4. MEP Training
Power
Examples of Questions we will answer:
1. What is AIC?
2. If 3-phase is better, why not 4 or 5?
3. Do all tenants have a transformer? Why or
why not?
September, 2007
5. MEP Training
Power
Electricity is created
by:
Nature - lightning
Battery - chemical
Generatormechanical
Creates alternating
current as it rotates
Why three phases,
why not four?
Motor in reverse September, 2007
6. MEP Training
Power
Generation:
Heat > steam > turbine > generator
Voltage boosted for distance
Substations step down to 7,200-15,000v
On-site transformer steps down to 480 or 208v
September, 2007
8. MEP Training
Power
Grounding and bonding:
Ground is a connection to the earth that only
carries current when there is a short (green wire)
Bond is connection between components to the
ground
Grounding is required:
At service initiation
At low voltage side of transformer
Initiation of new voltage (480v to 208v)
Grounds need to be where inspectors can find
them
September, 2007
10. MEP Training
Power
Fault Current:
How much current will flow when a fault (short)
occurs?
What limits the current?
Common panel/breaker rating is 10,000 amps
How do you know?
Utility company transformer
available fault current
Fuses in landlord or tenants main
September, 2007
13. MEP Training
Power
Types of Landlord distribution equipment:
Utility metered
Wireway
Switchgear
Meter Center
Landlord redistribution, no meters
Distribution Panelboard / Switchboard
Buss duct
September, 2007
20. MEP Training
Power
Load Center
Residential
Not bolt-on breakers
Breakers not switch duty rated
Panelboards
Grouping of 20A breakers
Distribution Panelboards
Larger breakers, 60A – 400A
Switchboards
Distribution panelboard with fused switches
Switchgear
Large utility rated equipment
September, 2007
22. MEP Training
Power
Ground fault:
Hot conductor connected to
ground
Create arc/spark
Caution: intermittent fault will
not trip breaker
Ground fault interrupters
September, 2007
23. MEP Training
Power
Common voltages:
Single phase:
120v – household outlet
Three phase:
240v = 120v x 2 wires for larger load (dryer)
208v/120v/3ph/4w
480v/277v/3ph/4w
What’s the last number?
Three wires are power
Four wire – neutral
Ground isn’t counted, it’s 2007
assumed
September,
24. MEP Training
Power
Neutral
Another path to ground (white
wire)
One hot (black) wire plus
neutral = single phase
277v or 120v
Why not 277v x 3 or 120v x 3
for three phase?
Power triangle, really is
geometry.
September, 2007
25. MEP Training
Power
Load calculation:
NEC rules
No consideration for diversity permitted.
Actual load rarely exceeds 65% of service size
Actual rating of equipment/lighting is required
Equipment nameplates
Lighting ballast inefficiencies must be considered
Continuous loads (operating for three + hours)
Rated load x 1.25
Lights
What about AC and heat?
Track lighting considered 75 watts per foot for service size,
but not branching size
September, 2007
27. MEP Training
Power
Wire size:
Sized like pipe (velocity, diameter and pressure)
Continuous loads de-rate wire capacity
Heat
Defined as 3+ hours (lights)
Multiple conductors de-rate wire capacity
Heat (more wires = more heat)
Voltage drops
Wire acts as resistor based on
length of wire
Long distance with continuous
load = larger diameter
September, 2007
29. MEP Training
Power
Wire size / Voltage Drop:
How long?
Wire can run as far as voltage (ex. 480v = 480 feet)
Single phase in out and back
Three phase is out only
Don’t ask why
Conduit sizing
Never consume more than 40% of cross-sectional area
Heat
Pulling wire through conduit
40%
Max
September, 2007
30. MEP Training
Power
Electrical formulas:
Energy = power for a period of time
Energy = usage, watts x time
Electric meters use this measurement
Units are watt hours
Power = voltage x amperage
Similar to auto horsepower
Power = demand, kilowatts (1 kw = one thousand watts)
Voltage = the pressure (pushing) on the current
Amperage = current, the flow rate
September, 2007
31. MEP Training
Power
Electrical formulas continued
Examples of calculations
Single Phase: 2.4 kw = 120v x 20 amperes
What is 115v or 110v?
