calculating breakers etc

[SGKent]

It is late and forgive me if this question is asked 10 times a week.

I have a main panel. This is single phase residential. On it are a pair of 30 amp breakers that supply power to a sub-panel breaker box in a shed that is about 10 feet away in wire length. In the shed is a locked pair of 15 amp breakers for a compressor and a single 15 amp 120V breaker for a light and a 120V 15 amp socket.

Q1. At 240V. Do that pair of 30 amp breakers in the main panel supply 30 amps each breaker = sixty amps combined or would one or more trip if the current exceeded 30 amps at 240V? Example - could I run two 10 amp 240V devices in the shed and still have a 10 amp safety margin?

Q2. At 120V. Do that pair of main breakers supply 30 amps each at 120V = 60 amps at 120V or because they are a pair locked together are they 15 amp a breaker and just marked30 combined? Each reads 30 amp with a bar between them.

Q3 What would be the optimal wire gauge used between the main breaker panel and the sub-panel with that 30 amp marked pair of breakers going to the sub-panel? 10/3 with a ground plus a grounding rod?


What I am having trouble figuring is that the breaker pair in the main panel are marked 30 amps. Is this 30 amps across each one to neutral so really this is 60 amps total at 120V in the shed? For example, when I have a 125 amp panel coming into the house, is this 125 amps on each leg of the 240 or 125 amps across the two legs so really only 62.5 amps between each leg and neutral?

 

[macdabs]

The amperage labeled on the breaker is the amount of currant it is capable to pass before it trips . How much currant is fed depends on the size/type of wire fed to the breaker and the size of breaker feeding the voltage. 120 volts is on each leg with a single phase residential service . How you use the breaker to supply 220v vs 120 volts is always going to be limited by the amperage rating on the breaker and most importantly for safety the NEC rules. I hope I don't sound like a **** but electricity is something you need to understand 100 percent cause you do not get second chance. A basic electricity how to book and pocket NEC Manuel is something you should read and study before touching anything in a breaker panel.
Mac

 

[1grnlwn]

10/3 is the correct wire size for 30 amp. Theoretically each side would allow 30A but in a 220 situation, I am not sure if 10 amps is 5 per side or 10 per side . I would guess it is 10 per side. If you pull more than 30 amps on one side both breakers should trip. As long as the wire is sized for the breaker it is safe for the amperage. Now if you start blowing that breaker, you obviously need a bigger sub panel. It depends on what your plans are.

 

[Falcon67]

10/3 is the correct wire size for 30 amp. Theoretically each side would allow 30A but in a 220 situation, I am not sure if 10 amps is 5 per side or 10 per side . I would guess it is 10 per side. If you pull more than 30 amps on one side both breakers should trip. As long as the wire is sized for the breaker it is safe for the amperage. Now if you start blowing that breaker, you obviously need a bigger sub panel. It depends on what your plans are.

X2 - 10-3 with ground is what you need for the panel. The breaker handles the labeled current per leg. For example, an amp probe on my 5kW heater shows 20.5 amps draw per leg (on 10-2 wire - no neutral required)

 

[SGKent]

The amperage labeled on the breaker is the amount of currant it is capable to pass before it trips . How much currant is fed depends on the size/type of wire fed to the breaker and the size of breaker feeding the voltage. 120 volts is on each leg with a single phase residential service . How you use the breaker to supply 220v vs 120 volts is always going to be limited by the amperage rating on the breaker and most importantly for safety the NEC rules. I hope I don't sound like a **** but electricity is something you need to understand 100 percent cause you do not get second chance. A basic electricity how to book and pocket NEC Manuel is something you should read and study before touching anything in a breaker panel.
Mac

FYI to all - when it was put in it was signed off as per code and an electrician looked over my calculations. That was 7 - 10 years ago so forgive me if I have forgotten what the specifics are.

X2 - 10-3 with ground is what you need for the panel. The breaker handles the labeled current per leg. For example, an amp probe on my 5kW heater shows 20.5 amps draw per leg (on 10-2 wire - no neutral required)

What I am thinking is that when it is 240V there is no neutral involved so the amperage to trip would be 30 amps as the current flows in series through both 30 amp breakers. However I am thinking that when it is 120V then each leg would support 30 amps in parallel to neutral so the total circuit could be up to 60 amps if 30 was pulled on each breaker. Why then is not the neutral wire designed to hold that 60 amps? Is this not the case?

 

[ishiboo]

What I am thinking is that when it is 240V there is no neutral involved so the amperage to trip would be 30 amps as the current flows in series through both 30 amp breakers. However I am thinking that when it is 120V then each leg would support 30 amps in parallel to neutral so the total circuit could be up to 60 amps if 30 was pulled on each breaker. Why then is not the neutral wire designed to hold that 60 amps? Is this not the case?

You're correct, each breaker allows a maximum amperage of say 30A... but it's in series on a 240v circuit, so you're allowed 30A @ 240v. Those same breakers with a neutral could do 60A at 120v, but that's the same quantity of energy... 30A @ 240v = 60A @ 120v.

