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Old 11-11-2008, 09:34 AM   #1
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Service Panel Basic Questions


Given: A service panel rated at 200 amp max at 240 volts.

Question #1: I see five double pole (240v) breakers near the top. Though they are placed on the right or left side, these breakers pull from both legs. Thus, a double pole breaker with 50 amps per side will have leads from each side (120v per leg) and will have a max of 50 amps per leg or 100 amps total, right?

Question #2: Now, there is a double pole breaker that has "50" printed across the bar connecting the two switches. I assume this is a 50 amp total or 25 amp per leg breaker -- rather than a 50 amp per leg breaker, right?

The actual draw of my circuits is far less than the cumulative total of the rated amperages of the circuit breakers (which is 520 amps). I know this b/c I do not trip breakers.

Question #3: So, it really isn't important what the cumulative total of amperages is for the circuit breakers. What's important is the actual draw on the individual circuits and the draw on each of the 120v legs of the service panel. And as long as I do not exceed the rated amps of any individual circuit breaker, or the rated amps of a leg (200 amps) and as long as I do not melt a wire with excessive draw for that gauge of wire -- then I shouldn't have any problems, right?

Question #4: So, to add another 40 amp, double pole breaker (and circuit), I should calculate the actual draw (in watts and convert to amps) of my home to make sure I am not going to overload either leg, right?

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Old 11-11-2008, 10:01 AM   #2
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Service Panel Basic Questions


You are mistaken about the ratings of double pole breakers. The current on both legs of a circuit is equal, so each half of a double pole breaker is rated at 50 amps. If 50 A flows out of one side of the breaker, then 50 A is flowing into the other side.

A circuit is sized to the load, and the conductors are sized to the breaker. I don't think adding a 40 A circuit to your panel will cause any problems, because not everything in your home will be drawing at the same time.

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Old 11-11-2008, 10:37 AM   #3
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1. true mostly. 50 amp double pole is 50 amp. You don't say 100 amp total.
2. False. 50 on the handle is 50 amps per leg.
3. Correct. The breakers is what prevents the wires from melting.
4. You do what is called a demand load calculation. There are many online versions to help you out.
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Old 11-11-2008, 11:27 AM   #4
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Thanks guys for your quick and valuable responses. I found a good (I think) online calculator here:
http://www.zenfixit.com/load_calculations.shtml
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Old 11-11-2008, 11:37 AM   #5
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1. A double pole breaker typically serves, "from its point of view", a 240 volt load with current in amps up to one breaker pole's rating. For example it may be a 240 volt 50 amp load protected by a 50 amp. double pole breaker. But there could be 120 volt loads such as when the breaker protects a subpanel. So a 50 amp double pole breaker ends up serving loads cumulatively drawing up to 100 amps of power at 120 volts although it trips if more than 50 amps are drawn on one leg.

Last edited by AllanJ; 11-11-2008 at 11:40 AM.
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Old 11-11-2008, 11:46 AM   #6
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It might be useful to point out that 100 amps 120v is the same watts as 50 amps 240v.

I'm curious though what do you need 40 amps 240v for? That's a *lot* of power. Welder or something?
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Old 11-11-2008, 12:08 PM   #7
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You wll never draw more than 50 amps on each leg on a 50 amp two pole breaker...
On a multi wire branch circuit, the two hot wires are out of phase so that the neutral wire will never see more than 50 amps.
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Old 11-11-2008, 12:17 PM   #8
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RE: AllanJ and Gigs
That's what I was thinking -- great feedback.

I'm planning on installing a tankless water heater that requires 3x 40amp 240v breakers!!! and uses 26.85kW. I will eliminate the oil furnace which uses a 30amp 240v breaker so I get that as a start. I have a 20amp 240v breaker for a window unit AC that I don't use, so I can use that if I need to.

I'm looking to see the best way to end up with 120amps (3x40a 240v). That's a big draw on a 200 amp service panel, I know. Especially since it will be drawing when the dishwasher and laundry washer (and dryer) will be drawing as well.

It may be too much. I may need to downsize the tankless hot water heater. That's what I'm determining now -- how much tankless I can have on the existing service panel.
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Old 11-11-2008, 12:25 PM   #9
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Erg.

Just so you know, consumer reports says that electric tankless produce "generally unacceptable results" for most people's expectations.

And they don't save much money at all.

