rewire table saw

[Josiah Bartlett]

Another EE here, although I design microelectronics for a living...

We get into a terminology quagmire when discussing mains AC in the US. Much of it doesn't apply to other power distribution systems.

The way a house (or small shop) is wired in the US is generally single-phase AC. There is a transformer up on the pole. It takes the the AC on the power companies' distribution network and transforms it down to 240 V RMS AC. (RMS means Root-mean-square, its a mathematical transform used to make Ohm's law work with AC systems). So how do we ever get 120V out of that? It's rather clever, really. There is a center tap on the transformer, which is bonded to a ground rod. This gives us the availability of either 120 or 240 volts, depending on how we wire the circuit. The "Neutral" is the center tap, bonded to ground at the panel. The "Red" hot is one side of the transformer, and the "black" hot is the other. The red and black are out of phase with each other with respect to the neutral, which is how they become 240V and not zero when you connect a load across them. Note that this does not apply in certain parts of old urban cities, which use an actual 2-phase (120 degrees out) 120V system to get 208V rms and don't offer 240V.

Unless something is really wrong with your equipment, all the current that flows in has to flow out. AC stands for alternating current, so it reverses direction 120 times per second. Thus, "hot" means its driven by the generator and "neutral" means it goes back to the generator. The neutral current can get to be pretty significant in a shop, given all the 120v tools that can be turned on. Depending on how the circuits are balanced between the red and black hot, the neutral at the panel isn't necessarily exactly ground, especially if the earth ground is poor or the subpanel is far from the main. Therefore you can get some variations in supply voltages. Wiring as many tools to 240V as you can removes the ground from the picture (other than safety) and provides more margin in a somewhat marginal panel.

In my opinion, 200A panels are a bit undersized for most modern homes. I have a 50A air conditioner, an electric dryer, an RV hookup, and an electric range. Many people also have electric hot water heaters. This doesn't leave much margin for running 2 or 3 horsepower motors.

[Chris Friesen]

In my opinion, 200A panels are a bit undersized for most modern homes. I have a 50A air conditioner, an electric dryer, an RV hookup, and an electric range. Many people also have electric hot water heaters. This doesn't leave much margin for running 2 or 3 horsepower motors.

Careful when you talk about "most modern homes"...

The majority of people in north america do not have 50A A/C units or RV hookups. I live in the Canadian prairies, and up here its very common to see 60A panels on older houses, and even most new houses only have 150A.

I have a 100A panel in my house, and I don't anticipate any problems unless I ever upgrade to a 5HP dust collector.

[Pete Bradley]

[quote=[B]Matt Meiser[/b]]
No one has mentioned that many Delta contractor saws have an extra winding. When wired 110, they hare a 1.5HP motor. When wired 220, they have a 2HP motor. ]

Nobody has mentioned it because it is not true. The motor in question was a 2 hp motor, and as such, is too large to run from a 15 amp circuit at 120 volts. So the motor was derated for the 120 volt configuration in order to get past UL requirements.

Rick,
Thanks for finally explaining where the "extra winding" urban legend came from. I saw the a similar reference on SMC a few weeks ago, but associated with Craftsman.

I'm curious , did they actually do anything to limit the draw at 110 or just change the rating? I recognize that unless it was loaded the motor wouldn't draw the full load amps anyway.

Pete

[Mike Seals]

Another EE here, although I design microelectronics for a living...

We get into a terminology quagmire when discussing mains AC in the US. Much of it doesn't apply to other power distribution systems.

The way a house (or small shop) is wired in the US is generally single-phase AC. There is a transformer up on the pole. It takes the the AC on the power companies' distribution network and transforms it down to 240 V RMS AC. (RMS means Root-mean-square, its a mathematical transform used to make Ohm's law work with AC systems). So how do we ever get 120V out of that? It's rather clever, really. There is a center tap on the transformer, which is bonded to a ground rod. This gives us the availability of either 120 or 240 volts, depending on how we wire the circuit. The "Neutral" is the center tap, bonded to ground at the panel. The "Red" hot is one side of the transformer, and the "black" hot is the other. The red and black are out of phase with each other with respect to the neutral, which is how they become 240V and not zero when you connect a load across them. Note that this does not apply in certain parts of old urban cities, which use an actual 2-phase (120 degrees out) 120V system to get 208V rms and don't offer 240V.

