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 1 
 on: February 22, 2018, 08:55:12 PM 
Started by 357Smith - Last post by PVAndy
You are saying, then, that the answer given to the second question on the waterheatertimer.org page is actually incorrect - i.e. (ignoring appropriate derating for code, for the moment) that a '200A' split phase panel is designed to deal with loads drawing up to 400 A at 120 V, and that the '200A' rating refers to its load capacity at 240 V?

Yes what you said above is correct.

Andy

 2 
 on: February 21, 2018, 08:58:50 PM 
Started by 357Smith - Last post by pfletch101
You are saying, then, that the answer given to the second question on the waterheatertimer.org page is actually incorrect - i.e. (ignoring appropriate derating for code, for the moment) that a '200A' split phase panel is designed to deal with loads drawing up to 400 A at 120 V, and that the '200A' rating refers to its load capacity at 240 V?

 3 
 on: February 21, 2018, 08:31:38 PM 
Started by 357Smith - Last post by PVAndy
From the point of view of CT sizing, your guideline probably makes sense, even assuming that PVAndy is correct, though certain large loads may reasonably and predictably draw more than 50% of the rated current of the circuit they are on (I am thinking of HVAC systems and electric stoves and/or ovens).

In terms of the rating of double breakers, the waterheatertimer.org link that you cite does seem more definitive, but there is one remaining inconsistency that had already struck me, and that turns up explicitly further down on that page, where there is another question and answer:
Quote
Question:
If you load a panel up with only single pole breakers can you put 200 amps on each leg?
Answer: No.

200 amps on each leg would be 400 total amps. If you draw 400 Amps, the main breaker will trip. {emphasis mine} A 200 Amp 120/240Volt panel has maximum 200 Amp, no matter if loads are 120Volts or 240Volts.

A 200 Amp panel has a main breaker rated (or, at least labelled) at 200 Amps - or, at least, mine does and have done in the past. By the logic of PVAndy's statements and of the originally cited answer on the waterheatertimer.org page, this should presumably mean 200 Amps per 'side' (since the main breaker is a double breaker), so 400 Amps (if perfectly balanced) would not necessarily trip it. I am fairly sure, however, that a panel's main breaker does trip if the total current through it is more than its rating. Would PVAndy like to comment on this apparent inconsistency?

I don't think its an inconsistency, I think  its a misunderstanding of how Single Phase (often called split phase) circuits work.  You may find this interesting as it appears to be technically correct  https://en.wikipedia.org/wiki/Split-phase_electric_power

In a 3 wire 240 V single phase circuit, as I tried to show in some of my earlier crude diagrams There are 3 wires,  L1 (Line 1), L2 (Line 2) & N (Neutral)


L1   200 A Breaker --------|
                            120V  Loads
N ---------------------------|
                            120 V Loads
L2  200A Breaker----------|


Remember L1 & L2 are 180 Degrees out of phase with each other, so that the potential (voltage) between them is 240V (nominal)

If the 120V loads on L1 & L2 are Balanced (equal) there will be no neutral current, the Neutral only carries the unbalanced current.


A 200A Panel can supply 2 x 200A at 120V (400A of 120V loads) or 200A of 240V loads  (remembering that the actual loading by code should never exceed 80% )

Regarding the question of a 50A 2 pole breaker being used at lower current.  A good example of where this is not the case is an electric range with all burners &the oven on.  Also a electric water heater.

The primary type circuits I supervise the design of daily are PV Inverter where we backfeed existing main panels.  The majority  of those residential inverters are backfeed into 2 pole 240V breakers running at 80% of max amperage.  They use a Neutral that carries no current but serves as a  voltage reference.

Also keep in mind that a breaker trip curve varies quite a bit.  Breakers don't actually trip at rated current.  In the event of what is called a Bolt Fault (dead short circuit) a breaker will trip very quickly.  All breakers and panels have what is called an AIC rating.  This is the max current the breaker is rated to trip with a bold fault.  (if a Bolt Fault exceed the AIC Rating of a panel, it may explode into molten metal)  .  Typically for residential equipment it is 10,000 Amps.  The Bolt Fault Current is determined by the impedance of the utility transformer,  wire size and material, and  secondary line length.

