texas calculations

[lenny44]

So I took the Texas state electrical Masters exam and these are questions that were on there. I want to check my work and see what i got wrong. i currently hold 2 states for electrical and i have 5 other trades in Florida. I have passed all my exams on the first try and this on part of the exam is giving me a problem. can someone answer these so i can check my work and see where i went wrong. Thanks in advance!!


These are some of the questions on the test.
You have 25,375 sq ft house. The service is 120/240 volt single phase. What is the minimum 15 amp 120 volt branch circuits?
You have a 535 sq ft house with 1 electric close dryer. What is the minimum size neutral conductor?
You have an office building with 50kw of lighting that operate at 15 min intervals. The service is 120/208. What is the feeder size?
You have 3 - 208volt 3 phase motors sized at 10hp, 5hp and 25hp. What size tapped conductor do you need?
You have a service with 4 100 amp rated disconnects. You have 2 disconnects at 83amps, 1 Disconnect at 92.5 amps and the last one at 45 amps. What size conductors and over current device for your feeders?
You have a garbage disposal and a dish washer connected to 1 20 amp 120 volt circuit what is the max amperage for the Dish washer?
You have 10 electric cars rated at 30 amps a piece at 120/240 volt. What size conductors do you need for the feeders?
You have a air conditioning rated at 65 amps. What is the size molded case switch
You have 9#12 thhn and 5 # 3 thhn. What size liquid flexible conduit do you need?
You have 21 dryers at 4.5kw what is the calculated power?

 

[Dennis Alwon]

For the first one you use 3 VA/sq.ft

So 25,375 (one heck of a house) x 3 = 76,125VA

For a 15 amp cir you would have-- 15 x 120 = 1800va per circuit

76125/1800= 42.29 or 43 - 15 amp circuits

 

[lenny44]

thanks!

 

[Dennis Alwon]

For the second one you have to calculated the dwelling load but the dryer neutral is rated differently. Look at 220.61(B)

 

[lenny44]

yea i read that by im thinking its not enough info so i would say a #8

 

[Dennis Alwon]

Given the small amount of load this service would be less than 100 amps but 100 amps is the minimum for a residence. The neutral for a 100 amp service may calculate to be small but it cannot be smaller than a number 8 which is the size required in 250.66 for the grounding electrode conductor

The calculation using standard method comes to 31 amps on the neutral so it would still require a number 8 neutral. I am assuming a #$ or even a number 2 service conductor