.

Thanks for the post, but I wonder about the 240 part. Most commonly we use 7200VAC for the input to our residential supplies, That's 7200VAC to ground. The output is 240VAC across the taps of the transformer. The 240 would be floating, but it would be 240 none the less. Is that correct?

Now add the center tap and you get a reference to ground. Looking at one half cycle across the whole of the transformer, one tap of the transformer would be positive and the other negative. With the center tap referenced to ground, would it not be that one side of the transformer is going to be negative in respect to ground and the other positive in respect. The next half cycle we change polarity.

[Art Mann]

Amperage is only "halved" on an individual hot conductor. The same total current is in play, just split over two conductors...I'm only mentioning this to clarify.

I stand corrected, Rick. I forgot about the phase relationship between the two legs of the 240v circuit. Thanks for the reminder..

There is an important sense in which Jim was correct, whether he still believes it or not. I will illustrate with a hypothetical example. Suppose you have a shop with a 50A, 240V sub panel. Further suppose you have a tablesaw that draws 20A @120V that keeps popping your breaker. By using a clamp on current probe, you observe that 20A is flowing in one leg and 0A is flowing in the other. You have reduced the available capacity in one one leg by 20A and 0A in the other. Now, rewire your saw for 240V operation and fire it up. Use the same current probe and you will measure 10A in one leg and 10A in the other. It doesn't matter that in the second case, the same electrons are flowing in both legs. From all outward appearances, what you have done is split the original 20A in one leg into half of the current in each of the two 120V legs and reduced their individual capacities accordingly. That is the point I thought Jim was trying to make. I don't need to read a text on AC circuits to know this is true.

I have said all this because I have seen the claim on this board that you can double your power capacity at the service entrance by running your tools on 240V, which isn't true.

[Greg Funk]

There is an important sense in which Jim was correct, whether he still believes it or not. I will illustrate with a hypothetical example. Suppose you have a shop with a 50A, 240V sub panel. Further suppose you have a tablesaw that draws 20A @120V that keeps popping your breaker. By using a clamp on current probe, you observe that 20A is flowing in one leg and 0A is flowing in the other. You have reduced the available capacity in one one leg by 20A and 0A in the other. Now, rewire your saw for 240V operation and fire it up. Use the same current probe and you will measure 10A in one leg and 10A in the other. It doesn't matter that in the second case, the same electrons are flowing in both legs. From all outward appearances, what you have done is split the original 20A in one leg into half of the current in each of the two 120V legs and reduced their individual capacities accordingly. That is the point I thought Jim was trying to make. I don't need to read a text on AC circuits to know this is true.

I have said all this because I have seen the claim on this board that you can double your power capacity at the service entrance by running your tools on 240V, which isn't true.

Your comments are correct if you assume that all of the 110V equipment in your house is balanced and divided equally on both phases. There is, of course, no guarantee that this will be the case particularly if you have a small service and a few large motors. In pretty well all cases you will have more power available if you wire all major power consumers with 220V.

Greg

[Josiah Bartlett]

Careful when you talk about "most modern homes"...

The majority of people in north america do not have 50A A/C units or RV hookups. I live in the Canadian prairies, and up here its very common to see 60A panels on older houses, and even most new houses only have 150A.

I have a 100A panel in my house, and I don't anticipate any problems unless I ever upgrade to a 5HP dust collector.

I'm guessing you have either natural gas or an oil furnace. In the Pacific NW, its common for homes to have electricity be the only energy source. My power costs me 4.9c/kwh.

