Understanding electrical loads.

[dbhost]

I am trying to get an idea of what to present to the electrician, and I want to make sure I am not getting taken for a ride here, so bear with me...

My main panel has a 150 amp main breaker, and two rows of 40 and 20 amp breakers thusly...

----------
| 150 A |
| |
----------
---------- ----------
| 40 A | | 40 A |
| | | |
---------- ----------

---------- ----------
| 20 | 20 | | 20 | 20 |
---------- ----------

---------- ----------
| 20 | 20 | | 20 | 20 |
---------- ----------

---------- ----------
| 20 A | | 20 A |
---------- ----------
---------- ----------
| 20 A | | 20 A |
---------- ----------

(sorry about the alignments, I hope you get the idea... I laid this out in ASCII with whitespaces but the forum software is removing whitespace for me...)

Now as I do the math, the load under the main adds up to 360 amps.

How on earth does a single 150 amp service main protect all of that?

[LCHIEN]

clearly not every breaker is maxed out all the time.
Many of your 20 A breakers will have only 2-3 amps on them.
As long as your house pulls under 150 Amps the main brekaer will not trip.
Each individual circuit is protected to 20A or 40A which is the capacity on the wire on that branch, so if you do draw a large current on that line it will not catch fire.

The basic point is that you can't have every breaker carrying their max current. But normal usage is that your house will be well under 150A total even though a couple of branches may carry near full load, most of them will not and in fact be quite a bit under.

If in fact you pull more than 150A total, then your main input wiring is at risk of overheating and catching fire and the main breaker will trip to protect that, even though you only pull 7 amps or so average on the branch breakers.

P.S. Master Yoda makes an excellent point further down that the 150 main Breaker supplies 240V, and the branch breakers (both single and double gang) you are adding up are all supplying one leg of 120V, so 20Amps through each pair of them still draws only 20A from the main breaker. So its more like 150A main breaker supplies 180A of branch circuits. You should have capacity to add a lot more branch circuits.

[dbhost]

Gotcha... Kind of makes sense...

How would I fit a sub panel for the shop into that assuming that the entire panel is full?

[LinuxRandal]

Gotcha... Kind of makes sense...

How would I fit a sub panel for the shop into that assuming that the entire panel is full?

Some of the breakers would be removed (normally 20 amp ones) and 220 ones placed in their place, that go to a daughter panel. The removed ones as well as the additions would be in the daughter panel.

[tseavoy]

Another approach is to identify the most lightly loaded circuits, usually lighting circuits, and to have one breaker serve for both of them, e.g. if your living room, dining room or bedroom circuits take less than, say 10 amps each with everything on and they are on a 20 amp or more breaker, they can be doubled up. You might be able to free two breaker positions this way. Watch out for heavier loads like electric space heaters though. Figure 100 watts per amp, conservatively. Don't do this with kitchen, dishwasher, or laundry circuits.
In my shop, which is not insulated, I often have a space heater on. When I turn a power tool, like a jointer or TS and it's on the same circuit, I turn off the heater first.

Tom on Marrowstone

[master53yoda]

one other thing that you are missing is that the 150 amp breakers are on 240 volts

if one 20 amp breaker comes off of one leg and another 20 amp breaker comes of the other leg the main only sees 20 amps not 40.


the breakers listed at full load would only load the mains @ 160 amps
not 320

a double pole breaker with two 40 amp single pole breakers only draws 40 amps at full load on 240.

I hpe this helps

[master53yoda]

to feed the sub-panel you could get some 1/2 size 20 amp breakers and replace 4 full size breakers with the 1/2 size breakers this will leave you with 2 full size slots open to run the sub-panel feed from. you would still have all the breakers that you have know. This will work with most panels such as Sq D, GE, Siemens, sylvania etc. Cuttler hammer does not offer this option on their older panels.

[crokett]

Another approach is to identify the most lightly loaded circuits, usually lighting circuits, and to have one breaker serve for both of them,

If you do this, be aware that the lugs on breakers are not normally rated for two wires. So assuming you were to combine two light circuits onto a single breaker you would have to splice them to a pigtail then land the pigtail on the breaker lug. A better approach than this is to either use half-high breakers or feed a smaller sub. If this is going to be inspected then you need to make sure the panel is rated for half-high breakers. If it isn't, the sub is your best option.

[Tom Slick]

If you do this, be aware that the lugs on breakers are not normally rated for two wires. So assuming you were to combine two light circuits onto a single breaker you would have to splice them to a pigtail then land the pigtail on the breaker lug. A better approach than this is to either use half-high breakers or feed a smaller sub. If this is going to be inspected then you need to make sure the panel is rated for half-high breakers. If it isn't, the sub is your best option.

Or the "two" circuits could be connected in a daisy chain, very common in house wiring.

Or you could combine multiple circuits in a J-box and make a "home run" to the breaker. Not common in house wiring but very common in commercial/industrial.

[crokett]

Or the "two" circuits could be connected in a daisy chain, very common in house wiring.

Or you could combine multiple circuits in a J-box and make a "home run" to the breaker. Not common in house wiring but very common in commercial/industrial.

True, but the pigtail at the breaker box is probably the easiest. No pulling new wiring or figuring out where the existing circuits are to daisy-chain them. Of course that also assumes he has access to the wiring.

Personally, in his situation I would take one of his existing 240V double-pole circuits like the dryer, pull the breaker and install a larger one in that spot that feeds a small sub. Extend the dryer circuit to the sub and put a breaker for it there. Then feed the shop off the sub. That way he is covered for more breaker spots if he wants to add circuits for anything else in the future.

[woodturner]

True, but the pigtail at the breaker box is probably the easiest.

Pigtailing in the service panel requires a wire nut. While I read nothing in NEC that prohibits this, I have heard heard that some inspectors will not allow wire nuts in service panels. I don't see anything wrong with it and have done it, just a "heads up" that some inspectors may balk at this.