[harum]

Hello, wondering if this is true: A 100 Amp electrical system in a house (photo attached) means that it can power about 24 kW worth of electrical devices?

The system I am looking at is a two-pole (most probably: there used to be an electric range and electric dryer hooked up); 100 Amp main service means that I have 100 A x 240 V = 24kW of max power, that is, each 120 V pole can have maximum current of 100 A.

thanks, h.

[stanfortyman]

Basically yes.

[ma0641]

A more common consideration is breaker Amperage. If you add up the total breaker rated Amps in your box, you will find they likely exceed the capacity of the main by 35-40% or more. That is because you will never, or at least not likely, have everything on. I have a Siemans 200 Amp box with almost 300 Amps on the breakers. However, you might only have a 40 or 60 watt light on a 15 amp circuit.

[harum]

A more common consideration is breaker Amperage. If you add up the total breaker rated Amps in your box, you will find they likely exceed the capacity of the main by 35-40% or more. That is because you will never, or at least not likely, have everything on. I have a Siemans 200 Amp box with almost 300 Amps on the breakers. However, you might only have a 40 or 60 watt light on a 15 amp circuit.

Yes, the 100 Amp breaker box shown contains 205 Amp total of individual breakers ranging from 15 through 50 Amp.

Is this true that all electric devices plugged into home 120 V receptacles are always connected in parallel due the need to keep the input voltage for all devices on the same circuit at 120 V? I think it is. Otherwise, specifying a max current for a given circuit wouldn't matter.

[ma0641]

Yes, the 100 Amp breaker box shown contains 205 Amp total of individual breakers ranging from 15 through 50 Amp.

Is this true that all electric devices plugged into home 120 V receptacles are always connected in parallel due the need to keep the input voltage for all devices on the same circuit at 120 V? I think it is. Otherwise, specifying a max current for a given circuit wouldn't matter.

Correct. All 240 VAC are connected to both sides. If you pull the breakers, you can see the buss bar layout. For 120VAC, breakers alternate even if on 1 side. That way you don't get unbalanced loads. Go to an electrical supply store and look at the guts of a box, you will then understand. So you have a 100% "overload" but not used wattage load. If you take a 15 Amp circuit for a room, you could have 18 X 100 watt lights running. Ever happen? Very doubtful. However, I gather you are concerned of overloads. 100 Amps is minimal today, smaller houses are 150, larger are 200 or 400. Most utilities install larger wire even if you are putting in a small cabin with a 100 Amp panel. That way if you want to build a bigger house and upgrade, you bear the cost of the install, not them putting in larger service cables. I was a bit intrigued by the term 100 AMP "Mains". I would guess it is an older box. I have never seen a 100 Amp service in a house built after the 60's.

[harum]

Correct. All 240 VAC are connected to both sides. If you pull the breakers, you can see the buss bar layout. For 120VAC, breakers alternate even if on 1 side. That way you don't get unbalanced loads. Go to an electrical supply store and look at the guts of a box, you will then understand. So you have a 100% "overload" but not used wattage load. If you take a 15 Amp circuit for a room, you could have 18 X 100 watt lights running. Ever happen? Very doubtful. However, I gather you are concerned of overloads. 100 Amps is minimal today, smaller houses are 150, larger are 200 or 400. Most utilities install larger wire even if you are putting in a small cabin with a 100 Amp panel. That way if you want to build a bigger house and upgrade, you bear the cost of the install, not them putting in larger service cables. I was a bit intrigued by the term 100 AMP "Mains". I would guess it is an older box. I have never seen a 100 Amp service in a house built after the 60's.

Yes, the plural in "MAINS" is unclear to me too. Another thing is that I can't find the switch for main power shut-off, the one upstream the main breaker box. It is not anywhere around the box.

Yes, when I say "100 Amp service" most people suggest upgrading to 200 Amp. But I guess 100 Amp is plenty considering several major appliances are fueled by gas. However, the current codes require a lot of dedicated high amp circuits.
Besides, there is no ground wire in the system, which is a project in itself.

Is this true that the top and bottom breakers on the photo (40 and 50 Amp) are for 240 V circuits because they are doubled and their halves can only be moved together (i.e. when you pull one of the switches the other follows)?

Correct. All 240 VAC are connected to both sides. If you pull the breakers, you can see the buss bar layout. For 120VAC, breakers alternate even if on 1 side. That way you don't get unbalanced loads.

Do you mean that some 120VAC circuits are connected to one pole of the two-pole system and some to the other to balance the load? And that different orientations of the 120 VAC breakers on the board indicate different poles? They look quite balanced on the photo then -- 150 Amp pulled from the left side, 145 Amp from the right side.

[ma0641]

There is a ground wire, just not in the wall circuits, the neutral is grounded, probably outside or to a water line. Do you have NMC or BX wiring? BX sheath is grounded. Look for a main disconnect at the meter box. In GA, where I live, all newer homes have the disconnect outside so a fire dept can disconnect all power before entering a building. Yes, those are 240 VAC breakers. What I mean by balanced is if you were to put everything only on 1 side of the box, you would expect a major imbalance. Well, the manufactures took that into account by switching the buss bars across both sides so some are actually connected to the other leg even if it doesn't look it. What circuit does the double 50 AMP control?

If you Google "Square D XO load center", everything I saw indicated "obsolete".

[harum]

There is a ground wire, just not in the wall circuits, the neutral is grounded, probably outside or to a water line. Do you have NMC or BX wiring? BX sheath is grounded. Look for a main disconnect at the meter box. In GA, where I live, all newer homes have the disconnect outside so a fire dept can disconnect all power before entering a building. Yes, those are 240 VAC breakers. What I mean by balanced is if you were to put everything only on 1 side of the box, you would expect a major imbalance. Well, the manufactures took that into account by switching the buss bars across both sides so some are actually connected to the other leg even if it doesn't look it. What circuit does the double 50 AMP control?

If you Google "Square D XO load center", everything I saw indicated "obsolete".

The double 50 Amp controls a 240 VAC (black, red, white and green wires) circuit that used to power something in the kitchen but hasn't been used in a long while -- it was hidden behind the cabinets and is just tape insulated wires in an open wall box. Controls nothing at the moment.

What is more interesting is that the 240 VAC all electric range (four-wire BX cable), 120 VAC ventilation hood (two wire BX cable), refrigerator with a regular plug, garbage disposal with a plug, and 5 outlets in three different areas of the house are on the same 20 Amp circuit. Do not know if this is common. Looks like the 240 VAC BX is split somewhere to have a 120 VAC loop for 120 VAC outlets and devices. I don't know if this setup allows simultaneous use of all the devices and outlets at the same time, but if it does, then this is another argument in favor of staying with the current 100 Amp system.

I guess, I am still not clear on circuit planning for a 100 Amp two-pole system. Is this true that each 120 VAC pole can provide a max of 100 Amp, and that the individual circuits should be distributed evenly between the two poles to make a full use of the provided power? E.g. can one exceed the max amperage on one pole but still have enough unused amperage on the other pole?