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240V circuit breaker amperage question

Posted by Vrtigo1 (My Page) on
Fri, Aug 26, 11 at 9:49

I was wondering how the amperage of 240V circuits works. I need to add a few new 240V circuits in my garage for tools and was not sure how CB labeled amperage differs from total circuit amperage and the amperage marked on the tool.

First, if you have a 240V double pole CB, for instance a CB that is labeled 20A, that means 20A on EACH of the hot legs, so the total circuit amperage would actually be 40A. Is that correct? I understand that folks say when voltage doubles, amperage is halved. Is that because you're dealing with two legs instead of only one?

The reason I ask about total circuit amperage is I am trying to determine what wire size I need to use and the chart I am looking at goes by length, voltage, conductor material and total circuit amperage, so I need to know if they is different from the amperage labeled on the CB.

Next, one of the tools is an air compressor, and it is labeled 15A on the motor. Is there any rule of thumb for sizing a CB other than match the label on the load? So in other words, if the motor is labeled 15A, I use a 15A CB?


Follow-Up Postings:

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RE: 240V circuit breaker amperage question

On each leg of 240V circuit, you have a 20A breaker. That means that the amperage of the circuit is 20A at 240V. The amperage is not additive.

In a simple world (say resistance heat), when the voltage applied is doubled, the amount of current is halved. It has nothing to do with the number of legs used in the power supply, but simply with the voltage that is being supplied. Look up Ohm's law on the web for a detailed explanation.

The circuit breaker protects the wire, so in almost all situations, the breaker and all the conducting wires in the circuit that it protects must match. So if a 20A breaker is installed, all wires in that circuit must be at least 12 gauge.

Your question about the compressor depends on what the manufacturer requires. If it requires a 15A circuit, then that's what you must install. If the compressor actually draws 15A while running, I'd install at least a 20A circuit so that I wasn't exceeding 80% of the circuit on a regular basis. The challenge is that motors typically require a significant amount of additional current when starting. So if your compressor requires 15A to run, it might require a 30A circuit to start. Let us know the make and model of the compressor, and we can probably figure out what you really need.


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RE: 240V circuit breaker amperage question

Ok, I understand what you're saying. The amperage of the 240V circuit on a 20A CB is 20A at 240V. If you measured each of the 120V legs, you would see 20A on each one, but that doesn't have any bearing because at 240V the amperage would be halved.

The compressor is a Coleman 60 gallon which doesn't have a model number. The 12 gauge wire size is where I am getting confused, because I get a lot of differing results looking at different guides that I found online. Is there a handbook I can buy or online reference where I would be able to find the code requirements in my area? Would they contain that info?

As to your point about the starting amperage, if I cannot find any info from the manufacturer, should I just start with a CB of the same amperage and progressively size it up if it trips when starting?


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RE: 240V circuit breaker amperage question

"Ok, I understand what you're saying. The amperage of the 240V circuit on a 20A CB is 20A at 240V. If you measured each of the 120V legs, you would see 20A on each one, but that doesn't have any bearing because at 240V the amperage would be halved. "

You are confusing what a load draws with what a breaker measures.

While a typical load that can operate at 120 V or 240 V draws half the current at the higher voltage, ALL the current is passing through EACH of the breakers in a two pole setup.

The breakers are NOT sensitive to the voltage (beyond they must be deigned to open correctly and not fail) but ONLY to the current passing through them.

Since the current in a 240 V circuit passes through BOTH of the breakers, a 20 amp, 2 pole breaker will only allow 20 amps to pass.

If you want to think in DC battery terms, a 240 V circuit is the same as having two batteries stacked up.
You can get the battery voltage from each one, and twice the voltage from the stack.
The current is the same through any circuit loading the batteries (one or two) but the voltage available is twice as much across both compared to one.

What the Edison circuit we use in the US does is make the point between the two sources 0 V and ground it.

For batteries you would now have a negative voltage on one end of the stack (relative to the middle) and a positive voltage at the other end (again, relative to the middle).

This allows us to have the benefits of 240 V for larger loads, but not have a voltage that is greater that 120 V to ground.


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RE: 240V circuit breaker amperage question

"Ok, I understand what you're saying. The amperage of the 240V circuit on a 20A CB is 20A at 240V. If you measured each of the 120V legs, you would see 20A on each one, but that doesn't have any bearing because at 240V the amperage would be halved. "

I think you have to toss this "would be halved" idea out to the curb. Any of your tools will be designed to work at a given voltage. If you double the voltage, you don't just half the currrent - you fry the tool!

"As to your point about the starting amperage, if I cannot find any info from the manufacturer, should I just start with a CB of the same amperage and progressively size it up if it trips when starting?"

You can't do that. The wire needs to be rated for the breaker size. Once you install the wire, putting in a larger breaker than it is rated for is a fire hazard.


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RE: 240V circuit breaker amperage question

Another way to think of it is that a 240V circuit is two 120V circuits end-to-end. The voltage adds up, but the current does not.

Most places in the US follow the National Electrical Code, which is published by the National Fire Protection Association. You'll need to check with your local government to see which version of the NEC is in place (it changes every few years), and if there are any local modifications.

Generally, the rules for which wire size to use are governed by the maximum amount of current expected to flow. So, if you need a 15A circuit, use 14 gauge wire, and if you need a 20A circuit, use 12 gauge wire.

If you change the breaker, you need to change the wire. So you can't just arbitrarily "size it up" if the breaker trips.

Are there no labels on this compressor? The only Coleman brand compressor that I see with a 60 gallon tank is a model CL7006016, which has a running rating of 3.1 horsepower, which translates to about 9.7A. According to the user manual for that compressor, a 15A circuit should be installed (and dedicated) to the compressor.

