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

By | June 18, 2019


Notes for FSP0008 – Power Distribution Inside Buildings
I’m in the US. I’m going to talk about US voltages and use US terminology because it’s what I am most familiar with. The exact details might be different for other parts of the world, but electricity works the same everywhere, so the concepts will apply universally.
Voltage classifications
  • high voltage, medium, and low voltage
    • low voltage is anything up to 1000V (IEEE says 600V, NEC says 1000V) – used in buildings to power devices and appliances. From our point of view inside the building, 1000V certainly seems like high voltage, but it’s at the low end from the point if view of the power distribution industry
    • medium voltage 1kVAC –  ~60kVAC (exact values vary) to describe the voltage used in regional distribution (that’s the part of the power distribution system that branches throughout populated areas
    • high voltage is anything above medium voltage 1000VAC or 1500VDC – used in long-range power transmission and distribution
    • High voltage is used for transmission and distribution because higher voltage requires less current to deliver the same amount of power (power=voltage times current). Higher current causes more energy loss in the transmission line due to heating.
    • Low voltage is used inside buildings because it is safer and easier to manage at the scale of appliances and human interaction.
  • In most places, utilities transmit and distribute 3-phase AC. (I’ll cover the details of 3-phase power, probably on a later episode…unless you are listening from a future where I already did it, in which case I already I already covered this in that #such and such.).
  • Electrical power is generally distributed as AC – 60Hz north america, 50 Hz Europe.
  • Need to understand what a transformer is.
  • A transformer for our purposes here is a device that we use to convert from one voltage to another.
  • A transformer uses inductance to accomplish the conversion. What does that mean?
    • When a charged particle travels through a wire (which is another way of saying that electrical current flows through a wire) it creates a magnetic field around the wire. This magnetic field can act on things near the wire and the strength of the magnetic field is proportional to the current flowing in the wire. A magnetic field that changes strength over time will create a voltage (which will then cause a current to flow).
    • We happen to distribute power using alternating current, which means that the current constantly changes directions so that it’s flowing first one way, then the other way. This means that AC, by creating a constantly changing magnetic field will “induce” a current (an AC) in a nearby wire (Assuming that nearby wire is part of a circuit in which current can flow).
    • A transformer is made up of 2 coils of wire (which aren’t touching each other). A coil is just what it sounds like, wire wrapped around and around and around. The wire is wrapped around a core that has a high magnetic permeability (which just means that it transmits the effects of a magnetic field very well). The 2 coils and the core are are arranged in relationship to each other to maximize the effects of the inductance. I hope you’re still following me here.
    • Connect one voltage to one coil of the transformer When AC flows in that coil, it induces AC current in the other coil. When the other coil is connected to a circuit, the induced current proportional to the current flowing in the first coil will flow through the circuit.
    • The amount of induced voltage is proportional to the number of turns or windings in the coil. You can have a certain number of turns in one coil and  a different number of turns in the other coil you’ll get a different voltage in the second coil. The voltage is proportional to the number of turns so you can calculate the voltage you’ll get on the second coil given the input voltage and the number of turns in each coil.
    • Magnetic fields interact with each other so that, when you connect a circuit to the output coil, the amount of current that flows in the input coil is exactly the right amount to induce the current necessary in the circuit connected to the output coil. Magic!!
  • (terminology) The primary side of the transformer is the part where we connect the input voltage, or the voltage we are converting from.
  • (terminology) The secondary of the transformer is the part where we connect to get the output voltage, or the voltage we are converting to.
Entry into Building/Premises
  • Premises is connected to medium or low voltage utility line depending on the power requirement of the site and proximity to other buildings. Almost always low voltage, and only medium voltage for facilities with very high power requirements.
  • Typical in dense residential area for a low voltage feed to serve multiple lots.
  • In a more sparsely populated area or in a commercial or industrial area, it is typical for the utility to bring a medium voltage line to the premises and convert to the proper (usually low) voltage with a transformer onsite or very close. Most often the transformer is owned by the utility, but sometimes can belong to the building.
  • I’m going to describe the typical connections used in the US because that’s what I’m most familiar with.
    • A residential or light commercial building will usually be connected in a split-phase 3-wire configuration. The secondary side of the transformer is 240V with the grounded neutral connected at the center of the secondary winding. The customer gets both conductors from the secondary side of the transformer (so that’s 240VAC between them) and also the neutral which means they can also use 120VAC from the neutral to each of the other lines. Most of the loads (lighting, wall receptacles, etc) use 120VAC. 240 VAC is typically used for higher power loads such as water heater, HVAC, clothes dryers, ovens, large power tools, etc.
    • Commercial or industrial buildings are typically connected with 3-phase systems. In the US, this is typically 120/208 or 277/480 (and rarely some others) (4 wires, 3 lines, 1 for each phase, and a neutral). 120/208 3-phase means that you can get 120VAC between any of the 3 lines and the neutral, you can get 208 V between any 2 of the 3 phase lines (so 3 possibilities) and you get use all 3 lines to get 3-phase 208V. Again,  I don’t want to get deep into 3-phase here, I’ll cover that on a later episode.
