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My main breaker panel is feeding a panel in my basement. The main panel feed to the basement panel is 2 60amp breakers that have the thing across them so they trip together. My question is am I feeding 120amps to the basement panel or still only60 amps. My main breaker in the basement is a 100amp, how does that affect the double 60 amp feed to it?

marked as duplicate by Fiasco Labs, DMoore, Niall C. Oct 14 '14 at 11:44

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Am I feeding 120 amps to the basement panel or still only 60 amps?

That you are asking the question at all strongly indicates that you do not understand what current is, so step one is to understand what current is.

Imagine you have a bag of marbles sitting on top of a table. You drop one marble per second off the table. Each hits the ground with a certain amount of energy.

Now imagine that you up the rate to two per second. Each hits with the same amount of energy, but there are twice as many arriving on the floor.

Now imagine that you lower the height of the table to half its previous height. Now the marbles are all hitting with less energy, but they are still arriving at two per second.

The marbles in our analogy are charges, which are measured in coulombs. The height that they are falling from is potential, which is measured in volts. The number of marbles falling per second -- charge per unit time -- is current, which is measured in amps.

A breaker at 60 amps means that if you exceed 60 coulombs of charge per second in the circuit -- if you are dropping more than 60 marbles per second -- then the circuit will break automatically. This is true regardless of the voltage; if the breaker is labeled 60 amps then it will not allow more than 60 coulombs of charge per second to be delivered to any point in the circuit. Remember, the current is the same everywhere in the circuit. If 60 marbles per second are leaving the table top, then 60 per second are arriving on the floor, and 60 per second are falling past every point between the table and the floor.

A single breaker gives you two wires that are 120 V apart: a hot wire and a white wire. The maximum current on both wires -- remember, the current is the same everywhere in a circuit -- is the rated ampacity of the breaker.

A double breaker gives you two wires that are 240 V apart: the table is twice as high, so the marbles deliver more energy when they hit the floor. The two wires are red and black. Again though, the amount of charge per second passing through any point in the circuit is limited by the rated ampacity of the breaker.

My main breaker in the basement is a 100amp. How does that affect the double 60 amp feed to it?

To clarify: you have a main panel with a 60A breaker. That breaker feeds a subpanel which has a 100A main breaker and then some smaller circuit breakers in the sub-panel.

If more than 100 coulombs of charge per second flow through the sub-panel main breaker, it will trip and cut power to the subpanel breakers. If more than 60 coulombs of charge per second flow through the subpanel breaker in the main breaker box, it will trip and cut power to the sub panel. All the charge that passes through the subpanel breaker also passes through the main breaker, so the 100A breaker in the subpanel box is redundant. It will never trip because the 60A upstream of it will have tripped first.

A question you did not ask but which is highly relevant is:

What is the relationship between volts, amps and watts?

Volts is potential: how high the marbles are falling from, so, how much energy each marble delivers to the floor.

Amps is how many marbles per second.

Power is amount of energy delivered per second, and therefore must be volts times amps.

So if you have a 60 W light bulb, a 120 V circuit, how many amps are needed to power the bulb? amps times volts is watts, the bulb needs 60 watts, so what times 120 is 60? one half. So a 60 W bulb draws half an amp in a 120 V circuit.

Now you should know the answer to this question:

Why do we use 240 V circuits instead of 120 V circuits for loads that requires a lot of power, like heaters?

Because if you double the voltage but keep the wattage the same, you can halve the amperage. If you double the height of the table then you can halve the number of marbles per second you drop off it, and the energy delivered per second stays the same.

I had already put the 100 amp main breaker in the basement panel when an electrician put the 60 amp in the main breaker panel and ran the wire to the basement. I just thought he wouldn't have put a 60 amp breaker that fed the 100 amp I put into the basement panel.

He could have put in a 100A breaker, yes, provided that the wire run to the subpanel was safe for that amperage.

Am I right in assuming that the 60 amp in the main breaker would trip before the 100 amp it is feeding in the basement panel.

Yes. The 60 will break when more than 60 C of charge per second is flowing past it.

I thought he should have put at least the size of breaker in the main as in the basement panel.

The size of the breaker should depend on (1) the expected maximum load from the subpanel and (2) the size of the wire feeding the subpanel. 60A at 240V is enough to light over 200 standard incandescent light bulbs and probably thousands of compact fluorescent bulbs, why do you think you need half again as much as that?

It just seems there now was no point putting in the 100 amp breaker.

A better question would be why did you put in a main breaker in the subpanel at all, when it is already protected by a breaker at the main panel? I have wired in several subpanels in my house and none of them have main breakers; they are not necessary.

I thought that you had to have a way to kill the power to the sub panel at the sub panel.

Let's clarify that.

A breaker interrupts the hot wire(s) going to the load. In your present configuration there are four breakers between the power coming from the street and the lamp you're plugging into the wall:

  • The main breaker for the main panel. Probably 200A.
  • The breaker for the subpanel in the main panel. 60A.
  • The main breaker for the subpanel. 100A.
  • The subpanel breaker for the circuit you'll be running. Probably 15A or 20A.

Each breaker interrupts the hot wire(s) cutting power going downstream. So when you turn off the main breaker at the main panel, there is still voltage coming into the house but it stops at the main breaker and does not go to any of the main panel breakers. If that one is on and you turn off the 60A breaker to the subpanel, there is now no voltage on the lines going to the subpanel. If that one is on and you turn off the 100A breaker on the subpanel, voltage is still being provided on the hot wires from the main panel, but the 20A subpanel breaker is getting no power.

