Electrical question.

[GLENN THOMAS]

Hi folks.

I usually hang out in the turners forum but look in here form time to time to see what folks are doing with there shops. This looks like a good place to post a question that I cant find an answer to anywhere else.

If you have for example a 60 amp sub feed does that mean that each hot wire coming into the panel could in theory deliver 60 amps for a total of 120 amps or would each hot only be capable a delivering 30 amps for a total of 60. I have a good understanding of how voltage and amps would work on a circuit for wired for 120 but not for 240. A friend of mine and I were discussing this the other day and no one seems to have an answer. Its really just a matter of curiosity more than anything.

Thanks in advance

GT

[Bryan Wellman]

Each leg is 60 amps. You want to try and balance your load in the panel though. In theory you want to have say 40 amps on one leg and 40 on the other. Everything works best in a balanced situation.

[George Bregar]

Each leg delivers 60 amps. Only the voltage is doubled.

[Anthony Whitesell]

Interesting. I never thought of this.

So a 60A subpanel can handle 120A @ 120V or 60A @ 240V? (this is a simple example where everything from the subpanel uses the same voltage. I realize this isn't likely or feasible, but I just want to make sure I understand the concept)

[Bob Coleman]

In a circuit, the current is the same everywhere. In a 120V circuit, your device is connected to one hot leg (hot1 = 120V) and to neutral (neutral = 0V). In a 240V circuit, the device is connected to both hot legs (hot1 = 120V, hot2 = -120V) giving you a voltage difference of 240V.

In 120V, current flows from hot1 (or hot2) to neutral. For a 60A panel, you are limited to 60A in the neutral, meaning that you could have 60A in one hot leg and 0A in the other, or 30A in both. Even though both feeders will be rated for 60A, you can't use 120A since the neutral will only be rated for 60A. Ideally you want the load balanced between legs.

In 240V, current flows from one hot to the other and are necessarily the same, so you are limited to 60A by the panel size.

For more detail and pictures, visit Cliff Holmes excellent article: www thewoodnerd com/articles/circuitSizing.html.

[Bruce King]

In a circuit, the current is the same everywhere. In a 120V circuit, your device is connected to one hot leg (hot1 = 120V) and to neutral (neutral = 0V). In a 240V circuit, the device is connected to both hot legs (hot1 = 120V, hot2 = -120V) giving you a voltage difference of 240V.

Correct but....

In 120V, current flows from hot1 (or hot2) to neutral. For a 60A panel, you are limited to 60A in the neutral, meaning that you could have 60A in one hot leg and 0A in the other, or 30A in both. Even though both feeders will be rated for 60A, you can't use 120A since the neutral will only be rated for 60A. Ideally you want the load balanced between legs.

No, the neutral current is only the unbalanced amount, for example if you have all 120V loads with 40 amps on leg 1 and 60 amps on leg 2 the neutral current will only be 20 amps. You can load each leg to 60 amps if you want with all 120V loads. The neutral current on the service cable will be almost zero with 60 amps on each leg. This is the beauty of split phase 120/240 systems. It is incorrectly referred to as two phase usually by industrial types that have always heard it called that. 120/240 is single phase or can be called split phase due to the 180 degree phase shift used. You can get 120v x 120 amps = 14,400 watts out of the 60 amp panel if only 120V loads are connected.


In 240V, current flows from one hot to the other and are necessarily the same, so you are limited to 60A by the panel size.


Correct, you can get 240V x 60 amps = 14,400 watts of power. notice how it is the same power capability of the 120V load example above.



For more detail and pictures, visit Cliff Holmes excellent article: http://www.thewoodnerd.com/articles/circuitSizing.html

I did not read the above article, I hope it did not have the mistake in it.

[Bryan Wellman]

In 120V, current flows from hot1 (or hot2) to neutral. For a 60A panel, you are limited to 60A in the neutral, meaning that you could have 60A in one hot leg and 0A in the other, or 30A in both. Even though both feeders will be rated for 60A, you can't use 120A since the neutral will only be rated for 60A. Ideally you want the load balanced between legs.



Not exactly.

The neutral carries the unbalanced load. This is very different than the entire load. What I had posted previously touched on this a bit.

Say you have a 120v dustcollector running and is pulling 14A. At the same time your table saw is running at 12A. If they are on opposite phases, you will only have 2A on the neutral in the panel. If they were on the same phase, you would have 26A on the neutral at the panel. The neutral picks up the unbalanced load. It is physically impossible therefore to have 120A on your neutral, because your main breaker will only allow 60A to ground/neutral per leg.

This is why trying to balance your panel is important. Try and plan out what loads will be running at the same time frequently and try and get them on opposite phases.

