Subpanel

Reply

  #1  
Old 04-10-11, 08:28 AM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Subpanel

I have been researching this for a while now and could use some advice.

I have 200A service and I calculate my main panel load as 98A. Main panel has 8 open 1" slots (half breakers not an option).

I want to add the following circuits:

- Attached Garage
- Air Compressor (16.0 x 240 = 3840 VA)
- Heater (20.9 x 240 = 5016 VA)
- Central Vac (14.0 x 240 = 3360 VA)
- 20A GFCI Outlets (16.0 x 120 = 1920 VA)

- Basement
- Table Saw (14.0 x 240 = 3360 VA)
- 20A GFCI Outlets (16.0 x 120 = 1920 VA)
- 20A GFCI Outlets (16.0 x 120 = 1920 VA)

The total demand is 21,336 VA. Multiply by 1.25 = 26,670 VA. Divide by 230 = 116A.

My initial thought was to add a 100A sub but then realized my demand was too high.

I see two reasonable options:

1. Install a 125A sub - Is this really an option? It would appear that 125A breakers are available for my Cutler Hammer Main (BR2125)? Is this common practice?

2. Install a 100A sub for everything except run 2 of the 3 20A outlet circuits from the main. This would tie up 2 for the sub and another 2 for the outlets, leaving me 4 open slots in the main. Was hoping to leave more open for future use. Right now I am leaning this way.

Am I missing something? What would you do? Thank you in advance for your help.
 
Sponsored Links
  #2  
Old 04-10-11, 08:37 AM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 12,277
Since you can get a 125 amp breaker for your main panel that is a good option. It is done all the time.
 
  #3  
Old 04-10-11, 10:00 AM
Justin Smith's Avatar
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
I would put garage sub in main panel.
 
  #4  
Old 04-10-11, 10:01 AM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Thank you for the reply Tolyn Ironhand.

I believe I could use a main lug panel but I think it would be good to have a breaker in the subpanel as well. What options are there for this? Could I purchase a kit with a 150A main breaker (to use as my sub) and put a 125A in my main? I can't tell if the neutral and ground can be separated or not but that may kill this idea. Would I then have to size the feeder for 150A?

Maybe I'll just go with the main lug...
 

Last edited by wrbeyer; 04-10-11 at 10:25 AM.
  #5  
Old 04-10-11, 10:45 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
Since this is all in the same building there is no need (nor reason in my opinion) to use a main breaker sub-panel. Nor is your assumption of a 116 ampere load correct as it is extremely unlikely that you would ever fully load all the circuits you have detailed.

I have two 125 ampere MLO panels on my Service panel, one fed through a transfer switch that has all my critical loads that I want to be usable from my generator and the other sub-panel is just for expansion purposes as I have a "short" service panel. The first sub is fed from a 60 ampere circuit breaker and the second from an 80 ampere circuit breaker.

If it is a significant distance from the service panel to either the garage or basement you may want to install the sub-panel in that area rather than adjacent to the service panel. Installing two sub-panels is also an option. I used the Siemens 125 ampere, 8/16 circuit MLO panels as they are relatively inexpensive.
 
  #6  
Old 04-10-11, 11:03 AM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Thank you Furd. I completely agree that my assumption of 116A load is high. I am just trying to do it per code. I was not aware that I could decrease the load based on my own judgement. Any opinions on this?

The more I think about this though... another option is floating to the top. Install a 100A subpanel for everything above EXCEPT the central vac. This will allow me to shut power off to everything in the garage and basement shop at once (kid proof). This will eat up 4 of the open slots but I think this is a good option. The subpanel load (very conservative as Furd pointed out) is 98A.
 
  #7  
Old 04-10-11, 11:34 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
Sure, you could do that but I would then advise getting a main breaker panel and a lock for the panel door. You would need to leave certain lights on the service panel for safety in the garage and shop areas and if you park your car in the garage then you would want the door opener (if installed) on a circuit from the service panel. But placing all the power tool receptacles (or hard-wired) on a separate panel where you can cut all power might be a very good idea if your areas are subject to inquisitive children.

The sub-panel(s) do need a four-wire (hot, hot, neutral, equipment ground) feeder from the service panel and the neutral MUST be isolated from the enclosure (remove or don't install the "bonding" screw or strap) and a separate equipment ground bus will need to be purchased and installed in the sup-panel enclosure.

