New Sub for Split Bus main


Old 01-31-05, 10:15 AM
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New Sub for Split Bus main

I'm considering adding a new sub panel to my existing 35 year old Square D split bus main panel. The existing panel is full (no tandems) save for a double pole slot in the "top six", which I am planning to use to feed the sub.

The sub would be used to power a new basement apartment. Although the square footage will be small, it will include kitchen, living room, bedroom and bathroom, possibly with laundry closet. Probably six new 20 amp circuits in total, plus a 50 amp stove circuit. All new circuits will be in the basement space currently served by a general purpose 15 amp circuit.

The house has two gas furnaces, and a gas water heater.

At the same time, I am remodeling my kitchen/dining room on the main floor and wish to add 3 or 4 20 amp circuits. The dining room is becoming the office, so I want a dedicated 20 amp circuit. Right now, the dining room shares (originates) one of the 20 amp SA circuits, so if I terminate the existing circuit in the dining room(office), I need a replacement for the kitchen. Also, I'm installing a dual refirgerators, dual dishwashers, and a dedicated microwave.

The laundry/utility circuits ( 2 20 amp furnace, 1 20 amp laundry ) currently pass under the kitchen to get to the utility room, so I am considering using these circuits for the kitchen, and wiring replacement circuits from a new utility room sub.

So, the upshot of all of this is I'm considering a 100 amp sub panel in the utility room with feed through lugs to feed another 100 amp sub for the basement apartment. The apartment panel would need to be seperate from the utility room panel in order to be accessible from within the apartment. The utility room panel would serve the replaced furnace and laundry circuits and perhaps a future small workshop ( one or two 20 amp circuits).

The existing panel is configured like this:
60 50
40 40
40 Empty

20 15
20 15
20 15

The sixty feeds the bottom half of the panel. The two forties on the left feed two heat pumps (air conditioning). Fifty feeds the existing stove and the forty on the right feeds the dryer circuit. Side question - the air conditioning forties feed seperate 30 amp fusible disconnects ten feet away via 10/2. Is this legit? I thought 10/2 should be protected with 30 amp breakers. I guess the load can't exceed 30 amps per circuit because of the disconnects?

Anyway, on to my main questions:

1) Can I install a new 100 amp double pole feeder in the free slot? I'm concerned because I've read that the sum of the six disconnects cannot exceed my 200 amp panel rating. Technically, they already exceed 200 amps.

2) I will be consulting with a master electrician in the next couple of weeks about all of this, but I want to know my options in advance. I think I want to do most of this work myself, but I don't want to work in my live main panel. Is the simplest/cheapest option here to ask the electrician to install a seperate 200 amp disconnect immediately before my main panel? Will this allow me to exceed 200 amps in disconnects in my main, while working safely in the unpowered main?

Looking forward to any answers or discussion about my plans.


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Old 01-31-05, 12:35 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
I suggest you replace the old split-bus panel before adding the subpanel. The issue is that the upper breakers collectively act as your main disconnect. The wires leading into this box are unfused except for the upper breakers. If you add or increase an upper breaker, you are compromising the protection of the entrance wires. This could be very bad.

Any time a kitchen is involved (including cooking appliances), power requirements are substantial. It's almost impossible to add a kitchen, even a small one, without an increase in your service size. This is another reason you likely need a new main panel.

The remodel of your existing kitchen is yet another nail in the coffin of your existing panel.

The dual refrigerators and dishwashers are more nails. So is the air conditioning.

Before you do anything, and before you even worry about the answers to your questions, you should do (or have done) a "demand load calculation" to compute how much power you will need when your are all done. If you google that term, you'll find information on how to do it. It is a fairly complex calculation, so you may prefer to have the electrician do it.
Old 02-01-05, 10:34 AM
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Alright, I took your advice ...

I did a demand load calc. I used a worksheet I downloaded from

To begin with, the base demand load (before renovations) is 155 amps.

The upstairs kitchen renovation adds a second fridge, second dishwasher and a dedicated micro. This takes the total to 170 amps.

The numbers the spreadsheet used for each additional appliance were:

Qty Appliance Watts
1 Refrigerator 1,400
1 Freezer 600
1 Dishwasher 1,030
1 Microwave 1,630

Now, for the basement apartment, I added 2 small appliance, one refrigerator, one freezer, one dishwasher, and one range circuit(s). Total demand climbed to 196 amps.

Still in the basement apartment, adding a laundry circuit takes the demand load to 199 amps. The only way to add the dryer circuit to the apartment is to take it from the main house, without replacement. This is an option, because we already have a gas line to the dryer location in the main house and could easily convert it to gas. Also, I suppose I could install a gas dryer in the basement apartment.

So now come my questions:

1) The basement apartment laundry/dryer is optional. But even without these circuits, the demand load is 196 amps. Is this too close to my 200 amp service to realistically implement, or do the adjustment factors and safety margins used in the demand load calc mean I'm ok at 196?

2) Given that gas is an option for the apartment dryer and even the range, and that having both on gas would bring the load down to 186 amps, is this a more realistic option? Basically, what maximum load would you accept in this remodel scenario?

And now for the real question:

The way I ran the numbers was to increase the appliance counts in the spreadsheet, say from one fridge to three fridges, and from one range to two ranges. Because of diversity adjustments, I suppose, the total additional load is about 30 amps, give or take. If I run the numbers seperately for a standalone apartment, it looks like I need up to 60 amps. Is the code specific on how these numbers should be run (ie. as a standalone apartment ) ?

And one last question, if I have only gas dryer(s), can I omit the dryer ciruit from the load calc?

Old 02-01-05, 12:19 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
(1) I don't know of any requirement that you "leave room". So if your demand load is 196, your 200 service is fine. Just don't plan to add anything significant in the future.

(2) Your choice. I'd lean towards gas.

Running the numbers in aggregate is the correct approach.

From memory, the calculation does not seem to allow you to omit the dryer circuit. I'm not sure the reasoning for this.

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