Can I put a 100 amp sub panel off a 100 amp panel for an addition?

Question

I plan to add an addition that adds more to the kitchen a living room, master bedroom, master bath, and two other smaller bedrooms. I have 100amp service now and was wondering would it be ok to just to add a 100amp subpanel sense I have 4 open breakers left to do it or should I just change over to a 200amp service? I plan on doing alot of the work myself but I could'nt do the 200amp change over. Help what do you think?

Answer 1

Wow! I am surprised to see another electrician telling you this can be done. You can't just add onto your existing electrical service and hope that you don't overload it. This is bad advice, especially coming from a licensed electrician.

It is possible to add the 100 amp sub-panel. However, if your existing 100 amp panel is nearly full, chances are your 100 amp service is nearly maxed out.

Keep in mind, if you overload your electrical service, you have a fire hazard on your hands and no electricity for a couple of days.

To determine if you can do this or not, you need to do a load calculation. To determine what size electrical service your need, get a copy of the 2005 National Electrical Code and read article 220.

I have an example on my blog that was posted today in part 2 of my series on wiring a new house.

If you need further assistance, please visit:
http://electricalblog.gilchrist-electric.com

Answer 2

Yes you can add a 100 amp sub panel and it is pretty common and if you find for some reason that you are tripping the main breaker then you could also change the main panel and service to a 200 amp down the road but that is very rare unlees you have a very large house with alot of large load drawing appliances.
You will need a 100 amp double pole breaker
# 4 copper wire SER cable or # 2 alumminm which is alot cheaper and most commonly used and make sure you get SER cable and not SEU ( The SER will have a red,black,white,and bare ground wire )
A 100 amp main lug panel ( no main breaker in this panel)
The 100 amp breaker for the sub panel goes in the main panel
And make sure in the sub panel you keep the neutral wires and ground wires on different bars ( you will notice in your main panel that grounds and neutral usually go on the same bars unless your main breaker is at the meter)
In the sub make sure the bar that you use for the grounds is also bonded to the panel itself ( a little piece of metal from bar to metal of panel)
Neutral bar does not bond to panel
Any ? Feel free to email me

Answer 3

Amp your main feeds with everything electrical in the house on at the same time. If the total on each main leg is between 90 and 100 amps you're probably better off going to a 200 amp service. Technically you can add a 100 amp breaker and feed a sub-panel for branch circuits, but the increased load might cause problems in the existing wiring circuits if they're loaded to near max. already. Depending upon the size of the service entrance cable feeding the meter, you might not be able to go up to 200 amps. without up-sizing there as well. Good luck.

Answer 4

Yes, maybe. A 100 amp sub panel is good for up to 100 amps, but you don't need to use it at a 100 amp rating. The current limit is established by the breaker in the service panel for the sub panel. In other words, if you put a 60 amp breaker in your service panel for the wires going to the sub panel, you now have a 60 amp sub panel. Make sure the wires going to the sub panel are properly sized (4 AWG for 60 amps).

However, you've got a significant addition, and 100 amp service may no longer be adequate. Your major loads are air conditioning, electric range/cooktop/oven, electric clothes dryer, swimming pool and/or spa. In addition, you may now have 5-6 bedrooms and perhaps three bathrooms. You're investing a lot in your addition, and it would be a shame for your electric service to be undersized. I recommend that you upgrade to a 200 amp service.