Author Topic: Best CT size for 2 pole breakers  (Read 1481 times)

chabotd

  • Newbie
  • *
  • Posts: 4
  • Karma: +0/-0
Best CT size for 2 pole breakers
« on: January 01, 2018, 11:05:49 PM »
Trying to determine the best CT size for 2 pole breakers.  We have 30, 35 and 50 amp 2 pole breakers. Assuming we put only one CT on one leg of the circuit, do we size CT to half the rated amperage or size them for the full breaker amperage?  For example, for the 30 amp 2 pole breakers, do we use the 60 amp CT or do we use the 20 amp CT? In this exmple, we know the 60 amp CT will work, but, if appropriate, the 20 amp will yield greater current reading sensitivity.

Support7

  • Administrator
  • Sr. Member
  • *****
  • Posts: 467
  • Karma: +1/-0
Re: Best CT size for 2 pole breakers
« Reply #1 on: January 02, 2018, 09:08:01 AM »
If the double pole breaker is rated at 50A, than it would be 25A per leg of the breaker and using the closest rated amperage CT to that without exceeding it would yield the most accurate readings so you can size your CT's based on half the double pole listed amperage as long as the CT is only going around one leg of the double pole breaker.

PVAndy

  • Newbie
  • *
  • Posts: 6
  • Karma: +0/-0
Re: Best CT size for 2 pole breakers
« Reply #2 on: February 17, 2018, 10:02:31 PM »
?  How do you figure that each pole of a 2 pole 50A Breaker is only carrying 25A.

Typically 2 Pole residential breakers are used for 240V and each pole is rated and will trip at 50A.

Andy



Support7

  • Administrator
  • Sr. Member
  • *****
  • Posts: 467
  • Karma: +1/-0
Re: Best CT size for 2 pole breakers
« Reply #3 on: February 20, 2018, 10:58:05 AM »
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

PVAndy

  • Newbie
  • *
  • Posts: 6
  • Karma: +0/-0
Re: Best CT size for 2 pole breakers
« Reply #4 on: February 20, 2018, 10:04:48 PM »
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


Support7

  • Administrator
  • Sr. Member
  • *****
  • Posts: 467
  • Karma: +1/-0
Re: Best CT size for 2 pole breakers
« Reply #5 on: February 21, 2018, 10:07:04 AM »
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.

pfletch101

  • Sr. Member
  • ****
  • Posts: 422
  • Karma: +0/-0
    • My home page
Re: Best CT size for 2 pole breakers
« Reply #6 on: February 21, 2018, 03:17:52 PM »
See my response in the other thread that bears on this (http://forum.theenergydetective.com/index.php/topic,4463.msg10692.html#msg10692).
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits