Previous in Forum: IGBT Gate Drive Selection   Next in Forum: Underfrequency Trips and Overcurrent
Close
Close
Close
24 comments
Power-User

Join Date: Mar 2011
Location: oz
Posts: 246
Good Answers: 10

Circuit Breaker Size

01/21/2021 8:54 PM

Hello All,

Just checking what's the max. size circuit breaker you will use in this example.

Rated current is 228A, Load Current is 36.9A

Login to Reply
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#1

Re: Circuit Breaker size

01/21/2021 10:22 PM

A really big one...

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#6
In reply to #1

Re: Circuit Breaker size

01/22/2021 2:27 PM

Your load current would be fed off a distribution box, because obviously you're not going to run that size wire for such a low load...So probably in the future there will be more equipment, your distribution box would be centrally or conveniently located closest to the largest anticipated loads...The equipment will be marked with breaker requirement type and recommended settings if any....This will determine the wire size required...

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29931
Good Answers: 808
#2

Re: Circuit Breaker size

01/22/2021 3:48 AM

The maximum is determined by the size of the receptacle into which it is to fit. And if it isn't going to fit then either a larger receptacle or an alternative, or additional, larger receptacle needs to be provided.

However, there is also a minimum:

  • It has to be at least large enough to get the upstream connection to attach to it.
  • It has to be at least large enough to get the downstream cable conductors attached to it.
  • It has to be at least large enough to disconnect a downstream short circuit like that shown in the calculation without sustaining damage.

Its operating rating, which is not the same descriptor as <...size...>, is determined by the calculation in the original post. Calculation styles vary across the globe.

Local standards at the point of installation must prevail; in the UK it is covered by British Standard 7671.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29931
Good Answers: 808
#3

Re: Circuit Breaker Size

01/22/2021 5:12 AM

One needs to remember that the breaker is there to protect the cable, not the load.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply Score 1 for Good Answer
Guru
Engineering Fields - Electrical Engineering - Been there, done that. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 14850
Good Answers: 913
#4
In reply to #3

Re: Circuit Breaker Size

01/22/2021 11:42 AM

I never liked this often repeated mantra. What bothers me is it implies an incomplete list of things possibly protected for more than just the cable itself is protected. The breaker protects cable installation costs, also whatever flammable material contacts the cable from a short circuit fire and even a possible maintenance personnel electrical safety. These added items are obvious for a seasoned EE or electrician but those who ask a public forum how to size a circuit breaker don't understand what also needs to be known for correct sizing of a breaker.

__________________
"Don't disturb my circles." translation of Archimedes last words
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29931
Good Answers: 808
#5
In reply to #4

Re: Circuit Breaker Size

01/22/2021 12:03 PM

What is astonishing, agreed, is the number of posts that appear in this forum for cable sizing and breaker selection, suggesting that there are an awful lot of individuals in practice who are either unsure, or unable, or unwilling, or too embarrassed to carry this out. The task is fundamental to the practice/discipline, and ought to be written on the inside of the cranium and formally assessed for readability and indelibility before the practice commences.

Still, not doing so does make work for the emergency services, Insurance Loss Adjusters, and the legal profession, among others. So it can't be all bad.

<unsubscribes>

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru
Popular Science - Cosmology - Let's keep knowledge expanding Engineering Fields - Retired Engineers / Mentors -

Join Date: Dec 2006
Location: North America, Earth
Posts: 4313
Good Answers: 104
#8
In reply to #4

Re: Circuit Breaker Size

01/23/2021 1:38 PM

It is true that the breaker is there to protect the wire, but the bigger issue is the home/business catching on fire. Using the wrong breaker for the wire could cause your insurance agent to reject the claim when the home/business burns down. The electrical code for your area must be followed.

__________________
“I would rather have questions that can't be answered than answers that can't be questioned.” - Richard Feynman
Login to Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20860
Good Answers: 774
#7

Re: Circuit Breaker Size

01/22/2021 9:29 PM

Insufficient information. What is the load? Motors, transformers, etc., have additional stipulations for sizing overcurrent devices.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Login to Reply
Power-User

Join Date: Mar 2011
Location: oz
Posts: 246
Good Answers: 10
#9

Re: Circuit Breaker Size

01/23/2021 3:22 PM

Thanks everyone, this is a job I have given to an Electrical contractor , they are a lot more familiar with the standards,

My understanding is the same , breakers primary role is to protect the cable, usually I follow the simple rule, breaker current needs to be less than cable current and more than load current.

On the above example a 32 amp breaker is used on a 95mn2 cable for a 22kw motor, project is to upsize the motor to 45kw, to me this was an easy job, all was needed is to upsize the breaker accordingly and select the correct motor protection.

The Electrical contractors view was different , he said cause the cable is 1000m, the max Breaker can only be 32amp and I will need to run another cable, hence the reason for my question, I will have to read my standards once I get back to work.

