EE's and EE students, help with a personal problem

[javier]

Hello fellow Electrical Engineers and Electrical Engineering students,
I’m in middle term Electrical Engineering so my experience so far can’t help me really, this is a problem I’m having in the house that my wife and I bought this year, it’s not a big house so the load is not that big, let me tell you so you can get a better idea, it has 2 bedrooms, 1 living room, 1 bathroom and the kitchen feed by 120V line the problem here is that in the backyard we have an additional building which is kind of a small apartment so it has an independent 120 V line (without contract with the electric company) for 1 bedroom, kitchen and bathroom, we have no intentions into renting that apartment or anything so I want to connect the electricity from the main house to that building in the back, I believe it is worth mentioning that this semester I took a class that was about DC so I’m not familiar with AC yet, I would appreciate if you want to help me with some advice on how to connect, I already searched for the main switch boxes for each building so my first idea was to connect the cables from the main box to the secondary box (I haven’t performed any action yet) in order to control everything from a single main switch box, maybe I can use a Theorem so if you know about one I would appreciate your help.
I include a fast sketch I made in Paint to show you what’s my first idea to solve my problem.
Have a good day and thanks in advance.

[furqan]

Im no electrician but you should probably contact one as electricity shouldnt be played with. And since you are still a student you probably aren't qualified to do it. If a problem occurs you might even get in trouble woth the authorities

[Farit]

Hi Javier,

Regarding to your problem,let me tell you that what you thought is the best way to solve it. Your case is like a building that have a second floor, second's floor usually have a second electrical box, but this box is linked to the main box, all the charge of the second floor should be calculated and a new switch that fulfill that charge amount must be placed on the main box. For example, if your second floor has a demand of 40A, you must place a switch of 40 to 50A in the main box.Do not forgot thath based on the demand of current you must find the correct wire gauge.

[javier]

Thank you very much, you have been very helpful, I will start gathering all the data to determine the gauge of the cables and to figure out if the Switch that I currently have will help me with the load addition, furgan i'm not playing with electricity that's why i'm asking for advice, also the fact that I'm studying Electrical Engineering shows that this is not a game, and regarding to problems with the authorities that's not going to happen because I always pay my bills, i will not steal the energy I just want to set it up for my second building. Have a great day or night depending on the side of the globe you are. Regards from Mexico.

[FPEPro]

I was a licensed electrician once upon a time for my fathers firm. The normal way to connect these two buildings is via a sub-panel box. You would connect the sub-panel back to the main distribution box. The main cut-off breaker size for the sub-panel would usually be determined by adding up the "required loads" (i.e. water heater, air-conditioning, major appliances) then calculating the estimated wattage load for the receptacles and lighting (which is often averaged at 3W per square foot of area). So if the small apartment was 500 sqft in size your estimated receptacle/lighting load would be about 1500W. So since, A = W / V, 1500W would be 12.5 A for the receptacle/lighting load for a 500 sqft apartment. But I always round up to at least the nearest unit of 5 for main cutoff breakers. So I would use 15 A in this case for the receptacles. The same method is used again for lighting. Then sum up your "required loads" that could easily be running simultaneously, and add your estimated receptacle/lighting load to that. Now only in regards to receptacles and lighting, it is often a code requirement (and just good practice) to limit them to only 15 A circuits. 20 A and up is usually only utilized for dedicated lines for one piece of equipment. Furthermore, another good practice that is often a code requirement is to avoid loading a breaker to more than 80% of it's actual/estimated load. Therefore, in the case above if I was the electrician, I would split this estimated 12.5 A load between two 15 A breakers, because 12.5 A exceeds 80% of a 15 A breaker. Once you add up all these values, this will be the value of your main-cutoff breaker for the sub-panel, that will be back in your main-panel.

Now, for the individual breakers inside the sub-panel at the apartment (fyi, it does need to be in the apartment). There should be dedicated breakers to each required load. So things like a Cooking Stove, FCU unit, ACU unit, Washer/dryer, hot water heater, hot tub, jacuzzi tub, etc. should all have dedicated power lines and a dedicated breaker for them. Good electrical practice would always place the lighting on a separate breaker from the receptacles. Both the lighting and receptacles are usually estimated as I said before. If the apartment is very small then it's entirely possible that you could be fine with a single 15 A breaker for all receptacles in the apartment, and another single 15 A breaker for all lighting. I would never use under 15 A for either of these two, regardless of the apartment size though. So to check how many you need, take 15 A * 120 V = 1800 W. At 3 W per square foot on average, you get 1800 W / 3 W per sqft = 600 sqft. But as said before, good practice would say to reduce this to 80% of that value (i.e. 480 sqft)  So one 15 A breaker would be technically sufficient for upto a 480 sqft area. But good electrician practices (and often a code requirement) usually dictate that all bathrooms and the kitchen be isolated. And that the bathrooms be GFI protected. So, optimally you would have a 15 A breaker on each of the bathrooms, and the kitchen. Then the remainder of the receptacles/lighting would go on separate breakers per the method I already explained.

You really have to make sure your estimated actual load is not higher than your main cut-off for the sub-panel, or the main cut-off will be tripping constantly. But you do not just sum up all the breakers in the box to get the value of your main cut-off either. This is also unsafe, because doing so could very well exceed the capacity of the sub-panel and/or the cables feeding them (and possibly even your main panel). This is why the estimated load is calculated for determining the cut-off breaker size. But you must always keep in mind the rated capacities of each component when doing all of this.

I do recommend that you get professional help for this project. Being an electrical engineer does NOT qualify you as a electrician, not even if you already had the degree in your hand. They will not teach you anything about the electrical and fire codes associated with residential electrical work. And this is the stuff that could get you in big trouble with your local government potentially.




Chris

[FPEPro]

I made a few edits above. It didn't read right to me. If you have questions, just let me know.



Chris

[javier]

Thank you very much for all that information, it helps a lot!

[javier]

Maybe is not big deal because all I need is to illuminate a small kitchen, bathroom and bedroom, tomorrow when I have day light I will measure the area, I believe there's no more than 3 light bulbs and maybe 4 out sources (to connect stuff into walls), all the instalations look in good conditions, I just need to take the electricity to that building, let me know what you think now that I'm giving more details. I appreciate your professional help. Good night.

[Robinet]

According to the law we have over here you are not qualified to do this, neither should. You're better off contacting a professional electrician. I worked as an electrician before I would always sigh out loud when people had done these kind of jobs on their own without any experience. They usually mess up, and if they wouldn't you usually need documentations on these kind of stuff and permission from the electricity company.
Just to save yourself from trouble call for help, it pays off in the end