always what the building will need in the future. Sanjay asks owners how long they expect to occupy the space, how often they reorganize and what they expect to add: more access points as wireless standards advance, more cameras, PoE lighting, building sensors. He then decides where to spend on spare capacity now, usually pathways, rack space and backbone strands, which are expensive to add later, and where to wait, usually extra horizontal cables in areas whose use is uncertain. A design that leaves room to grow is cheaper over the building's life than one that fits today's needs exactly.