The central vacuum needs a dedicated 20 ampere 120 volt circuit.


De-rating from the installed circuit breaker maximum capacity is done all the time, it is called a "demand load calculation". Add up everything that might be operating at the same time and that will be the maximum load. It could be the air compressor (auto start), the heater, the table saw and the vacuum cleaner for a total of less than 12kW. Since a 100 ampere sub has a capacity of 24kW you can see that you haven't even surpassed 50% of the sub-panel capacity.
 
  #8  
Old 04-10-11, 11:53 AM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Thank you.

I prefer the main breaker any way. Lights and door openers are on separate circuit for reason you mentioned.

Actually the central vac is 20A 240V. Very large unit with two motors.

I believe the central vac, heater, compressor, table saw and dust collector (15A) is worst possible scenario. Unlikely but possible. Wife uses vac inside, heater is on, compressor kicks on (auto start) and I'm using the table saw and dust collector. This calculates to 95A. This uses the 1.25 factor - which I thought was required. Is the 1.25 factor required?
 
  #9  
Old 04-10-11, 12:12 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
First, reduce everything to a kW load, do NOT go by the size of the circuit breaker but use the actual kW rating from the nameplate. If it has only a amperage at X volts then do the multiplication and add 20% IF it is a primarily motor device. Add up the entire load this way and see what you get.

Then, calculate the worst-case scenario when everything might be in operation at the same time. This is the maximum demand. You do NOT need to multiply by 125% unless this load will be in continuous service for more than three hours. I'm going to state that your entire load will be maybe in the neighborhood of 15-16 kW. Yes, I am ignoring the 120 volt loads right now and you need to "balance" those 120 volt loads (as much as possible) across the two "hot" busses. You didn't state the power requirements of the dust collector so I will assume it is also 240 volts at maybe 10 amperes. How often will you be using any significant 120 volt loads while also using the table saw, dust collector and other items?
 
  #10  
Old 04-10-11, 01:11 PM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Everything in my first post is in W load except for the 120V outlets. The load is directly from the motor nameplates. I did not add 20% for primary motor devices (not sure what that is).

I do have some room then because I did multiply by 125%. It is very unlikely that anything would be on for more than 3 hours.

Dust collector is 120 volt and I was planning on it being covered by one of the 120 volt outlets. It is 15A. I would only use the dust collector with the table saw - no other 120 volt devices with the table saw. The other option INSTEAD of the table saw is that I run the dust collector with another 120 volt device (planer) that is also 15A. These would be on two separate circuits.
 
  #11  
Old 04-10-11, 01:46 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
What I mean by a primarily motor device is one where the electric motor is the major user of electricity. For example, a fan-forced heater might have a 5 kW heating element and a 1/20 horsepower motor. Clearly the heating element is the primary load so I just ignore the motor. On the other hand, a table saw might have 1-1/2 motor (mine does) and no other use of electricity so I take the watts of the motor and multiply by 20% to allow for the inefficiency of electric motors and also the starting surge. Since an electrical horsepower is equal to 746 watts if the motor is rated in kW instead of horsepower (many imported motors and tools) the actual electrical draw is greater than the kW rating would at first lead you to believe. As long as you are using the amperes and voltage to determine the kW draw you will be sufficiently accurate without the additional 20%.

Unless you need to have the planer and dust collector portable I would strongly suggest that (if possible) they be re-connected to use 240 volts. The more items that use 240 the easier it is to balance the load (240 volt loads are inherently balanced) and the less problem you will have with voltage drop and overloading one leg of the service.

Bottom line, I think that a 100 ampere feeder to a 100 or 125 ampere panel will be more than adequate for your uses. Since you have several 240 volt loads you might want to get a longer (more breaker spaces) panel than the 8/16 I previously suggested.
 
  #12  
Old 04-10-11, 01:56 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
You may run into limitations on the amount of load that can connect to the buss. You should be able to find out on the panel label.
 
  #13  
Old 04-10-11, 02:08 PM
Member
Thread Starter
Join Date: Feb 2006
Location: USA
Posts: 26
Thank you very much for the help.
 
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