[Art Mann]

Greg,

I don't understand what you are saying. The effects of one load - in this case the saw - are simply added to the effects of other loads. Whether the loads on the two legs are balanced doesn't matter. Perhaps you are talking about current in the neutral wire? Can you provide an example? Maybe you ae referring to the current problem i have, which is that I only have a 50A subpanel and need to balance the load on the two legs carefully?

[Bryan Rocker]

Lots of good points here. I am glad the smart people pointed out the right way to do things. I always learn lots of useful information here.

I have a different take on things. The whole point of 220 is to buy bigger and more powerful 220 powered equipment. To me it means my 3 HP X5 saw runs like a champ. My old 110 saw never ever had the power of my X5 and it was a 15 amp motor. There is no way my 3 HP X5 could run on 110........More power....bigger tools.....that's what we all want.......

He who dies with the most tools wins!

Bryan

[Greg Funk]

Greg,

I don't understand what you are saying. The effects of one load - in this case the saw - are simply added to the effects of other loads. Whether the loads on the two legs are balanced doesn't matter. Perhaps you are talking about current in the neutral wire? Can you provide an example? Maybe you ae referring to the current problem i have, which is that I only have a 50A subpanel and need to balance the load on the two legs carefully?

A 50A subpanel should be able to support a total load of 12kW (50A x 240V). The actual load is likely less due to some derating factors which an electrician would be familiar with. The maximum one phase can support is half this or 6kW so if you want to take full advantage of your subpanel you would need to ensure that the loads are evenly distributed.

If you had a 110V air conditioner running along with a 110V dust collector and then you wanted to start a 1 1/2 HP tablesaw and they were all connected to one side of the service you would be drawing significantly more than 50A while the saw started. Depending on how far your subpanel was from the main panel you may notice a reasonable voltage drop at your saw and it would start a little slower. In any case you would be operating at the design threshold of your service. On the other hand if all three pieces of equipment were running on 220V you would only running at less than 50% of the design threshold and your saw would start quicker.

In the above scenario you would be fine if you had balanced the 110V equipment as best as possible but with a small service you need to make a little effort to determine which side each piece of equipment is connected to. With a 200A service it's not so critical since there is usually a lot more margin and any large loads like air conditioners, dryers and ovens would already be on 240V.

Hopefully this makes sense.

Greg

[Art Mann]

What you say makes sense, but I still don't understand what you are disagreeing with me about. Apparently, you are reading more into my little example than what I put there. What I said about dividing the current between two legs is true whether the load is balanced or not.

[Greg Funk]

What you say makes sense, but I still don't understand what you are disagreeing with me about. Apparently, you are reading more into my little example than what I put there. What I said about dividing the current between two legs is true whether the load is balanced or not.

You indicated earlier that you couldn't double your capacity by re-wiring everything for 220. My point is that you will generally increase your capacity if your equipment is wired for 220 and in the case where your 110V equipment is badly balanced (i.e. all on one leg) you will double your capacity.

Greg

[Rick Christopherson]

You indicated earlier that you couldn't double your capacity by re-wiring everything for 220. My point is that you will generally increase your capacity if your equipment is wired for 220 and in the case where your 110V equipment is badly balanced (i.e. all on one leg) you will double your capacity.

Greg

You are not increasing your overall capacity, all you are doing is forcing the loads to be balanced between the incomming phases. Aside from this clarification, I believe (without reading everything) that everything else you stated is correct.

The same results are also achieved by simply balancing your 120 volt loads. Most importantly, is that you and other readers of this discussion, understand that this is really only applicable when you have a very small subpanel.

By the way, many people believe that all of the circuits on the right-side of the panel are on one phase, and all of the circuits on the left are on the other phase. This is not correct. Every other breaker position (vertically) alternates between the two incoming phases. While I am sure you already knew this, I am sure that other people reading this discussion may not have known this.

[Greg Funk]

You are not increasing your overall capacity, all you are doing is forcing the loads to be balanced between the incomming phases.

Agreed, but I suspect there are a reasonable number of home/shop owners who may not know to balance their loads and if they are running a number of high current 110V machines off a small subpanel could end up with a perceived diminished capacity.

Greg