FSP0008 – Power Distribution Inside Buildings – Facility Science Podcast #8

so in a 3-phase panel, if you call the phases A, B, and C, a breaker connected to phase B will have an A-phase breaker above it and a C-phase breaker below it. Similarly in a a split phase 240/120 system the breakers will alternate between each side of the split phase.

This is done so allow the use of multiple phases to power a high-power appliance. A 2-pole breaker can be installed anywhere in a split-phase or 3-phase panel and be guaranteed to contact 2 different phases. Similarly a 3-pole breaker can be installed anywhere in a 3-phase panel and be guaranteed to contact all three phases and not contact any single phase more than once.

the 480V service will enter the building and land on some type of switchboard (some people call this the main switch gear, other reserve the term switch gear for medium voltage and higher). The switchboard will likely include a a disconnecting device (probably a circuit breaker, maybe a remotely operable circuit breaker) that will allow disconnection of utility power.

From this switchboard, power will be distributed to all of the electrical rooms in the building (generally at the higher voltage 480/277).

At each electrical room, the 480 lines will enter a switchboard. This switchboard will then feed the distribution devices in the electrical room (usually through a circuit breaker sized appropriately for the downstream load).

Power will generally be distributed throughout the building at the higher voltage (in this case 480/277) both because we want to be able to power high power equipment and lighting at each part of the building and also to minimize power losses in the feeder lines. (Not sure if I covered the concept of line losses before, so I’ll do it now…When current travels around a circuit, it consumes all of its potential energy, which means it “drops” all of the voltage. Ideally, energy is only consumed in the devices we intend to power, but in real life, the wires themselves consume some energy by warming up from all of the electrons traveling through them. So if the wire run to a device is really long, a device on a 120V circuit might only see 115V on the wire because 5V were consumed by the wire. The amount of loss is proportional to the current, so higher current means more loss (more voltage drop). Since power is voltage times current, higher voltage means lower current which means less power consumed in the wires (which is one reason we choose to distribute power at the highest available voltage).

At each electrical room. we’ll have a panel to serve the higher power loads with 480/277. This might be the same switchboard the power lands on in the room. It’s also typical to have a 277V panel for lighting.

The switchboard will also power a transformer to convert the 480/227 to 208/120 for all of the plug loads and “regular” appliances and etc…

If many 208/120 circuits are needed in this part of the building, the output of the transformer might feed a second switchboard which will then feed multiple breaker panels, each protected by a circuit breaker of appropriate size.

These are long skinny pieces of metal, usually copper or aluminum. Sometimes a conductor (or a wire) is a solid piece of metal and sometimes it is made up of many individual strands twisted together. The metal conductors are usually insulated with plastic of a type and thickness appropriate for the voltage and environment.

The amount of current a conductor can carry is dictated by its size (cross-sectional area, or diameter for a round cable). A larger conductor can carry more current than a small one. If a conductor carries too much current it will heat up and potentially cause a fire or melt. The current carrying capacity of a conductor is called ampacity (for amp capacity). The voltage doesn’t affect how much current a particularly sized conductor can carry current at any voltage. Instead, the voltage affects the type and thickness of insulation and the amount of space required between conductors. A cable carrying a higher voltage requires thicker insulation or more separation from other conductors.

The conductors have to be protected from damage and accidental contact so they are usually run inside a raceway or conduit. This typically a pipe made out of steel or plastic, but could be other things as well. A raceway can only be filled to a certain percentage capacity to allow for proper dissipation of heat generated by the current flowing in the wires.

Some conductors aren’t run in conduits but are instead bundled into cables. The cables are protected by some kind of sheathing typically metal (steel or aluminum), plastic, or rubber. You might have heard terms like MC cable (metal clad), Romex (that’s a brand of plastic-sheathed cable typically used in residential construction) or SO-cord (a rubber clad cable). Cables aren’t run in conduit so they have to be supported either by strapping them to appropriate surfaces or running them in cable trays.

There is a practical limit to the size of an electrical wire that can be used inside a building. An individual wire has to be able to be bent and pulled through conduit and pulled into enclosures and secured to terminals. So when the current requirements get very high, multiple conductors are used rather than continually increasing the size of the conductors. The largest flexible conductors in general use can carry in the range of hundreds of amps (400-600)

In some facilities, with high power requirements we use busbar systems. Rather than flexible cables, a busbar is a rigid piece of conductive metal (usually aluminum or copper).

Busbars can be individual conductors or multiple conductors (ex 1 for each phase and a neutral and a ground) bundled together as an assembly.

Busbars are used when we want to move a lot of power at a high current (up to several thousand amps). Since they are self-supporting and don’t need to bend, they can be made much larger than wires or cables. In this case larger means higher ampacity.

Busbars typically have provisions to allow tapping into the busbar at regular intervals often without the need to de-energize the busbar. This allows for flexible reconfiguration in certain situations.

Busbars trunking systems are typically used in factories, high rise buildings, data centers.

To illustrate the benefit of this, imagine if you need to send 3000 amps up 50 floors in a tall building. Doing this with say copper cable would require probably 30-ish separate conductors and all of the conduit and support structures that would be required. Or, you could use one busbar assembly which can carry the entire 3000 amps in one set of conductors and has built in support structure. Additionally, at each floor passed by the busbar, devices could be clamped to the busbar to draw power from it, and this could be reconfigured much more easily that separate conductors inside conduit. YOu can probably imageine how the same flexible xonfiguration benefit would be useful in a dense data center or manufacturing facility.