Breaker Rating vs Line Current
- Thread starter EZ94DS
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Breaker Rating vs Line Current
I'm not quite sure I understand the question. The current in each phase will be the same as the current flowing through the breaker.
Most of the breakers we see in daily use have what is called an "inverse time" tripping scheme. The breaker will trip when it exceeds its rating. But the more current it sees (i.e., the higher the current gets above the trip setting), the faster the breaker will trip. If you put 21 amps through a 20 amp breaker, it might not ever trip. Put 40 amps through the same breaker, and it might trip in 5 seconds (I made that number up). Put 100 amps through the same breaker, and it might trip in one quarter second (I made that number up too).
The square root of three shows up in a great many electrical calculations. But this is not one of them.
If I didn't answer your question, could you rephrase it for me?
I'm not quite sure I understand the question. The current in each phase will be the same as the current flowing through the breaker.
Most of the breakers we see in daily use have what is called an "inverse time" tripping scheme. The breaker will trip when it exceeds its rating. But the more current it sees (i.e., the higher the current gets above the trip setting), the faster the breaker will trip. If you put 21 amps through a 20 amp breaker, it might not ever trip. Put 40 amps through the same breaker, and it might trip in 5 seconds (I made that number up). Put 100 amps through the same breaker, and it might trip in one quarter second (I made that number up too).
The square root of three shows up in a great many electrical calculations. But this is not one of them.
If I didn't answer your question, could you rephrase it for me?
Re: Breaker Rating vs Line Current
Sorry. I did confuse the question. Stated another way:
Is the breaker rated in what I've heard termed "3 phase current", or, is the rating the same as the current in a single phase, or line?
For example, my breaker is rated 1600 Amp. Does this imply each phase could carry up to 1600 A? As you point out, there are time-current considerations, but here I need the line current in order to check the size of the individual feeder conductors. I will leave breaker coordination to others more knowledgeable.
Many Thanks.
Sorry. I did confuse the question. Stated another way:
Is the breaker rated in what I've heard termed "3 phase current", or, is the rating the same as the current in a single phase, or line?
For example, my breaker is rated 1600 Amp. Does this imply each phase could carry up to 1600 A? As you point out, there are time-current considerations, but here I need the line current in order to check the size of the individual feeder conductors. I will leave breaker coordination to others more knowledgeable.
Many Thanks.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Breaker Rating vs Line Current
The rating of an overcurrent protective device is the amount of current per pole. There is no math involved.
Your 1600A breaker can carry 1600A per phase at its rated ambiemt temperature (usually 25C).
Remember to correctly account for continuous and non-continuous loads.
The rating of an overcurrent protective device is the amount of current per pole. There is no math involved.
Your 1600A breaker can carry 1600A per phase at its rated ambiemt temperature (usually 25C).
Remember to correctly account for continuous and non-continuous loads.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Breaker Rating vs Line Current
Since you are an engineer (albeit of the mechanical persuasion, and therefore are deceived into believing that closing something will stop flow
), you will understand the following math. The three phase currents (presuming the loads are reasonably well-balanced, which is a good bet), can be represented by:
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</font>
Do whatever mathematical manipulations are most familiar to you, but add those three curves together. Use trig identities, or use vector addition, or use a spreadsheet to calculate values point-by-point. You will discover that they sum to zero. They have to. It is obvious that they have to.
What this means is that you can measure 1600 amps on phase A, 1600 amps on phase B, and 1600 amps on phase C, and that the sum of these three is zero.
Now I have to ask, "Why do you ask"? What type of project are you doing, and what is your role? I must pry in this manner because this Forum has rules that prohibit our giving "how-to" information that would assist persons who are not electricians to perform electrical installation work at their own homes. I suspect that you would not be installing anything with 1600 amps worth of load at your own home. But may I still ask for the nature of your interest?
Since you are an engineer (albeit of the mechanical persuasion, and therefore are deceived into believing that closing something will stop flow
), you will understand the following math. The three phase currents (presuming the loads are reasonably well-balanced, which is a good bet), can be represented by:</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Ia = Im cos(wt)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Ib = Im cos(wt -120)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Ic = Im cos(wt-240),</font>
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Im is the peak current, and</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">"w" (we usually use the Greek small-case letter omega) is 2 pi times the frequency (i.e., 2 pi times 60 hertz).</font>
Do whatever mathematical manipulations are most familiar to you, but add those three curves together. Use trig identities, or use vector addition, or use a spreadsheet to calculate values point-by-point. You will discover that they sum to zero. They have to. It is obvious that they have to.
What this means is that you can measure 1600 amps on phase A, 1600 amps on phase B, and 1600 amps on phase C, and that the sum of these three is zero.
Now I have to ask, "Why do you ask"? What type of project are you doing, and what is your role? I must pry in this manner because this Forum has rules that prohibit our giving "how-to" information that would assist persons who are not electricians to perform electrical installation work at their own homes. I suspect that you would not be installing anything with 1600 amps worth of load at your own home. But may I still ask for the nature of your interest?
Re: Breaker Rating vs Line Current
Thanks Charlie. I don't mind your asking.
I am with a oil drilling company, and we have a rig in a shipyard for modifications. We are replacing an old 1750 kVA 600V/480V transformer with a 2000 kVA unit.
Since our 'sparks' guy is out just now, I was asked to review the feeder breaker and cable design (by the yard.) I noticed they set the secondary bkr trip at 3200AT, or 133% of rated current (2400A). I found this is greater than the IEEE/NEC/ABS accepted 'norm' of 125%. And then questioned size of the secondary cables to the breaker.
Was embarrassed to ask, and got hung up trying to determine line current at the 125% trip of 3000A. If I've understood correctly, the line current will be 3000A (RMS). So, cables they call out would be correct, as 5c x 535MCM (per phase) to IEEE/IEC 95C core in 45C ambient.
Thanks again.... EZ
PS- You're right, I do have to stop and think... "Closed" means GO, and "Open" means STOP.
Forget all those cables. Just put a drive shaft on it!
Thanks Charlie. I don't mind your asking.
I am with a oil drilling company, and we have a rig in a shipyard for modifications. We are replacing an old 1750 kVA 600V/480V transformer with a 2000 kVA unit.
Since our 'sparks' guy is out just now, I was asked to review the feeder breaker and cable design (by the yard.) I noticed they set the secondary bkr trip at 3200AT, or 133% of rated current (2400A). I found this is greater than the IEEE/NEC/ABS accepted 'norm' of 125%. And then questioned size of the secondary cables to the breaker.
Was embarrassed to ask, and got hung up trying to determine line current at the 125% trip of 3000A. If I've understood correctly, the line current will be 3000A (RMS). So, cables they call out would be correct, as 5c x 535MCM (per phase) to IEEE/IEC 95C core in 45C ambient.
Thanks again.... EZ
PS- You're right, I do have to stop and think... "Closed" means GO, and "Open" means STOP.
Forget all those cables. Just put a drive shaft on it!
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