Short circuit analysis is a very complex subject ( the reference text I use is over 400 pages)   If you want some interesting reading  Google  Short Circuit Coordination Study

Andy






 4 
 on: February 21, 2018, 03:17:52 PM 
Started by chabotd - Last post by pfletch101
See my response in the other thread that bears on this (http://forum.theenergydetective.com/index.php/topic,4463.msg10692.html#msg10692).

 5 
 on: February 21, 2018, 03:15:06 PM 
Started by 357Smith - Last post by pfletch101
From the point of view of CT sizing, your guideline probably makes sense, even assuming that PVAndy is correct, though certain large loads may reasonably and predictably draw more than 50% of the rated current of the circuit they are on (I am thinking of HVAC systems and electric stoves and/or ovens).

In terms of the rating of double breakers, the waterheatertimer.org link that you cite does seem more definitive, but there is one remaining inconsistency that had already struck me, and that turns up explicitly further down on that page, where there is another question and answer:
Quote
Question:
If you load a panel up with only single pole breakers can you put 200 amps on each leg?
Answer: No.

200 amps on each leg would be 400 total amps. If you draw 400 Amps, the main breaker will trip. {emphasis mine} A 200 Amp 120/240Volt panel has maximum 200 Amp, no matter if loads are 120Volts or 240Volts.

A 200 Amp panel has a main breaker rated (or, at least labelled) at 200 Amps - or, at least, mine does and have done in the past. By the logic of PVAndy's statements and of the originally cited answer on the waterheatertimer.org page, this should presumably mean 200 Amps per 'side' (since the main breaker is a double breaker), so 400 Amps (if perfectly balanced) would not necessarily trip it. I am fairly sure, however, that a panel's main breaker does trip if the total current through it is more than its rating. Would PVAndy like to comment on this apparent inconsistency?

 6 
 on: February 21, 2018, 10:14:26 AM 
Started by 357Smith - Last post by Support7
@pfletch101 Your right, it doesn't say it specifically but we still stand behind the general rule of thumb of using half a double poles amperage to gauge the CT size that you will use. I address the reasons in another post so I will copy/paste below for easy reference. Although PVAndy is technically right regarding the breaker, TED's position based on it's use and engineering recommendations are as stated.

http://forum.theenergydetective.com/index.php/topic,4453.0.html
Thank you for the correction, you are technically right so I will make a note of it. Based on a quick search it does turn up misleading information but based on a little more research I was able to find some more convincing information to backup your argument than I was using for mine so I will cite that below and it does give examples/formulas based on testing to backup that each leg of a double pole breaker would be at the indicated amperage rating. So for clarity a 50A double pole breaker would be at 50A per leg according to:

http://waterheatertimer.org/are-both-sides-of-30-amp-breaker-15-amp.html
 
But, would you not agree that a 50A Double pole breaker would be installed on a much lower potential load so there is less chance of hitting a false trip/surge?
Would you also not agree that since there is no actual current passing through the CT's, only sensing of the current than there is little chance of any electrical damage to occur from having a lower rated CT on a higher rated breaker?
So for little to no negative side effect of using 2 20A CT's on a 40A double pole breaker you will get improved resolution of energy readings for the actual range of current that may be passing through that leg at any given time.

For those reasons we still stand behind the general rule of thumb of using half a double poles amperage to gauge the CT size that you will use.

Thanks again for the excuse/reason to increase my knowledge though.

