Ncarb exam guide #17 electrical distribution

Mechanical & Electrical Plan Vignette and Multiple Choice

Ncarb exam guide #17 electrical distribution

Postby detail4 » Tue Feb 18, 2014 7:56 pm

I was reviewing the exam guide at http://www.ncarb.org/en/ARE/~/media/Files/PDF/ARE-Exam-Guides/BS_Exam_Guide.pdf and Came across #17

The question is: A developer wants to construct a 15 story office bldg with approximately 10,000 SF of gross floor area per floor. Which of the following electrical distribution systems would be best to meet the developer's requirements and be most economical in terms of initial cost as well as later operational costs?

A. 120/208v, 3 phase, 4 wire
B. 120/240v, single phase, 3 wire
C. 120/240v, 3 Phase, 3 wire
D. 277/480v, 3 Phase, 4 wire

The answer is D.... Can anyone explain why?

Is it because feeder sizes are smaller?
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Re: Ncarb exam guide #17 electrical distribution

Postby detail4 » Tue Feb 18, 2014 8:04 pm

I saw the link at http://arecoach.com/coachforum/viewtopic.php?f=9&t=3842&p=11642&hilit=Constant+volume+reheat#p11642 but I was hoping for a more thorough explanation.
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Re: Ncarb exam guide #17 electrical distribution

Postby SoonArch » Tue Feb 18, 2014 9:44 pm

I had the same question. I thiiiiink :ugeek: that we want high voltage in commercial buildings, so conductors and conduits are smaller. Smaller conduits mean less expense, as well as easier installation.

Would love more thoughts on this one though !
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Re: Ncarb exam guide #17 electrical distribution

Postby detail4 » Tue Feb 18, 2014 10:25 pm

SoonArch wrote:I had the same question. I thiiiiink :ugeek: that we want high voltage in commercial buildings, so conductors and conduits are smaller. Smaller conduits mean less expense, as well as easier installation.

Would love more thoughts on this one though !


Exactly! I found in MEEB 9th edition page 1005 the comparison of different service distributions. It suggests this type of service is ideal for large multi-story buildings to reduce costs by smaller feeders, smaller conduit sizes, and smaller switch gears. This comparison also shows up in the Alikat Notes. Higher voltage power= smaller wire size
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Re: Ncarb exam guide #17 electrical distribution

Postby EndlessUrbia » Wed Feb 19, 2014 8:28 am

Additionally, here is what one of my flashcard says:

120/240 v single phase : This three-wire system is most commonly used in residences and small office buildings. It is used where the actual load is not more than 80 amps even though minimum service is considered 100 amps.

120/209 v three phase, four-wire : Used for small and medium sized commercial buildings. It differs from 120/240 v single phase in that it allows for various electrical loads.

277/480 v three phase, four-wire : Used for large commercial buildings.

2400/4160 v three phase, four-wire : Used for very large commercial and industrial buildings with a lot of machinery.
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Re: Ncarb exam guide #17 electrical distribution

Postby mimi » Wed Feb 19, 2014 8:36 am

re: correct answer D. 277/480v, 3 Phase, 4 wire
This answer is explained in the AMBER book, page 81 as follows:
" 277/480 volt 3-phase, four-wire power service is specified for large buildings with heavy lighting and equipment loads. 277-volt lighting is installed and small transformers are used to step down power to 120-volts for process loads, like DVD players."

On the archiflash cards, the information is as follows:
277/480 v 3 phase, 4 wire system is used for large commercial buildings.
2400/4160 v 3 phase, 4 wire system is used for very large buildings with a lot of machinery.
120/208 v 3 phase 4-wire system is used for small and medium sized commercial buildings-it allows for various electrical loads.
120/240 v single phase system is most commonly used in residences and small office buildings, where the load is approx 100 amps or less.
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Re: Ncarb exam guide #17 electrical distribution

Postby tammy2329 » Sat Feb 24, 2018 1:15 pm

Two part question:

Is single phase used only for single family homes? What if you have multi-family dwellings?

Also, would 120/208 three phase, four wire be most appropriate for a multi-use facility? I believe this services varying loads.
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