This is the concept that confuses people with the MWBC... say you run 14/3, you have a dual-pole 15A breaker. You can draw up to 15A on either or both legs. The neutral carries only the imbalance between the two, since they are opposite phases - one half of the 120v circuit is at 170v at the peak of the waveform, while the other is at -170v. IE, the maximum current on the neutral is actually when ONE circuit is drawing peak current, and the other is doing nothing... there's LESS current (theoretically zero) when both circuits are drawing 15A. If you put two single pole breakers in a "MWBC" on the same phase, you'd overload the neutral two-fold.

 

[SGKent]

You're correct, each breaker allows a maximum amperage of say 30A... but it's in series on a 240v circuit, so you're allowed 30A @ 240v. Those same breakers with a neutral could do 60A at 120v, but that's the same quantity of energy... 30A @ 240v = 60A @ 120v.

This is the concept that confuses people with the MWBC... say you run 14/3, you have a dual-pole 15A breaker. You can draw up to 15A on either or both legs. The neutral carries only the imbalance between the two, since they are opposite phases - one half of the 120v circuit is at 170v at the peak of the waveform, while the other is at -170v. IE, the maximum current on the neutral is actually when ONE circuit is drawing peak current, and the other is doing nothing... there's LESS current (theoretically zero) when both circuits are drawing 15A. If you put two single pole breakers in a "MWBC" on the same phase, you'd overload the neutral two-fold.

So if I understand this, effectively with two 120V devices connected to circuits coming from both legs to neutral, the current flows through one breaker, through device one and then to neutral and back to device two through its neutral and out through the other leg. Only the imbalance between the two devices is carried on the neutral wire back to the main panel which would be at max 15 amps. All three wires are carrying current with the load on one wire never exceeding the 15 amps that the breakers, one on each leg, were designed for. Is that correct?

 

[Charles (in GA)]

So if I understand this, effectively with two 120V devices connected to circuits coming from both legs to neutral, the current flows through one breaker, through device one and then to neutral and back to device two through its neutral and out through the other leg. Only the imbalance between the two devices is carried on the neutral wire back to the main panel which would be at max 15 amps. All three wires are carrying current with the load on one wire never exceeding the 15 amps that the breakers, one on each leg, were designed for. Is that correct?

You are correct.

Charles

 

[SGKent]

Ok that part is now firmly understood. One last question on this. In the articles and codes I've read the neutral bus on the sub-panel must be lifted off the sub panel ground. At the same time a ground should be run back from the sub-panel to the main panel and also to rods in the ground. While I have done that I don't understand the logic of it since the main panel is grounded and the neutral and ground are tied together on the main panel. I know it accomplishes something or eliminates a risk but I don't understand what it does.

 

[Falcon67]

Note that the MWBC works because - if I have this right - each 120V leg is on a different side of the line (phase). The single neutral can carry the "two" loads because the 60 cycle wave peaks occur opposite each other. It's a "trick" LOL.

 

[SGKent]

Ok that part is now firmly understood. One last question on this. In the articles and codes I've read the neutral bus on the sub-panel must be lifted off the sub panel ground. At the same time a ground should be run back from the sub-panel to the main panel and also to rods in the ground. While I have done that I don't understand the logic of it since the main panel is grounded and the neutral and ground are tied together on the main panel. I know it accomplishes something or eliminates a risk but I don't understand what it does.

anyone?

 

[Charles (in GA)]

The purpose of an equipment ground, earth ground, that green or bare wire, is to provide a path to ground in case of a fault. It does not normally, and should never carry current.

Each circuit has its own neutral, and those neutrals are separated back to the panel, the grounds are many times combined, and cross connected, and in the case of EMT, the whole conduit is grounded. You do not want any of this to be hot.

The purpose of a grounded conductor, the neutral, the white wire, is to provide a "return path" for current that begins at the breaker and travels thru the appliance and then back down the neutral to the transformer. It is a hot or current carrying wire.

You want the neutral to carry current, and you do not want the ground to carry current, thus you keep them separated. If anywhere after the one point where they are combined, at the service entrance, they again touch, then the ground could be carrying current, which is dangerous to persons using devices and appliances, and to anyone working on the system.

Charles

 

[SGKent]

The purpose of an equipment ground, earth ground, that green or bare wire, is to provide a path to ground in case of a fault. It does not normally, and should never carry current.

Each circuit has its own neutral, and those neutrals are separated back to the panel, the grounds are many times combined, and cross connected, and in the case of EMT, the whole conduit is grounded. You do not want any of this to be hot.

The purpose of a grounded conductor, the neutral, the white wire, is to provide a "return path" for current that begins at the breaker and travels thru the appliance and then back down the neutral to the transformer. It is a hot or current carrying wire.

You want the neutral to carry current, and you do not want the ground to carry current, thus you keep them separated. If anywhere after the one point where they are combined, at the service entrance, they again touch, then the ground could be carrying current, which is dangerous to persons using devices and appliances, and to anyone working on the system.

Charles

Thanks Charles. That makes sense. If I understand, one is removing the duplicate path, which if it was connected at the sub panel as a parallel return path, would be carrying roughly 1/2 the load so potential could exist. One wants that ground path available for 100% of any faults and no chance of potential on it.