I think you could easily trip your main pulling 120 amps, heat pump and washer and well pump if you have one kick on, and you might well be over 200.
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Old 11-11-2008, 12:45 PM   #10
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I had to look-up "multiwire branch circuit"
From: http://www.electriciansparadise.com/articles.html

"The main concept embodied in this definition is that the ungrounded conductors have to have voltage between them; that is they cannot be connected to the same leg. If they were connected to the same leg, the apprentice's worst fear would be realized - the shared neutral's ampacity would soar. Most electrical mistakes resulting in overloads will trip out the circuit breaker providing warning that something is wrong. But a misfed multiwire branch circuit will silently heat the neutral until it either burns clear inside an enclosure or ignites flammable building material resulting in property loss or worse."

So, the key is the two ungrounded leads come from different legs, and they can then share a neutral wire between them b/c they are out of phase. Thus eliminating the need for a redundant 2nd neutral wire. Right?
Learning...

As for tankless... I have done some research and am still doing more. Thanks for your feedback. The key seems to be to get a powerful tankless that can raise the temp at the desired flow rate to 110 degrees. Cheaper/less powerful ones only do well at 1.5 gpm flows. The one I'm investigating can raise 50 degree incoming water to 115 degree at 3 gpm (see link below). That would work for me. The problem is -- can I figure a way to balance the loads. If not, I'll be looking for a smaller one... and may end up having to shower when I'm not washing dishes : )

My current system is an oil-fired water heater... and that era is gone. Looking to move into the future and that's tankless, as I see it.

http://www.boschhotwater.com/StartPa...5/Default.aspx
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Old 11-12-2008, 01:32 AM   #11
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Quote:
So, the key is the two ungrounded leads come from different legs, and they can then share a neutral wire between them b/c they are out of phase.
This is how a 2 pole breaker already is. Each side is a different leg, there is 240v between them, and no neutral current flows unless the load between the two hots is uneven.


Quote:
The key seems to be to get a powerful tankless that can raise the temp at the desired flow rate to 110 degrees
110 degrees at the heater is potentially 100 at the tap. That's barely hot enough to shower/do dishes. Have you tried setting your current heater to output 100 degrees water at the tap to see if you could even deal with that? It might be cooler than you think.

Quote:
Looking to move into the future and that's tankless, as I see it.
I don't see it that way at all. I think it's a fad and a marketing gimmick, especially whole house electric tankless. Tanked electric are very efficient, nearly as efficient as tankless without all the drawbacks (I saw someone say 95% vs 99% for tankless). Throw some split-foam insulation on any hot water pipes, and maybe an extra blanket on the heater, and you get a couple more percentage points. I hope you will reconsider.

Last edited by Gigs; 11-12-2008 at 03:35 PM.
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Old 11-12-2008, 09:17 AM   #12
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I've wired four or five of these exact models....which bosch has discontinued by the way. I personally would avoid them and look for a gas heated system if possible. These things are energy hogs and of those I installed I've been called back to install repair parts on more than one occassion in less than one year of operation. One unit that I know of is no longer in service.

You will need 3 independent 40 amp 240 volt branch circuits to the unit....so 3 double pole 40 amp breakers. Each circuit supplies one heater (3 total) in the unit. The units terminal block allows for the connection of the 6 hots (L1's and L2's). Minimum 8 awg copper supply conductors no aluminum. This being in the USA.

In Canada the unit as I remember can have one 125 amp branch circuit and then the three heaters a tapped from that inside the unit at the terminal block. the taps are most likely done by the factory but I'm not sure on that.

There is no reason to be talking about multiwires and shared neutrals unless your just discussing these as the unit is strictly 240 volts.

The unit will not support more than one appliance at a time so forget showering and running the washer (hot or warm cycles) or the dishwasher at the same time.

Last edited by Stubbie; 11-12-2008 at 09:32 AM. Reason: Added last paragraph
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Old 11-12-2008, 03:34 PM   #13
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Quote:
Originally Posted by Stubbie View Post
You will need 3 independent 40 amp 240 volt branch circuits to the unit....so 3 double pole 40 amp breakers. Each circuit supplies one heater (3 total) in the unit.
Thanks. It makes more sense now. I was wondering where he got the 3x40 idea from.
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Old 11-12-2008, 05:41 PM   #14
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Monkey, If you want tankless give up the electric idea and go with natural gas or propane. With the cost of the unit, installation and amount of electricity it consumes, you might never see a return of investment.
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Old 11-13-2008, 04:55 AM   #15
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Monkey did you get the ZEN load calculator to work. No matter what I put in it said 10.xxxx service amps

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