# 3 phase breakers

Discussion in 'General Electronics Chat' started by Rocky Howell, Nov 27, 2012.

1. ### Rocky Howell Thread Starter New Member

Nov 27, 2012
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If I want to know the maximum amperage per phase on a 30 amp 3 phase breaker, don't I say 30 X 1.73 = 51.9 / 3 = 17.3 maximum amps per phase? Thanks

2. ### mbohuntr Active Member

Apr 6, 2009
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Where I work, we test them at rated current on each phase, that is, in your case, 30 A per phase. We also test them at 300% rated and instantaneous current. Hope this helps.

3. ### Rocky Howell Thread Starter New Member

Nov 27, 2012
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Thanks for the reply, but maybe I should have asked what is the maximum load that I can put on each phase of the 30 amp 3 phase breaker? I didn't think that you could put a 30 amp load on each phase??? Don't you calculated by 30 x 1.73 - 51.9 / 3 = 17.3 on each phase??? Is this the correct way to calculate the maximum load per phase?

4. ### BillB3857 Senior Member

Feb 28, 2009
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Many 3 phase breakers are simply 3 single phase breakers in one box with a common handle, or possibly 3 handles tied together with a tie bar. Each can carry the rated current of the breaker.

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
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1,625
The NEC specifies the over current device shall be at least 1.25 times the largest continuous current (where continuous is defined as 3 hours or more). Note 1/1.25 = 0.8 so this is an 80% derating.

So a 30 amp circuit is built to handle a 24A continuous load.

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6. ### Rocky Howell Thread Starter New Member

Nov 27, 2012
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I appreciate everyone's reply and I do know the 80% rating rule for most breakers, but I'm still confused. I am pretty sure that a 30 amp 3 phase breaker is not designed to carry 30 amps on all 3 phases, that would be 90 amps. I also know that a 30 amp 3 phase breaker can carry more than 10 amps on each phase which equals 30 amps. I thought and I repeat thought, that when dealing with 3 phase breakers, you multiplied the amp breaker by the square root of 3 and then divided this by 3 Example 30 x 1.73 = 51.9 / 3 = 17.3 per phase. Can someone tell me if this in fact is incorrect? Remember we are talking about 3 phase breakers, not single phase breakers.

7. ### crutschow Expert

Mar 14, 2008
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The maximum current per phase is 30A. Whether it's 1 phase or 3 phase makes no difference for that parameter. Calculating the maximum power is different, of course, for 1 phase or 3 phase.

8. ### mbohuntr Active Member

Apr 6, 2009
413
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In a three phase "balanced load" system, the currents do not add together, rather cancel each other out. The measured neutral current in a balanced system is zero amps. You have to remember that the phases are 120 degrees apart.

9. ### gerty AAC Fanatic!

Aug 30, 2007
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As above, the breaker doesn't care if it's 3 phase or single phase.

According to your logic, a 3 phase 30a breaker will only carry 17.3 amps,(per phase) but a single phase 30 amp breaker will carry 30 amps
And if that were true, 3 pole 30amp breakers would be marked 17 amps..

you are introducing a formula that has nothing to do with circuit breakers.

Last edited: Nov 27, 2012
10. ### Rocky Howell Thread Starter New Member

Nov 27, 2012
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Thanks everyone, I just need education and clarification.

11. ### Rocky Howell Thread Starter New Member

Nov 27, 2012
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Again, I need some education. A 42 pole 225 amp 3 phase panel will accommodate 14 3 phase breakers. If all 14 breakers are 30 amp this will be 420 amps. I guess this means that the 225 amp panel can handle 225 amps per phase making it capable of handling 675 total amps not taking into consideration the 80% rule?? Again, I'm still learning. Thank you guys for helping.

12. ### crutschow Expert

Mar 14, 2008
13,472
3,361
If it's a 225 amp panel, then I assume the total allowed current per phase is 225A. Thus if you have 14 breakers, the total current of the breakers should add to no more than 225A per phase. Therefore you couldn't have all breakers be 30A. The average current should be no more than 16A per breaker phase.

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13. ### BSomer Member

Dec 28, 2011
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Not necessarily true. There are de-rating factors used to calculate the size of the panel needed, or going the other way, calculating the load you can put on a panel of a certain size. It really depends on what will be running at full load for how long with what ever else is running. You could have 420A per phase on a 225A panel just in circuit breakers. However, if somebody didn't realize that all of those loads could be on at the same time when designing the electrical installation, the panel main breaker (if used) will trip.

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14. ### GetDeviceInfo Senior Member

Jun 7, 2009
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225 amps per phase is not 675 total amps. Your error is not considering or understanding phase displacement, which impies angular, or vectorial additions.

your service panel bus is sized to service the loads attached, as are the branch circuit overcurrent devices, based on rules, calculations, and tables provided by the Electrical Codes. The methodology varies with the characteristics of the specific load.

Motor loads for instance can have a breaker up to 250% FLA, but the ampacity of the conductor only needs to be 125% of FLA. You will see what appears to be too much breaker for bus ratings.

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