Login to Reply
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#10
In reply to #9

Re: Circuit Breaker Size

01/23/2021 6:52 PM

It seems unlikely that the motor feeder for a 22kW motor would be big enough for a 45kW motor, so replacing the motor feeder would not be unusual. Not sure why 150mm2 wire is flagged at the beginning of your post? For the amperes you cite, 10 or 16mm2 wire is plenty big enough.

The circuit breaker size is based only on the ability to get the motor started, and short circuit withstand capability, it has nothing to do with the wire ampacity. The motor overloads protect the wire from overcurrent.

Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29931
Good Answers: 808
#22
In reply to #10

Re: Circuit Breaker Size

01/27/2021 6:21 AM

<...For the amperes...10 or 16mm2 wire is plenty big enough...>

Not for a cable that is supplying that size load <...1103m...> away. The volt drop criterion takes precedence over the cable's fault withstand criterion; that's what the table in the original post is saying.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#23
In reply to #22

Re: Circuit Breaker Size

01/27/2021 12:14 PM

Yes, I hadn’t done the math on the practically incomprehensible 1000 meter feeder length...

Login to Reply
2
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 290
Good Answers: 17
#11
In reply to #9

Re: Circuit Breaker Size

01/24/2021 12:45 AM

95sqmm cable for a 22kW motor was necessitated from voltage drop point of view, to ensure minimum required voltage is available at the motor terminals during motor starting.

But, 32A breaker for a 22kW motor seems on lower side. What is the full load current of 22kW motor - about 45A I suppose. How is it that the breaker is only 32A?

When you uprate the motor to 45kW, the distance is going to remain 1000m I suppose. That being the case, while 95sqmm cable may be good enough from current rating point of view, how about voltage drop in the cable during motor starting?? Has it been verified??

Breaker rating of 100A may be good enough for 45kW motor. However, the breaker protection characteristics to be verified with motor data to confirm the breaker is stable during motor starting.

__________________
Raghunath
Login to Reply Good Answer (Score 2)
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#14
In reply to #11

Re: Circuit Breaker Size

01/24/2021 2:06 PM

It looks like the 36 amperes is the full load current for your 22kW motor, you really need to state the voltage and current for your 45kW motor, likely 70 amperes.

A quick calculation shows 5% steady state voltage drop for 36 amperes on a 1000 meter length of 95mm2 wire.

For 70 amperes, the voltage drop is 9%. So if you have a 415 volt system, and a 400V motor, you are marginally acceptable at 7% voltage drop at full load. It’s unlikely that the motor will operate at full amperage for extended periods of time, but we can’t see that from here.

Assuming a conservative 6 x 36 = 216 amperes for starting current, your voltage at start is under 30% voltage drop, assuming 415 volts, 95mm2 wire, which is likely plenty to develop your needed starting torque for your load.

In this case, you really need to make sure you have enough voltage to run steady state, and to accelerate your load from rest.

In the US, your breaker can be up to 200A rating, based on type of breaker, and size of motor; it is not based on motor wire size at all. The required motor overloads are set to 125% of motor amperes, and the wire is also sized for 125% (at least) of motor amperes.

Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#15
In reply to #14

Re: Circuit Breaker Size

01/24/2021 4:12 PM

Maximum amperage ratings for wires are based continuous current at that level without the wire overheating with the insulation type being used....Starting current is only momentary and has little to do with the wire sizing, that's why there is a delay in the breaker tripping time to allow for temporary overcurrent in starting rotating bodies...

The voltage drop calculation is based on maximum amperage rating for that size and type wire, and the voltage drop diminishes as the amperage is lowered...95mm2 wire w/copper conductor is rated anywhere from 140 amps to 225 amps depending on insulation type...

The voltage drop of a wire this size with this load would be .93%, with a 415 volt supply, that would be 3.85 volt drop or 411 volt supply at the motor...

http://www.stabiloy.com/nr/rdonlyres/4c30707f-5ec7-46ef-a2ba-aa25028b0b5e/0/wiresizecomparisontable.pdf

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#16
In reply to #15

Re: Circuit Breaker Size

01/24/2021 5:34 PM

Starting current is more than momentary, it will persist if the motor rotor does not turn. If the voltage drop is too high upon start, the motor may not develop enough torque to accelerate the load beyond locked rotor. The mechanical characteristics of the load will determine if the wire is big enough to deliver the energy necessary to rotate the driven machine.

Login to Reply Score 1 for Good Answer
Power-User

Join Date: Mar 2011
Location: oz
Posts: 246
Good Answers: 10
#17
In reply to #16

Re: Circuit Breaker Size

01/24/2021 5:58 PM

We size cables on full load current, cables can handle the start up current , if this current last for an abnormal time, this is when the thermal overload will trip, but the primary protection for the cable is still the circuit breaker.

Login to Reply Score 1 for Off Topic
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#18
In reply to #17

Re: Circuit Breaker Size

01/24/2021 7:14 PM

That is incorrect for motor feeders. The circuit breaker or fuse or branch circuit protective device provides short circuit protection for the conductors, but not overload. The motor overload relay is the primary protection for the wire, as well as the motor, by design. In some cases, the circuit breaker or more commonly dual element fuses can serve as backup overcurrent protection.