  • Electrical service comes to the building either off a pole or underground. Passes through electric meter before entering switch gear. Electric meter is usually located outside but in most areas may be located inside a room that satisfies certain requirements.  The general requirements for electric meter installation location is that it must be easily accessible by both the electric utility service personnel and the utility customer (the property occupant, bill payer). Meter also has to be installed in such a way to allow space and conditions for safe work. Meter, service entrance
After passing through the electric meter, the electric service will enter the building and land on a some sort of distribution device.
Simple case, small building, 120 volts.
  • In the simplest case of a small building fed with split phase 120/240 or 3-phase 120/208 this might be a single panelboard (breaker panel or electrical panel) filled with circuit breakers that serve the branch circuits in the building. This is what you would commonly find in a small office or storage building or a single-family home.
  • A branch circuit is a circuit that provides power the devices in the building, such as light fixtures, plug loads, appliances, etc. The circuit breakers for branch circuits are sized appropriately for the expected load. Typical lighting or wall receptacle circuits are 15 or 20 amps. Circuits dedicated to appliances are sized appropriately for the appliance.
  • These electrical panels are laid out so that adjacent breakers are on different phases
    • 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.
  • In addition to the circuit breakers that serve the branch circuits, the panel will include a main circuit breaker that will allow disconnection of the entire panel from the electric service. This breaker will be sized to serve the entire load of all the branch circuits served by that panel, possible up to several hundred amps.
  • If more than one panel is necessary, it is common to install additional panels near the first and feed them from from circuit breakers in the first panel.
Larger building, multiple electrical rooms.
  • In larger buildings with multiple electrical rooms or electrical closets, the utility service might first connect at a distribution panel intended just to feed other panels.
  • This is often called a switchboard or sometimes switch gear (though the term switch gear is generally reserved for higher voltage distribution).
  • There might be a feed from this “main panel” to each electrical room in the building. The connections from the “main panel” to other panels are called feeders (as opposed to branch circuits).
  • The connections coming off the main distribution panel will be protected by an appropriately-sized circuit breaker, potentially (but not necessarily) eliminating the need for a main circuit breaker in the other panel.
Buildings served with higher than the standard plug-load voltage (240/120 or 208-120)
  • In the US, this usually 480/277V or sometimes 600V service
  • This is for larger buildings or buildings with higher power requirements.
  • A building with 480/277 will generally use the 480/277 for high power loads such as air conditioning, heating, pumping. It’s also common to use the higher voltage for lighting because, with 277V you can power about twice as many light fixtures with the same # of circuits and wire and etc.
  • OF course they’ll still need the 208/120 for “regular” office equipment, so a building like this will have transformers that step the voltage down to the “regular” line voltage in order to make the 120V available.
  • In a typical facility:
    • 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.
  • At each step of the distribution inside the facility there will be an appropriately sized over-current device (or circuit breaker). So, the main switch gear will feed each electrical room with a circuit breaker sized to accommodate every load served from that electrical room. The switchboard in each electrical room will feed the step-down transformer with a breaker sized to accommodate every load served by the transformer. The switchboard fed by the transformer will supply every panel connected to it with a breaker sized appropriately for the loads at each panel, and finally each branch circuit in each panel will be supplied through a circuit breaker appropriately sized for the load on that circuit. This means that a fault at any stage in the distribution network will (ideally) trip the nearest upstream breaker without taking down the whole building and also prevent creating a dangerous situation on an inadequately protected connection.
Buildings served with medium voltage (10-20kV)
  • Some facilities use a lot more power than can be reasonably provided at 480V or 600V. These buildings can be fed by the utility directly from the medium voltage distribution lines. This will be in the neighborhood of 10-60kV.
  • High-rise buildings, large campuses, industrial facilities.
  • These facilities will have a substation (or multiple substations) on premises to convert to usable voltages and distribute it throughout the facility.
  • In some cases the facility will use the medium voltage directly. This is true in the case of some industrial facilities or large pumping stations or the like.
  • In some cases the medium voltage will be distributed to substations at key points in the facility and then converted to the 480/277 and 208/120 in the typical configuration and used for the typical loads i described before. This is especially true for a large campus or a skyscraper.
  • In other cases, there might be a single substation that converts to the facility’s usable voltage and then all voltage will be distributed at the low utilization voltage (480 or 600 or whatever). This would be the case for some high-rise buildings or for buildings with a very high density of low-voltage equipment such as a large manufacturing facility or very large data center (and remember I’m using “low-voltage” here to describe anything under about 1000V)
Current-carrying conductors
  • Talked a lot about voltages and transformers and switchboards and stuff like that.
  • Haven;t really talked about the actually conductors that carry the electricity across the facilities and to the devices and appliances.
  • I’m talking about wires here
    • 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)
  • Busbar trunking systems
    • 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.
Why different voltages to buildings, what dictates choice? 2-main things.
  • The first reason might be the most obvious…you’ll want to get the voltage required by the highest demand device to be used at the facility. For example, a steel foundry might have an arc furnace that requires 50,000V. So we might as well feed that facility with medium voltage and step down from there.
  • The main reason, though has to do with ampacity.