Size of subpanel

Reply

  #1  
Old 08-07-18, 07:14 PM
Member
Thread Starter
Join Date: May 2015
Location: u.s.
Posts: 85
Received 0 Votes on 0 Posts
Size of subpanel

I have a 150 amp main electrical panel. I want to add a subpanel. How do I determine what amps I can put in a subpanel?
 
Sponsored Links
  #2  
Old 08-07-18, 07:40 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,909
Received 24 Votes on 20 Posts
The bus stab rating will determine the maximum size breaker that can be installed. Have you calculated the load the subpanel needs.to.serve?
 
  #3  
Old 08-07-18, 07:51 PM
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 4,644
Received 19 Votes on 18 Posts
Usually you're better off keeping the large load items in your main panel, then installing a reasonably small subpanel for the smaller loads. For example, if you keep your water heater, AC, range, etc. in your main panel, maybe all you'll need is a 60A feed to your subpanel for lighting, receptacles, etc.

Of course, that's not a hard rule, just a suggestion.
 
  #4  
Old 08-07-18, 09:13 PM
Member
Thread Starter
Join Date: May 2015
Location: u.s.
Posts: 85
Received 0 Votes on 0 Posts
Zorfdt and pcboss, all cable except two enter my main electrical panel from behind the panel. I have two cables that were added for outside use (30 amp double pole for A/C unit and 1 20 amp for water garden pump) I intended to move these two over to the subpanel because it would be the easy thing to do. I intended to use those empty spaces to put in breakers for the subpanel. The reason I need to add a subpanel is to supply electricity to three 1500 watt electric heaters in my basement. If I'm figuring this correctly I will have the 30 amp double pole breaker for the a/c and three 20 amp double pole breakers for heaters in the subpanel. So, what would be needed to supply the subpanel in that case? Considering that the a/c and the heaters will not be in use at the same time should that be considered? Do I have to take a close look at what my main panel is drawing before I can know what I'm able to add?
 
  #5  
Old 08-08-18, 10:05 AM
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 4,644
Received 19 Votes on 18 Posts
Sounds like you're good. You're not adding any huge loads, really just circuits. You have a good plan in place.

The heaters probably don't need three 20A circuits. If they are 1500w @ 240v, you're at 6A a piece. You can use 2 15A circuits. One circuit will have 2 heaters on it for 12A (within the 80% continuous rule), and the other would have 1 heater for 6A.

Your load on your subpanel will be roughly 20A @ 240v AC; 18A @ 240v heaters; 2A @ 120v garden pump (depending on size), so you'll have lots of headroom leftover for a 60A subpanel.
 
  #6  
Old 09-04-18, 10:13 PM
Member
Thread Starter
Join Date: May 2015
Location: u.s.
Posts: 85
Received 0 Votes on 0 Posts
Zorfdt, I'm close to beginning my project. One last question. I'm buying a 240V 1900watt and a 240V 700 Watt heater. Can these two heaters go on one 20 amp breaker? The Amps listed are around 8 and 3 so the answer seems obvious to me. However, I want to double check for the sake of reassurance.
 
  #7  
Old 09-04-18, 10:37 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 52,594
Received 337 Votes on 315 Posts
Just so you know for the future.....
To determine amps..... 1900 watts / 240v = 8 amps and 700 watts / 240v = 3 amps

Since heating is considered a continuous load...... you can connect up to an 80% load to the circuit.
If the circuit is 20A....... 80% is 16A max.
If the circuit is 15A...... 80% is 12A max.

So your load is fine on a 20A circuit...... with room to spare.
 
  #8  
Old 09-05-18, 04:35 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 3,502
Received 15 Votes on 15 Posts
" Do I have to take a close look at what my main panel is drawing before I can know what I'm able to add? "

Yes. You might want to do a load analysis. Sample calculation at the back of the NEC code book. This gives the expected total wattage or amperes for the whole house.

This is in addition to, as mentioned above, the maximum amperage for any one branch circuit breaker inserted into a panel (stab rating)..
 

Last edited by AllanJ; 09-05-18 at 04:50 AM.
  #9  
Old 09-05-18, 08:23 AM
Member
Thread Starter
Join Date: May 2015
Location: u.s.
Posts: 85
Received 0 Votes on 0 Posts
A big thanks to everyone who helped with this.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: