Tom Székely, P.E., LEED AP

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March 13, 2006

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The Shareholder/Unit Owner Board Member Survival Manual, or, Engineering for Dummies – Part 5


Why do people look for a 200 amp electric service when they buy a home? What is a 200 amp electric service? How do you figure out if you need it? Does all this apply to co-ops and condos? What about, say, an office building?


Funny you should ask.


Well, in the last issue we learned about how the flow of electric current is measured in amperes (amps, for short). This time we’ll tackle the related question, “What’s a Watt?”, but first, back to amperes.  


Because it’s called electric current we can paint an immediately understandable picture thinking of the flow of water, where drawing a glass of tap water and a broken water main convey the conceptual difference between turning on a light and a short circuit.


With water, the flow is usually (in the U.S.) measured in gallons per minute.  An ampere of electricity, however, is defined as the flow of an absurdly large number of electrons (6.28 x 1018, or about six and a quarter quintillion) per second.


Voltage is the force which pushes current on its way, while watts are the measure of electric power.  Power is the rate of doing work, and is why a 1200 watt microwave will cook stuff faster than a 700 watt microwave.  In electrical terms, watts are volts times amps, so a 120 volt window air conditioner that draws 7-1/2 amps, uses 7.5 x 120, or 900 watts of power.


So what does all this have to do with how much electric service a place needs? 


Look around you. 


Is every light on?  (This question doesn’t apply if you’re in your office – more later.)  Does every wall outlet have something plugged into it? Is everything that’s plugged in turned on?


Didn’t think so.


Electric Codes speak of something called “demand load” which is what is expected to be on at any given time.   That is, diversity in connected load leads to a peak co-incident demand load, and the electric service is sized for the latter, not the former.


Or, to put it in allegorical terms, a person can only wear one suit at a time.


So, how much is enough?


Electric Codes base their requirements on empirical experience, with the National Electric Code (or NEC) ascribing, as the connected load, so many watts of electric service capacity per square foot of occupied space to account for general lighting and appliances, and then applying a demand factor to the load to calculate the peak co-incident demand.  Before the application of the demand factor, however, the Code requires additional equipment loads and other dedicated loads to be added to this general lighting and appliance load.


As an aside, it should be noted that electric utility companies such as Con Edison use demand factors even more stingy than those prescribed by the NEC, with the result that Con Ed’s cable in the street (or transformer on the pole) is a quarter to half the minimum Code-calculated capacity for electric service within a building.  Utility companies, having more experience than anyone else in what constitutes adequate electric capacity, are exempt from Electric Code requirements, and if they screw up and their cables or transformers catch fire, that fire is not in your kitchen, and  is therefore of no concern to Code authorities.


To get back to how much is enough, the Code specifies 3 watts per gross square foot for residential premises, plus an allowance of 3000 watts for dedicated kitchen countertop appliance circuits, comprised by two dedicated circuits, each not permitted to serve more than two duplex outlets.  Other appliances, such as dishwashers and clothes washers must be provided for via dedicated circuits at 1500 watts per appliance/circuit, each circuit serving only the outlet for the appliance.  Major appliances such as electric clothes dryers are to be provided for at their nameplate rating, which in the case of something like a clothes dryer, is usually in the neighborhood of 6000 watts.


After adding all this stuff up, the Code tells you to figure the first 3000 watts of this connected load at 100% demand, with everything from there up to 120,000 watts at a demand factor of 35%.  Connected loads totaling more than 120,000 watts (think a high rise apartment building) are figured at 25% demand. Electric cooking is treated in a special manner, as is air conditioning, particularly if it’s central air conditioning. Office space is treated similarly, except the ascribed general lighting and receptacle load allowance is figured at 6 watts per square foot instead of 3, and the demand factor, because all the lights on an office floor are generally on at the same time,  is 100%.


What does this mean when the dust settles with regard to the electric service needed for a home?  Well, a 200 amp electric service, when fed from a 120/240 volt pole-mounted transformer provides the home with an electric service adequate to feed a peak demand load of 48,000 watts.  To put it in different terms, if all that electricity were used to provide heat, it would comfortably heat an entire 6 to 12 unit apartment building, and I’m talking about a 75 year-old urban structure here, not a modern building insulated to today’s Energy Code.


In fact, going through the demand calculations show that a 200 amp electric service is adequate to feed a modern home with all the bells and whistles (dishwasher, laundry, central air conditioning, etc.) comprising approximately 6000 square feet.


In urban environments apartment buildings are routinely fed by something called three-phase electric service, roughly equivalent to 1-1/2 pole-mounted transformers worth of the “single” phase power provided to single family homes.  Extend these three-phases to an individual dwelling unit, and a 200 amp service can supply a demand load of 72,000 watts.  We’re now talking about Taj Mahal-like spaces, and while circuit breaker panels mostly come in only two aggregate capacities, 100 or 225 amperes, the only reason one would need one of the latter’s capacity in any but the most outrageously large residences is that the former only has space to connect 24 circuits or so.


The latter has enough physical space for 42 circuit breakers, and while virtually no single family home nor any dwelling unit in a multiple dwelling needs 225 amps worth of panel capacity, segregation of loads for convenience of operation and equipment maintenance not infrequently ends up requiring a panel with more than 24 circuits in it.


200 amps in a dwelling is almost always like a 350 horsepower engine in an urban automobile.


Absolutely unnecessary.

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