Three phase: 16.6 kw = 480v x 20 amperes x 1.73
1.73 is the square root of three (for three phase)
September, 2007
32. MEP Training
Power
Rules of thumb:
20 watts per square foot for retail tenants with
their own rooftop units.
15 watts per square foot for retail tenants with
landlord central air conditioning.
40 watts per square foot for sit-down restaurants.
100 watts per square foot for food court type
tenants.
September, 2007
33. MEP Training
Power
1. What is AIC?
2. If 3-phase is better, why not 4 or 5?
3. Do all tenants have a transformer? Why or
why not?
September, 2007
Editor's Notes
We will answer all these questions in this presentation.
The questions will be repeated at the end.
Why 3 phases?
Structural loads are balanced on the generator shaft with 3 phases.
Balances the output of energy.
More than three is simply the law of diminishing returns.
Therefore a motor is as close to a “perfect” electric user as it gets.
Distribution voltage is 155,000 to 760,000 volts.
Transmission distances are up to 300 miles.
The top wire between poles is usually the ground wire.
Poles themselves are grounded at the base.
Short circuit is an unplanned connection between any of the hot wires and a grounded metal part.
Service initiation is also the end point of utility responsibility. “Demarkation”
Do not ground to water pipe. Water pipes snake all over a mall with only one connection to ground. Dangerous. Building steel is carefully grounded at many columns and even has wires connecting across expansion joints to ensure good ground.
Notice:
No wire connection from low to high voltage.
Ground.
Fewer “twists” on low voltage side.
Intro:
Let’s go back to ground. Earth has an infinite ability to absorb electricity and when given a chance, will do just that, attempt to absorb infinite energy.
How much current will flow when a fault or short occurs?
Can far exceeds breaker rating
30,000 – 65,000 common in malls
Each component must be selected for correct fault current or Ampere Interrupting Capacity (AIC)
Problem: Our systems can not deal with “infinite loads” and remember the breaker is designed for continuous load and allow higher loads for short periods.
What limits the current?
Resistance of wire and components
Transformers
Fuses (one of the best devices)
Panels may literally explode if not designed (rated) properly.
Metering is the key first off.
If redistribution, owner can not meter unless they are a “utility” registered with the state’s Utility Commission.
Cost is also a key. Order is by increasing cost to owner (not necessarily tenant)
Owner/Landlord installs trough if anything. Often trough is by the utility company.
Wireway is most basic of group distribution systems, multiple individual disconnect switches.
Wireways take a lot of space and are tough to coordinate space.
Often outdoors due the their size.
Simply wires laid in a trough.
Notice some have switches, some do not.
Switch before/after meter.
Often these were installed by the utility company and therefore not subject to NEC rules. Today NEC rules most of these installations. NEC limits the number of switches to make a service safe (shut off all power) to six (6).
Photo on left has six switches. One on right has ten meters and fewer switches. This one often needs to be upgraded if touched. Ok if left alone, but no new services or upsized services.
Big, solid units. Still less space and more organized than wireways.
Outdoor unit has meters inside doors.
Specialized form of switchgear
208V and 1600A maximum
Space saving
Note no meters! Redistribution of power by landlord.
Small space. 13 services in space of 3 or 4 on a wireway.
All owner purchased equipment.
Flexible. Easy to add a service, just plug in one more, or provide spares as in this photo.
Again, no meters. Redistribution here too.
Essentially no floor space is used. This one is overhead in service corridor.
Much like a wireway, except there is copper bars in the center section. Switches are the boxes on left and right.
Not rated for instantaneous, short overcurrents. Those happen often at startup and the breaker is designed to ignore them. Motors for instance can draw much more than rated amperage at startup, but we don’t want the breakers to trip.
A special condition of short circuit. Not any different, just defined for engineers.
Need ground fault interrupted rated breakers to trip on short duration overcurrents.
Equipment is often rated at 208v and 240v
Diversity for cooking equipment. Outlets in large spaces.
Services are “oversized” for safety factor when actual loads can not be measured. Who can be sure of diversity?
After the fact, NEC allows us to measure load on landlord main service to prove we can add more load.
Amperage is velocity. Diameter is wire size. Voltage is pressure.
Wire types, really speaking about the insulation. More H’s = higher rating. N = Nylon, slippery. W = Wet locations.
Ampere Interrupting Capacity. The rating of tenant panels, consistent with utility transformer rating.