 7 
 on: February 21, 2018, 10:07:04 AM 
Started by chabotd - Last post by Support7
Hey,

Thank you for the correction, you are technically right so I will make a note of it. Based on a quick search it does turn up misleading information but based on a little more research I was able to find some more convincing information to backup your argument than I was using for mine so I will cite that below and it does give examples/formulas based on testing to backup that each leg of a double pole breaker would be at the indicated amperage rating. So for clarity a 50A double pole breaker would be at 50A per leg according to:

http://waterheatertimer.org/are-both-sides-of-30-amp-breaker-15-amp.html
 
But, would you not agree that a 50A Double pole breaker would be installed on a much lower potential load so there is less chance of hitting a false trip/surge?
Would you also not agree that since there is no actual current passing through the CT's, only sensing of the current than there is little chance of any electrical damage to occur from having a lower rated CT on a higher rated breaker?
So for little to no negative side effect of using 2 20A CT's on a 40A double pole breaker you will get improved resolution of energy readings for the actual range of current that may be passing through that leg at any given time. 

For those reasons we still stand behind the general rule of thumb of using half a double poles amperage to gauge the CT size that you will use. 

Thanks again for the excuse/reason to increase my knowledge though.

 8 
 on: February 20, 2018, 10:22:59 PM 
Started by 357Smith - Last post by PVAndy
Sorry I don't have a reference to cite but I'm quite certain that PVAndy is correct.

Thanks

I'll try to find a source, but for now I'll be the source.  As an engineer with over 30 years experience who currently designs power systems and PV systems for a major utility, I can assure you the information I posted is accurate.

A double pole breaker or for that matter a tripe pole (3 phase)  is nothing more than 2 or 3 individual breakers mechanically interlocked so that when one trips the other also trip.  The current rating of the individual breakers in a multiple pole breaker does not change regardless of how many are ganged.

I tried to find  reference but like many things in engineer it may just be understood.

Sorry I can't be more definitive.  

BTW there are many other factors to be considered in specifying CT's.  Range (could be 1 ,2 3 etc  meaning a 60A CT with a extended range of 2 would be usable up to 120A)  Burden which is the load the CT puts on the circuit being measured.  Remember the CT acts as a transformer with a winding ratio.  One turn on the wire going through the center to the number of turns in the toroidal coil. It imposes a load on the circuit it is measuring called a Burden.  Normally not much of a factor but can be at very high accuracy.

Hopefully this helps a little

Andy




 9 
 on: February 20, 2018, 10:11:22 PM 
Started by 357Smith - Last post by tlveik
Sorry I don't have a reference to cite but I'm quite certain that PVAndy is correct.

 10 
 on: February 20, 2018, 10:04:48 PM 
Started by chabotd - Last post by PVAndy
Each terminal on a double pole breaker is protected by half the breaker which I interrupt as half the amperage. I could be wrong as I'm no electrical engineer but I linked info I found from thespruce.com that supports this.

How Double-Pole Breakers Work:

Inside your breaker box or main service panel, are energized metal plates, called "hot" bus bars. Each pole, or connection point, on the bars, carries 120 volts of electricity. When single-pole breakers are installed, they snap into one pole to receive 120 volts. Double-pole breakers snap onto two poles for a total of 240 volts. Circuit wiring that connects to double-pole breakers contains two "hot" wires. Each of these connects to a terminal on the breaker and is protected by half of the breaker. If a fault or other problem occurs along one of these wires, the corresponding half of the breaker will trip.

This causes the other half of the breaker to trip at the same time because the two halves are tied together by the single breaker bar or toggle. This effectively shuts off the connection to both bus poles, shutting down the entire circuit at once.



https://www.thespruce.com/what-are-double-pole-circuit-breakers-1152727


Hi   Actually I am an electrical engineer with over 30 years experience in the power field.  I read the description you posted above and can see the confusion.  It talks about a fault that occurs on one leg of the circuit that will trip 1/2 the breaker.  It does not mention 1/2 the current.  Each half of lets say a 50A 2 Pole breaker is a single pole 50A breaker.  Depending on the type of fault (L1 to N or G, L2 to N or G or Line to Line either one or both of the breakers will trip on over current,  in either case the other breaker will also trip due to the mechanical interlock.  I commented on your post because I saw the same misinformation being posted in numerous places in the forum and I wanted members to be able to correctly size CT's.  AS a commercial solar system designer for a major utility, I design in CT's on a daily basis.  Feel free to ask any questions you would like help on.

Andy


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