First a pannel is rated by the maximum about of current that it can safly carry. And that amount is limited by the main breaker in main pannel and the sup-pannel feeder breaker in main pannel. The sub-pannel does not need a main breaker, but a main pannel with a ground bus bar kit is often used, because they are available.
Now the size of the main pannel and the service is determined by a forumla that realizes that all circuits are not used at all the time to full capacity.
For the oven you need to get name plate rating. If it is a range (or cooktop/oven) and it is over a certain size then there are some derating factors that are allowed.
Next you need to get the actual nameplate current rating on the other fixed appliances (refigerator, dishwasher, etc).
You count the 2 20amp counter top circuits at full rating. For lights and utility outlets you figure a total of 3 watts/sq ft of the area that this serves.Then convert that to current.
The other mistake that you made was not taking into account that this 240 volt sub-pannel, but that except for the oven the loads are 120 volt and 1/2 of it would be on one leg and half on the other.
Replies
Get Black and Deckers (or anyone elses) Guide to Home Wiring. It has teh chart you are looking for which will tell you how much to rate individual appliances (in the absence of a nameplate) as well as various other types of circuits in your house.
Doug
"I'll use wattage rating and then plug to the equations they outline to determine my actual or practical amperage? And then throw in a safety-factor?"
The formulas have a loading factor with a "built-in" safety factor. You want to rate the usage per the formulas to determine the capacity you need for the sub-panel. I'm guessing (and it's just a guess) it'll come in at about 125.
I think its best to run a circuit breaker that size in the main panel and run the appropriate wire size to the sub.
Don't forget to keep the grounds and neutrals separate in the sub back to the main.
________________________________________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
"5,916 Total divided by 240 = 66 amps"
A little better than your orginal number <G>.
Yes, you can use a 75 amp sub-pannel or BIGGER. 100 and 125 amp or very common so you might get those cheaper.
The feeder has to be sized for you load, the 66 amps
The feeder breaker at the main pannel is them sized for the size of the feeder, but not more than the size of the pannel.
55 ft is no problem.
And you can't get an 85 amp breaker, but you can up-size to the next standard size, 90 amps.
One thinks of the amps as dollars or hundreds of dollars...sort of. Really the price difference between the 6, 4, and 2 gauge wire is not much in terms of price. Subpanels are much the same.
What I'm trying to say is that the size of the panel and the wires you use are not things you want to size at the minimum. The labor and the danger outweigh the cost you save on the wire size. 4 is fine by the way.
Make it easy on yourself and size large unless there is a reason you can't. 30 extra bucks spent on the materials will make you sleep easier.
Good luck.
Wiley -
Your load is rated 66 amps, yes, BUT even so you won't likely have both small appliance circuits loaded up , the DW heater, disposal motor, refrig compressor all on and be microwaving and cooking the Thanksgiving turkey (3 burners and the oven) simultaneously. Even if you did, the current-drawing elements and motors of these appliances mostly are cycling on and off during use.
So, relax and trust us. #4 copper and something around 90-100a breaker is fine. A lot of old houses run on less than that :)
Ed
Edited 12/18/2002 7:36:45 PM ET by Ed
"I mean you don't want the breaker to work harder to
break do you given my actual load is 66 amps?"
First lets look at what the breaker protects. It does not protect the 66 amp load. The feeder breaker in the main pannel protects the feeder wiring and the sub-pannel. For example you where putting a sub-pannel for a workshop. You knew that you where going to get a 2 hp dust collect, 3 hp table saw, and air compressor, some time in the future. But at the current time you only have a 15 amp lighting circuit and 20 amp circuit for outlets. You would still use the same breaker for the feeder.
The wiring to the oven, refigerator, etc is protected by their individual breakers in the sub-pannel.
There is enough "safety" in the code that when the capacity of a wire does not match a standard breaker then you can upsize to the next size breaker. In you case, if you wanted to you could down size to 80 amp breaker if you wanted.
Wiley -
In the case of 85a to 90a you don't HAVE to go up to the larger breaker size, you can go down to a 70 if you want. The 5a up is meaningless in terms of safety for conductors that size - go to an electrical supplier and get a catalog for the breakers you're using. Look at the amperage/time curve graphs and you'll understand why this is so. Also look at table 310-16 in the NEC and you'll see how imprecise a science the amapacity ratings really are.
As for going from 15a to 20a, this would be a 33% increase. The NEC specifically does not allow this.
You don't need a main breaker in the subpanel and most electricians wouldn't put one in since they take up space, aren't required, cost money and honestly to a pro wouldn't serve any useful purpose in terms of maintenance.
Just my thoughts..............Ed
Ed,
If the subpanel were a lengthy distance from the main panel wouldn't it be adviseable to have a breaker there to cut the whole subpanel without having to go back to the main? As I said, I'm no electrician just thinking of walking between panels might get frustrating?
Mike
Mike -
Why would you want to turn the thing off but still have the feeders live? If you want it off so you can stick your hands in it, far safer to turn off the feeders.
Ed
Ed,
I was just thinking that if you tripped the breaker or had to cut the power to the sub that if it were far from the main or even in a different building, it would be a hassle to have to go back to the main? Just looking at it pessimistically I guess?
Mike
Yeah sure it can be a pain sometimes. If in a different building then a main would be required within the panel since it would be functioning as a service entrance for that building (like for a detached garage at a home).
Even in a commercial or industrial facility subpanels don't usually have a main disconnect in the panel. When I see it in a residecne I figure that whoever put it in probably wasn't an electrician. Judgemental, huh............. :)
Ed
Uh, oh. I hope you're wrong. Unless the code changed since 1996 [110-16(a)], there is only 30" of side-to-side working space required, and not necessarily centered in front of the equipment, with overlapping of working clearances allowed between equipment, 90 degree minimum equipment door opening angle, and 36" required clearance from the front of the equipment to an ungrounded surface (behind you as you work in the panel). I've just layed out a basement room which includes an electric closet which meets this criteria, but I have a problem if I'm wrong. Since I'm submitting to the local authority prior to start of work, I guess I'll find out, but I'd like to know before finalizing the plans, and wasting everyone's time. I really should get a 2002 NEC.
Be seeing you...
Thanks for quoting the section of code.
Not to be picky, but I didn't actually quote the Code; I paraphrased. And what I did say wasn't complete. There are other sub-paragraphs relating to (b) Clear Spaces, (c) Access and Entrance to Working Space, (d) Illumination, and (e) Headroom, among other things. You should look that Article up yourself, and/or get an electrician or code authority person to give a definitive answer, but that's how I understand it. I was hoping a Knowledgable Person would confirm this before I take it to my local authority for review (they're not so local, so it's a pain). I hope it works out for you.
Be seeing you...
I did a quick search on Google and found this:
http://www.jhlarson.com/ind_tables/table310-16.htm
BTW - the 30" refers to total clearance needed, panel need not be centered. And it has to be clear to the floor, i.e. you aren't allowed to install the panel over a pantry counter-top or within some shelving.
I've read some of Rex Cauldwell's stuff and I think sometimes he just sits around thinking things up that sound good but aren't really anymore effective than the industry standard.
Ed
Edited 12/19/2002 7:02:09 PM ET by Ed
Edited 12/19/2002 7:07:39 PM ET by Ed