Low-voltage conductors, raceways and circuit sizing

Lesson 5 laid out the low-voltage chain from switchboards to rack PDUs. Connecting those pieces takes thousands of meters of cable, conduit, cable tray and busway, and each run must be sized correctly: large enough to carry its load without overheating, to keep voltage drop within limits and to clear faults safely, but no larger than necessary, because copper is expensive and space in electrical rooms and pathways is scarce. This lesson works through low-voltage circuit sizing as practiced in data centers: continuous loads and overcurrent devices, ampacity and its corrections, voltage drop, parallel conductors, neutrals and harmonics, copper versus aluminum, and the raceways that carry it all.

The sizing sequence

A low-voltage feeder or branch circuit is sized in a fixed order, and each step can only increase the conductor size found in the previous one:

1. Determine the design load current (Lesson 2), and whether it is continuous (expected to run for three hours or more, which almost every data center load is).

2. Choose the overcurrent device rating: for continuous loads, at least 125% of the load current, unless the device and its enclosure are listed for 100% continuous operation (Lesson 11).

3. Choose conductors whose ampacity, after all correction factors, is at least the larger of the overcurrent device rating (with permitted round-up rules) and 125% of the continuous load.

4. Check voltage drop and increase the size if needed.

5. Check that the conductors can withstand the available fault current until the protective device clears it, and size the equipment grounding (bonding) conductor.

Ampacity and termination temperature

Conductor ampacity tables give the current a conductor can carry continuously at a stated ambient temperature (30 °C) without exceeding the temperature rating of its insulation, for a small number of current-carrying conductors together. Insulation is commonly rated 75 °C or 90 °C. However, the terminals of breakers and equipment are usually rated only 75 °C (and 60 °C for small sizes), so the ampacity used for the final selection is normally taken from the 75 °C column even if the cable is rated 90 °C. The 90 °C column may be used as the starting point for applying correction factors.

Correction and adjustment factors

Two corrections are applied to the table values:

• Ambient temperature: above 30 °C, ampacity is reduced. For 90 °C insulation at 40 °C ambient, the factor is 0.91; for 75 °C insulation at 40 °C, 0.88. Electrical rooms in data centers can run warm, and cables routed above hot aisles or near heat sources may see ambients well above 30 °C.
• Number of current-carrying conductors: with more than three in one raceway or cable, mutual heating reduces ampacity: 80% for 4 to 6 conductors, 70% for 7 to 9, and lower for more.

Sizing a feeder for a continuous load

Large feeders are almost always run as parallel sets. Codes permit paralleling only for conductors of 1/0 AWG and larger, and require every parallel conductor of a phase to be the same length, material, size and insulation, terminated the same way, so that the current divides equally. Each raceway must contain one conductor of every phase, so that the magnetic fields cancel and steel conduits are not heated by induced currents.

Voltage drop

Codes and good practice limit voltage drop so that equipment receives adequate voltage: a common target is 3% on a feeder or branch circuit and 5% in total from the service to the load. For a three-phase circuit,

where \(R\) and \(X\) are the conductor resistance and reactance per unit length at operating temperature.

Neutrals and harmonics

In a balanced three-phase four-wire system with linear loads, the neutral carries almost no current, because the three phase currents cancel. The switch-mode power supplies of IT equipment, however, draw current in short pulses rich in harmonics, especially the third harmonic and its multiples (the triplen harmonics). Triplen harmonics from the three phases are in phase with each other, so instead of canceling in the neutral they add.

A neutral that carries significant harmonic current counts as a current-carrying conductor when applying the adjustment factors above.

Copper or aluminum?

Aluminum conductors are lighter and much cheaper than copper for the same ampacity, but have about 61% of copper's conductivity, so they must be about two sizes larger. Aluminum has a reputation for failures at terminations, mostly from older alloys and poor installation; modern AA-8000 series alloys with properly rated, correctly torqued terminations perform well. Aluminum is widely used for large feeders and busway; copper remains standard for branch circuits and wherever space is tight.

Raceways, cable tray and fill

Conductors run in raceways: rigid or electrical metallic conduit (EMT), cable tray, wireways and busway. Each has rules:

• Conduit fill: conductors may fill only a set percentage of a conduit's cross-section (40% for three or more conductors), so that they can be pulled without damage and can shed heat.
• Cable tray: suitable cable types only, with fill limits based on cable diameters and tray width, and separation between power and data cabling. In data halls, overhead cable trays for power and network cabling must be coordinated with busway, lighting, sprinklers, containment and cooling (DCE-320).
• Bending radius and pulling tension limits protect large cables during installation.
• Fire-stopping must restore the fire rating wherever raceways pass through fire-rated walls and floors (DCE-440).

In the US, conductor sizing follows Article 310 of the National Electrical Code (ampacity tables and correction factors), Articles 210 and 215 for branch circuits and feeders, Article 392 for cable trays and the raceway articles of Chapter 3.