So now let's examine your statement:

I thought that you had to have a way to kill the power to the sub panel at the sub panel.

The power to the subpanel is killed by either the main panel main breaker, which kills power to the whole house, or by the main panel subpanel breaker, which kills the power on the wires going to the subpanel. Turning off the subpanel main breaker doesn't stop power from coming to the subpanel, it stops that power from getting to the subpanel breakers.

So let's rephrase your statement:

I thought that you had to have a way to kill the power to the sub panel breakers at the sub panel.

That's not a requirement, because the breaker on the main panel that feeds the subpanel is perfectly capable of cutting power to the subpanel.

It might be convenient to have a main breaker on the subpanel to cut power to the subpanel breakers because then you can do work on the subpanel without walking over to the main panel, but personally that is not how I've wired my house. If I want a subpanel to be off, I turn it off at the main panel and then I know that there is no power anywhere in the subpanel. If you have a subpanel with a main breaker that is off, there is still power coming into the subpanel, it's just not making it to the subpanel breakers. Those big screws that hold the wiring from the main panel will be electrified, and when I work on a subpanel I want none of it to be electrified.

I do believe that I should not have put that large of breaker because the wire size coming from the main breaker isn't large enough.

Ensuring adequate overcurrent protection to the wire leaving the main panel and going to the subpanel is the responsibility of the main panel, not the subpanel. The important thing is that the wires leaving the main panel have overcurrent protection appropriate to the size of those wires. If the load at the other end of those wires has additional protections, that's great, but it's not a requirement.

Is this 100 amp breaker dangerous in any way and should be changed or just unnecessary?

I would consider the situation you're in as being somewhat confusing but not dangerous; provided that every wire leaving a panel is protected by a breaker appropriate to the size of that wire, you're all set for overcurrent protection. Your 100A breaker will never trip automatically because the 60A at the other end will trip first.

Thanks for your help

You're welcome! I note that of course I am not an electrician; I design programming language analyzers for a living, I just rewire old houses for fun in my spare time. If you want to know whether your electrical system is up to local code, ask a local electrician.

  • This is an excellent description, thanks Eric. I may need to read this a few times to understand it but you have helped a lot and I appreciate it. – Rick Oct 13 '14 at 20:13
  • Eric, I thought that you had to have a way to kill the power to the sub panel at the sub panel. I do believe that I should not have put that large of breaker because the wire size coming from the main breaker isn't large enough. Is this 100 amp breaker dangerous in any way and should be changed or just unnecessary? Thanks for your help. – Rick Oct 14 '14 at 15:37
  • @Rick: I've added some notes on your follow-up questions. – Eric Lippert Oct 14 '14 at 16:40
  • I love the detail that you have added about my situation and I know have a much better understanding. Especially your explananation about having all power off from another panel to ensure that all power in the sub-panel is off. I did not consider that the big screws would still have power. For that reason alone your logic makes perfect sense and adds to safety. Thanks so much, I have learned a lot from you. You have a gift for teaching and making things understandable for people. – Rick Oct 15 '14 at 5:18
  • @Rick: Thanks for the kind words, I appreciate it. An analogy often made to beginners working with wiring is that wiring is like plumbing: switches are valves, current is the amount of water that flows past a point in a pipe per second, voltage is water pressure. Hot wires are supply pipes and neutral wires are drain pipes. (Note that in our analogy there is current in a drain pipe even though the drain pipe is not pressurized. And similarly in electrics, there is current in the neutral wire, but no voltage.) – Eric Lippert Oct 15 '14 at 16:35
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Assuming that you are in the US and have typical residential electrical service, you would have split phase electric power service coming into your main panel.

The 60A 240V breaker servicing your basement sub-panel feeds up to 60A to the sub-panel. That your basement sub-panel has a 100A capacity and breaker doesn't change this.

Now, assuming that you have the basement sub-panel circuits balanced, you could have up to 60A of capacity per hot leg. If you are only running 120V circuits, that would be 60A available per leg. The first leg to exceed 60A would cause the main panel with the 240V 60A breaker to trip.

  • House current is split phase with only a single phase available across 240V. The circuit passes only 60A, there are two breakers to ensure each hot leg is shut off during a fault. – Fiasco Labs Oct 13 '14 at 15:47
  • Thank you for both comments. I had already put the 100 amp main breaker in the basement panel when an electrician put the 60 amp in the main breaker panel and ran the wire to the basement. I just thought he wouldn't have put a 60 amp breaker that fed the 100 amp I put into the basement panel. Am I right in assuming that the 60 amp in the main breaker would trip before the 100 amp it is feeding in the basement panel. I thought he should have put at least the size of breaker in the main as in the basement panel. It just seems there now was no point putting in the 100 amp breaker. – Rick Oct 13 '14 at 17:18
  • @Fiasco Labs: This is true, editing this in my answer – Joshua Berry Oct 13 '14 at 18:36
  • @Rick: you are right: under excessive load, the 60A breaker in the main panel would trip before the 100A breaker in your subpanel. The electrician may not have run a gauge of wiring that would safely allow more than 60A. – Joshua Berry Oct 13 '14 at 18:42

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