Here is a link that might be useful: Powermate user manual


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RE: 240V circuit breaker amperage question

This forum software confuses me, so I'll just reply to everyone at once.

"Are there no labels on this compressor?"

Yes, it's labeled 15A. I thought I included that but may have forgotten it. Thanks for the info.

"If you change the breaker, you need to change the wire."

Right, I understand that. What I should've said is install wire capable of carrying 30A and start by installing a 15A breaker, then size up if needed.

I'd planned on going a wire size up once I determine what the size I need is anyway, since from what I've gathered there is no reason not to do that, and if I ever need to repurpose the circuit, I can install a larger breaker if needed.

As far as the amps question, I think I understand the answers. I guess what I wasn't getting is that measuring the two legs of a 240V circuit independently wouldn't mean anything because without both legs together, it isn't a 240V circuit.

What I was asking is akin to if you installed two 20A single pole breakers right above/below one another and connected each to a 120V receptacle, you would have two 20A circuits, so total 40A. If you connected both of those to a 240V receptacle, then you would have a 240V receptacle, but the amperage would still only be 20A since the voltage has changed. I know that wouldn't be safe because 240V circuits breakers need to be tied together so if one trips they both trip, and I'm probably not wording that right, but that's what my original question was about. I just didn't get that the amperage doesn't add when you have two 20A breakers because the voltage also changes. It's my understanding that a 240V breaker is no different than two single pole breakers except for the bar that ties them together so they trip together, so my analogy seemed reasonable to me.

As far as your answer regarding wire size being governed by current, I understand that, but what I was saying is that I see multiple different wire sizes for the same voltage, current, distance and conductor material when I look at different charts. When you say a 14A circuit needs 14 gauge wire, I assume that could change depending on if the length of the circuit is 25' vs 250'?

Maybe it would be easier if I just tell you exactly what I have.

1) Compressor - labeled 15A on the motor. Wire distance from load center about 35'.

2) Table saw - Delta Unisaw model 36-829. Wire distance to receptacle, approx 45', with a 10' 12 gauge extension cord from receptacle to tool. The manual says "A separate electrical circuit should be used for your machine. This circuit should not be less than #12 wire and should
be protected with a 20 Amp time lag fuse"

3) Band saw - Grizzly model # G0513X2. Specifications say it draws 10A at 220V or 20A at 110V. I have mine wired for 220V. The wire distance from the load center is approx 35'.

These three items are what I am trying to determine wire size for.


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RE: 240V circuit breaker amperage question

""As to your point about the starting amperage, if I cannot find any info from the manufacturer, should I just start with a CB of the same amperage and progressively size it up if it trips when starting?"

You can't do that. The wire needs to be rated for the breaker size. Once you install the wire, putting in a larger breaker than it is rated for is a fire hazard."

As a matter of fact, you can do that.

Article 430 covers motor circuits.

Larger motors have thermal overload protection.
They cannot draw excessive current without shutting themselves off from overheating.

The breaker in a motor only circuit is only providing short circuit protection to the feeders, NOT overload protection.

You are allowed to increase the size of the breaker.

The wire size is based on 125% of the motors full load running current.

An inverse time breaker can be 350% of the motor full load current to allow starting, without changing the wire size.


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RE: 240V circuit breaker amperage question

You need 20 amp, 240v circuits for each tool using 12 ga. wire. In theory you could run the bandsaw and compressor off of lesser circuits but you might be hard pressed to find 15 amp double pole breakers and without looking there might be some prohibition against it.

In any case, keeping everything at 20 amps will give you flexibility should you find the need to rearrange your tools.


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RE: 240V circuit breaker amperage question

"might be hard pressed to find 15 amp double pole breakers and without looking there might be some prohibition against it. "

Perfectly acceptable and available.

The big box may not have them, but an actual electrical supply house should.


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RE: 240V circuit breaker amperage question

Using a Fluke "true RMS" clamp-on amp meter I want to know if I need to measure both legs (120V of a single phase 240v)for the inrush current. I know that for the Full running Amps or steady state current I have to measure both legs and add. My readings for steady state match the compressor specs. My readings for inrush on one leg match the LRA numbers of the compressor. I have seen one reference to inrush and only one leg, but would like another source to confirm.


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RE: 240V circuit breaker amperage question

It is best to create a new thread instead of piggybacking on an old one that is more than 2 years old.
For single phase 120 or 240 volts (not 120/240), checking just one conductor shows the full circuit amperage.

"I know that for the Full running Amps or steady state current I have to measure both legs and add"
What is your source for that claim?

This post was edited by bus_driver on Sun, Sep 1, 13 at 20:37


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RE: 240V circuit breaker amperage question

Using Ohm's Law ...When voltage is doubled (from 120 volts to 240 volts), and the impedance or resistance (ohms) of the load was not changed, the current (amps) through the load will double, and thus the wattage of the load is quadruple. In order to maintain the same wattage as before, the impedance (resistance) of the load will have to be increase four times the amount.


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RE: 240V circuit breaker amperage question

240 volts circuits use no neutral. 240 volts current travels through all the transformer windings in the secondary side of the single phase step-down transformer. The 120 volts circuits uses the center tap of the transformer windings and travel through only half the windings in the transformer, thus having only half the voltage. Which half of the windings in the transformer it travels on depend on which bus bar the circuit breaker is mounted on in the circuit breaker panel.


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RE: 240V circuit breaker amperage question

Oh my bad...I didn't know I was responding to a thread that was two years old. Teach me to not stay up too late on Labor Day weekends.


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