The reason for this is to allow for motor starting current, up to 6 times the rated current. The branch circuit protection must not trip for that locked rotor current, up to 20 seconds in some cases, so in certain instances and based on the actual protective equipment, it can have a trip rating many times higher than the cable rating. Dual element fuses, MCP, inverse time circuit breaker, fast acting fuses, all have different rating limits, none of which are directly related to the wire size.

With a motor feed as long as you have, the voltage drop upon starting needs to be examined, as well as your steady state voltage drop, as was done with your original installation. Your motor overloads will certainly be set to a lower trip setting than your wire size dictates, since the wire has been sized for voltage drop, giving you a generous margin for conductor overheating.

The breaker should be the correct size for the motor, to preserve standard engineering methods. This is so the next person looking at the installation is not confused by the application of non-standard methods. This helps to confirm that the size of the wire was selected solely for voltage drop alleviation.

Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#19
In reply to #18

Re: Circuit Breaker Size

01/24/2021 8:17 PM

Well we need the actual start curve of the proposed motor, the amperage draw how high how long is necessary for clarification....The type of starter could have a big influence...The motor if properly sized will be loaded at less than 75% of rated FLA...

https://www.plantengineering.com/articles/whats-in-a-nameplate/

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 290
Good Answers: 17
#20
In reply to #18

Re: Circuit Breaker Size

01/25/2021 1:04 AM

Motor starting current could be as high as 8 or 10 times the rated current if we assume the motor to be high/premium efficiency type.

This will make the starting situation worse.

__________________
Raghunath
Login to Reply
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#21
In reply to #20

Re: Circuit Breaker Size

01/25/2021 1:37 PM

That’s a good point, my rule of thumb 6x FLA is based on larger motors 10kW and up. I never realized smaller motors can draw up to 10 times FLC.

I had not run across a link between high efficiency motors and high starting current. With smaller air gap I would have perhaps incorrectly assumed a smaller locked rotor current.

Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#12
In reply to #9

Re: Circuit Breaker Size

01/24/2021 2:11 AM

You haven't said what the voltage is, that will have an impact on the wire size...a 60 hp motor(45kw) would draw 177 amps at 200volts but only 77 amps at 480...as we go up in voltage we go down in amperage and breaker size...Voltage drop over distance is used to derate the capability of the wire at different amperage load...When you are talking about wire this large, 150mm2 or 300 mcm awg, it's oversized enough for this load that the voltage drop would be negligible...this wire is good for 280 amps in a copper conductor at 75°C...That is why you would have a disconnect at the motor that stepped down the wire size and circuit breaker amperage...You can't have the disconnect 1000 ft away, it has to be close to the motor for service, that's code...

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Guru

Join Date: Oct 2006
Location: 44.56024"N 15.307971E
Posts: 6004
Good Answers: 240
#24
In reply to #9

Re: Circuit Breaker Size

01/29/2021 8:43 AM

Here is how we do it in the States.

Local codes may vary,and the Authority Having Jurisdiction has the final word.

All installations of this manner require a preliminary and final inspection by the local inspector.He is the local expert on such matters.

If you want to do the calculation(not the work) yourself,here is a link.There are many ,inks here to advance your knowledge in these and other electrical matters.

https://www.jadelearning.com/blog/motor-circuit-conductors/

I am a retired electrical contractor with many years of experience,so my advice has some basis on real world applications.

__________________
"A man never stands so tall as when he stoops to help a child." "Ignorance does not consist of what a person does not know,but of things he knows that just aint so"
Login to Reply
Guru

Join Date: May 2018
Location: Under the spreading Bunya Trees, South Burnett, Queensland, Australia
Posts: 532
Good Answers: 47
#13

Re: Circuit Breaker Size

01/24/2021 4:52 AM

From the Olex cable information 150 nsq mm cable in PVC copper three phase has a rating of 303 amps so a 300 amp three phase circuit breaker would protect the cable from the supply to the sub main board.

If you are using XLPE then 377 amps is the cable rating with copper cores.

The final sub circuit circuit breaker would be sized to protect the load depending on its type.

To suggest you need to double up on the cable seems like an early retirement fund or you need to supply us with more information.

Here is some helpful data; https://www.olex.com.au/eservice/Australia-en_AU/fileLibrary/Download_540225217/Australia/files/OLC15984%20Handbook%20July%202016%20FA%20V3%20LR.pdf

__________________
Hare today, goon tomorrow!
Login to Reply
Login to Reply 24 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

eltech (2); HiTekRedNek (1); PWSlack (4); raghun (2); redfred (1); rwilliams (6); SolarEagle (5); StandardsGuy (1); Stef (1); Tornado (1)

Previous in Forum: IGBT Gate Drive Selection   Next in Forum: Underfrequency Trips and Overcurrent

Advertisement