Spyder CTs

[357Smith]

Hey Guys,

I'm trying to configure my TED Home Pro order with the proper spyders and one thing I'm confused with is which spyder I need for stuff like the A/C which has two breakers that are sorta connected to each other (Sorry if my terminology is way off). 

For example here is my breaker box for the pool subpanel and two central A/C units I have: https://image.prntscr.com/image/-sHfY7S2RsuX0EAVfVo9wQ.png    Could I get away with three 60A Donut CTs since the highest I see on anything is 50A?  or do I need six 60A Donut CTs to monitor those 3 loads?

Regards,
Ryan

[pfletch101]

Generally speaking, you only need to monitor one 'side' of a domestic 240V circuit, so you will only need 3 CTs to monitor the 3 double breakers, with the respective input channels set up to double their readings. Also, breakers are rated by the total current which is needed to trip them, so the CT on one 'side' need only be rated for half the nominal breaker current. You would therefore need a 20A CT on your lower A/C breaker and a 60A CT on the upper one. I can't read the rating of the Pool subpanel breaker with certainty, but if it is > 40A and < 120A you would also need a 60A CT on it.

[PVAndy]

I have seen this misconception in several post in this forum.  When talking about a dual pole breaker which is typically used in 240V residential applications, A 50A 2 Pole breaker is 2 50A single 1 pole breakers mechanically interlocked.   Each leg of the breaker trips at 50A.  If one leg trips the other also is tripped.  In most 240V loads, there is very little neutral current.  Please explain why some appear to believe  that each leg of a 50A breaker only carries half the current and will trip at 25A

L1 
   120V
N            L1-L2  240V
   120V
L2 

Current from L1 going to a 240V load returns through L2  (not the neutral)

The breaker on L1 & L2 are both carrying the same current and will trip at the rating of the breaker.

When sizing CT's  unless the CT's are Extended Range CT (which many are) thr rating of the CT is the max current at which rated accuracy is maintained.

A 20A CT with an extended range  of 2 can be used to measure 40A loads.

Andy

Sr Electrical Engineer

[pfletch101]

I have seen this misconception in several post in this forum.  When talking about a dual pole breaker which is typically used in 240V residential applications, A 50A 2 Pole breaker is 2 50A single 1 pole breakers mechanically interlocked.   Each leg of the breaker trips at 50A.  If one leg trips the other also is tripped.  In most 240V loads, there is very little neutral current.  Please explain why some appear to believe  that each leg of a 50A breaker only carries half the current and will trip at 25A

I am one of those responsible for the posts you cite. I had believed that they were based on a published statement from what I regarded as a definitive authority, but I can no longer find my source, so you may well be right. Could I ask you, however, to cite an authoritative documentary source, so that this can definitively be put to rest? In particular, my impression had been that double breakers with a individual current marking on each 'arm' of the breaker were, as you say, rated at the stated current on each side, but that double breakers with a single rating embossed on the link bar were rated at that total current (divided between the two sides).

[Support7]

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

[pfletch101]

@Support7: I found that reference, too, but it does not clearly or unequivocally support what we believe and have previously written, since it does not actually specify the break current for each 'half' of the breaker - just says (correctly) that they each see half the voltage. Hopefully, PVAndy will be able to cite a clear authority.

[tlveik]

Sorry I don't have a reference to cite but I'm quite certain that PVAndy is correct.

[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

[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.

[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:

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?

[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:

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

[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?

[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