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Discussion Starter · #1 ·
Two questions.

The first: The dryer I'm looking at draws 43a at 240v. There are two hot wires and a ground. Does the dryer draw 43a @ 120v or does it draw 21.5a @ 120v from each hot wire?

Second question: All I want to know is the conditions under which a main circuit breaker trips. Assuming I have a 100a main...

1. If I have a three, two pole appliances (240v) drawing 30a, 20a, and 60a respectively then my breaker trips because I'm drawing 110a. Correct?

2. If I have three single pole appliances (120v) drawing 30a, 20a, and 60a respectively then my breaker DOES NOT trip, so long as the appliances are spread across two different bus bars. I am not drawing more than 100a from either bus, so the main breaker does not trip.

3. If I have one two pole (240v) appliance drawing 60a, a single pole (120v) appliance on bus bar A drawing 40a, and a single pole (120v) appliance on bus bar B drawing 50a my bus bar trips because I'm drawing greater than 100a
 

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Call an electrician. This forum is for electrical pros only. Because of liability we cannot answer your questions. I'm sure you understand.
 

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Discussion Starter · #3 ·
This is a theorhetical question. I am not a home owner, I do not have a dryer, I am not working in my panel.

I work for a general contractor. All of the electricians I have contacted have given me B.S rule of thumb answers and can't tell me what's what.
 

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This is a theorhetical question. I am not a home owner, I do not have a dryer, I am not working in my panel.

I work for a general contractor. All of the electricians I have contacted have given me B.S rule of thumb answers and can't tell me what's what.
It's good to wonder but not very likely that short answers would help your understanding, a better method would be to take some very basic classes either night school or correspondence course

The 43 amps is on both legs, not 21.5 each, and since alternating current, opposite

Adding up numbers on breaker handles won't give correct answer
 

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You are trying to work out what a breaker panel does but you have never googled what a breaker is and how it works.

A breaker controls the max load. A 20 amp breaker with only one amp of load is 1amp.
A breaker is to protect the wire not the load.
A breaker has a trip curve you need to understand how that affects its action

Once you understand this the rest makes way more sense.
 

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Discussion Starter · #7 ·
You are trying to work out what a breaker panel does but you have never googled what a breaker is and how it works.

A breaker controls the max load. A 20 amp breaker with only one amp of load is 1amp.
A breaker is to protect the wire not the load.
A breaker has a trip curve you need to understand how that affects its action

Once you understand this the rest makes way more sense.
NO. I am saying, in this theoretical scenario, the appliances are actually drawing those loads. I understand how a circuit breaker works. You can have as many 20a breakers on a panel as you'd like, it doesn't determine anything about how much the circuits are actually drawing.

What I'm asking is: IF you have a 100a main breaker, can you draw 100a @ 120v from each bus bar before the breaker trips? This would mean 200a @ 120v because there are two bus bars. This mathematically makes sense because it's the same amount of energy as 100a @ 240v.
 

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NO. I am saying, in this theoretical scenario, the appliances are actually drawing those loads. I understand how a circuit breaker works. You can have as many 20a breakers on a panel as you'd like, it doesn't determine anything about how much the circuits are actually drawing.

What I'm asking is: IF you have a 100a main breaker, can you draw 100a @ 120v from each bus bar before the breaker trips? This would mean 200a @ 120v because there are two bus bars. This mathematically makes sense because it's the same amount of energy as 100a @ 240v.
Yes, theoretically you could draw 100 amps from each leg because that is electrically equivalent to draw 100 amps at 240. The transformer feeding the panel, on the other hand, may not be able to keep up in reality.
 

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I will bet many electricians have difficulty doing load calculations based on the NEC. We all have a good working knowledge of what will work and what won't. Load calculations can be tricky unless you know the exact application and who is asking the question.
 

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Two questions.

The first: The dryer I'm looking at draws 43a at 240v. There are two hot wires and a ground. Does the dryer draw 43a @ 120v or does it draw 21.5a @ 120v from each hot wire?

Second question: All I want to know is the conditions under which a main circuit breaker trips. Assuming I have a 100a main...

1. If I have a three, two pole appliances (240v) drawing 30a, 20a, and 60a respectively then my breaker trips because I'm drawing 110a. Correct?

2. If I have three single pole appliances (120v) drawing 30a, 20a, and 60a respectively then my breaker DOES NOT trip, so long as the appliances are spread across two different bus bars. I am not drawing more than 100a from either bus, so the main breaker does not trip.

3. If I have one two pole (240v) appliance drawing 60a, a single pole (120v) appliance on bus bar A drawing 40a, and a single pole (120v) appliance on bus bar B drawing 50a my bus bar trips because I'm drawing greater than 100a
A 100 ampere circuit breaker could trip out with a dead short on a 15 amp circuit. This is where the electrician comes in and is suppose to design a complete system with selective co-ordination. I have tripped out a 200 ampere MCB when I shorted out a 20 amp kitchen circuit. Square D QO type. I am sure there are electricians here who can get into details.
 

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Discussion Starter · #11 ·
Yes, theoretically you could draw 100 amps from each leg because that is electrically equivalent to draw 100 amps at 240. The transformer feeding the panel, on the other hand, may not be able to keep up in reality.
Thank you for the insight. I'm focusing on the main circuit breaker, but it's good to think more broadly.
 

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NO. I am saying, in this theoretical scenario, the appliances are actually drawing those loads. I understand how a circuit breaker works. You can have as many 20a breakers on a panel as you'd like, it doesn't determine anything about how much the circuits are actually drawing.

What I'm asking is: IF you have a 100a main breaker, can you draw 100a @ 120v from each bus bar before the breaker trips? This would mean 200a @ 120v because there are two bus bars. This mathematically makes sense because it's the same amount of energy as 100a @ 240v.
In a real life situation the breaker may trip earlier than 100 amps on each leg or it may not trip even if you have 110 amps on each leg continuous.

Ambient temperature, load verses time and a bunch of other things affect when a breaker will trip (basic magnetic thermal breaker).

When ever we start a motor we are probably going to see a amp spike that exceeds the breaker rating for a short period of time. In this scenario the breaker will not trip even though its overloaded according to the handle.
 

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As said look into trip curves. It will surprise you how much over current can be carried for how long.

Example a BR breaker can go 4 hour at 125% load, before it trips and it may never trip.
 

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This is a theorhetical question. I am not a home owner, I do not have a dryer, I am not working in my panel.

I work for a general contractor. All of the electricians I have contacted have given me B.S rule of thumb answers and can't tell me what's what.
I use rules of thumb all the time.
 

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OP thank you for filling out your profile. You really need to talk to your GC's electrical contractor.
btw... Can I add an addition to my house?
 
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