Now....about the 60A main. All breakers are rated to handle 80% of a load consistantly. That being said, in this application, you could conceivably run 48A constant on either phase of the panel without tripping the main. It will hold up to 60A for a period of time, but you do run the risk of tripping the main. (Not likely, but possible). Anything over 60A per phase and it should trip in short time.



Here is a quick article on neutrals....

http://ecmweb.com/nec/code-basics/electric_code_basics_14/

[Bryan Wellman]

Sorry Bruce......

You hit it before I did and much better!! I started replying as soon as I read Mr. Coleman's post.

[Roger Frazee]

I like to explain this using an example that I call a maximum power equation.

If I have a 60 amp 120/240 volt feeder to a sub-panel and that feeder is protected by a 60 amp double pole breaker in another panel, then I can have 60A either 120 volts or 240 volts or a combination of both flowing on both hot legs of that feeder. Either hot leg exceeds 60A the feeder breaker will trip.

The feeder circuit breaker controls the available power to the sub-panel which is 240 x 60A = 14,400 watts. So If I install a 60 amp 240 volt double pole breaker in the sub-panel to power a 240 volt 60 amp load then I have equaled the maximum power allowed by the feeder breaker. I will have 60 Amps on each hot leg and the heater will be consuming 14,400 watts of power.

Now I want to power that same load with 120 volts because it says on the data plate I can rewire at 120 volts. Now I need a single pole breaker to make this happen. I can't change the power it remains 14,400 watts. For example a 2hp motor is a 2hp motor regardless whether I supply it with 120 volts or 240 volts.

I now need to determine how big of a single pole breaker I need to allow the use of 14,400 watts so the equipment can operate at its intended design. Using the power formula (Volt)(AMPS) = Watts I get 120(?) = 14,400 solving that I get 120 amps. I need a 120 amp single pole breaker.

The point being it takes 60 amps at 240 volts to deliver 14,400 watts and 120 amps at 120 volts to deliver the same power.

We know that the sub-panel utilizes two hot legs serving two buses in the panel and that single pole breakers utilize one or the other of those hot legs but not both while 240 volt breakers utilize both hot legs. We also know that 60A per hot leg is the maximum allowed by the feeder breaker. So single pole breakers installed on one hot leg of the sub-panel cannot serve more than 60 amps of load or 120V x 60A = 7200 watts or half that of a 240 volt circuit. The same with the other hot leg. Either leg serving over 7,200 watts of power at 120 volts (60A) the breaker trips protecting the feeder. So both legs together could serve 14,400 watts if supplying 120 volt loads only but it must be balanced over both hot legs.

So comparing ... one hot leg at 240 volts can power 14,400 watts or 60 amps. One hot leg at 120 volts can power 7,200 watts or 60 amps. Either example will trip the 60 amp double pole breaker protecting the feeder if those power limits are exceeded on either hot leg.

So thinking of it with both 120 and 240 volt loads operating at the sub-panel ... If install a 40 amp double pole breaker serving a 35 amp load and 2 single pole breakers utilizing the same hot leg and together these serve 25 amps of load. I have 35 amps + 25 amps on the same leg or 60 amps total and my feeder is maxed out.

Clear as mud ..... :)

[GLENN THOMAS]

Just want to thank everyone for taking the time to respond.

So, the best way for me to wrap my little pea brain around this, and correct me if Im wrong, is to think in terms of watts. So in a 60 amp sub feed I can get a max of 14400 watts and it doesnt matter if Im using 120 volts at 120 amps or 240 volts at 60 amp. So I guess if you had a panel configured the right way and your circuits were properly wired you could run 120 amps worth of power tools and other goodies as long as they were all 120 volts, or another way to say it is 60 amps worth of tools on one leg and 60 amps worth on the other. And also if the load on both legs were exactly the same the current in the neutral would be zero. Do I understand this correctly or not.

Thanks
GT

[Bruce King]

Thats right, but remember, you can't actually measure 120 amps anywhere when you are drawing 60 amps on each leg. AC waveforms are like a push-pull and take advantage of opposite peaks so even when operating six 20 amp loads at the same time (three x 20 on each leg) you will only be able to measure 60 amps on any one service feed wire.

[Anthony Whitesell]

From a group of master electrician's I got the following answer (and a demo/tour). The answer is yes. Each hot in a 60 amp subpanel carries 60 amps. Therefore a 60 amp subpanel wired with only 120V circuits can provide 120amps. BUT if the fewest circuits you could have is 2; each on a 60amp breaker. The same 60 amp subpanel wired with only 240V circuits can only supply 60 amps with the fewest circuits being one and on a 60 amp breaker.

Their (the electricians) suggestion was to "derate" the capacity of the subpanel by the 240V circuit requirements first, the remainder of which is half the about of 120V circuits available within the panel.