How to select the right Circuit Braker

[nidalls]

Hi,

If i have 4 inverters, TRIO-27.6-TL-OUTD, Maximum Output Current Iacmax= 45A.
The right way to select the main SDB circuit breaker is to sum all SDB MCB's ( 63*4)
or from the rated current from the inverters (45*4) * 1.25 (Safety Factor) ?

[SunEagle]

My first comment would be to be careful using 4 pole circuit breakers. Sometimes the neutral wire is bigger then the other 3 power wires based on how electrical circuits work with the neutral wire being used as the "return path" it can see amps that are 2 or more times higher then each of the other 3 legs so the oc rating of the neutral needs to be bigger or hardwired.

Sizing the main SDB would is based on the load amp rating and not the ratings of the other circuit breakers. Usually you would find the highest amp load (ie 45amp) and multiply that be 1.25. Then add the amp rating of the remaining loads (3 x 45amps) to get the minimum sized rating for the main SDB. Whatever you calculate for the main SDB amp rating make sure the wire from it is sized to handle more than that amp rating so that the main SDB protects it.

[nidalls]

My first comment would be to be careful using 4 pole circuit breakers. Sometimes the neutral wire is bigger then the other 3 power wires based on how electrical circuits work with the neutral wire being used as the "return path" it can see amps that are 2 or more times higher then each of the other 3 legs so the oc rating of the neutral needs to be bigger or hardwired.

Sizing the main SDB would is based on the load amp rating and not the ratings of the other circuit breakers. Usually you would find the highest amp load (ie 45amp) and multiply that be 1.25. Then add the amp rating of the remaining loads (3 x 45amps) to get the minimum sized rating for the main SDB. Whatever you calculate for the main SDB amp rating make sure the wire from it is sized to handle more than that amp rating so that the main SDB protects it.

mmmmmm
We always select the neutral in the same way we use to select the phases.
the same cross section :l

and for SDB main circuit breaker, in the photo i attached there are 4 inverter, each one of them the maximum current output is 45A. 45*1.25+45*3= 191.25A so in that way we will select 200A circuit breaker.
if we put 250A circuit breaker, Does this will create a problem in the future? Or that the difference would be at the circuit breaker price only.

[SunEagle]

mmmmmm
We always select the neutral in the same way we use to select the phases.
the same cross section :l

and for SDB main circuit breaker, in the photo i attached there are 4 inverter, each one of them the maximum current output is 45A. 45*1.25+45*3= 191.25A so in that way we will select 200A circuit breaker.
if we put 250A circuit breaker, Does this will create a problem in the future? Or that the difference would be at the circuit breaker price only.

Again the calculation of 191.25A is the minimum required. You can go with a 250A CB as long as the wire is rated above that amount and does not exceed any other maximum requirements.

I have seen the use of a 4 pole CB on some of the jobs my company gets from over seas. The problem is the neutral wires might need to be sized bigger then the phase wires. So the CB rating may be too low causing it to trip due to the neutral wire seeing double the amps that the phase wire will see. It all depends on the type of load you are connecting. If the loads generate triplen harmonics (3rd, 9th, 15th) then the chance of the neutral wire seeing higher amps then the phase wires is more than possible.

[nidalls]

Again the calculation of 191.25A is the minimum required. You can go with a 250A CB as long as the wire is rated above that amount and does not exceed any other maximum requirements.

I have seen the use of a 4 pole CB on some of the jobs my company gets from over seas. The problem is the neutral wires might need to be sized bigger then the phase wires. So the CB rating may be too low causing it to trip due to the neutral wire seeing double the amps that the phase wire will see. It all depends on the type of load you are connecting. If the loads generate triplen harmonics (3rd, 9th, 15th) then the chance of the neutral wire seeing higher amps then the phase wires is more than possible.

Thank you very much

[adoublee]

Hi,

If i have 4 inverters, TRIO-27.6-TL-OUTD, Maximum Output Current Iacmax= 45A.
The right way to select the main SDB circuit breaker is to sum all SDB MCB's ( 63*4)
or from the rated current from the inverters (45*4) * 1.25 (Safety Factor) ?

Inverters want protection at 46A? (Maximum AC overcurrent protection value)

[nidalls]

Inverters want protection at 46A? (Maximum AC overcurrent protection value)

No, from ABB manual the mention that we should put a 63A cicruit breaker. so the inverters don't need protection at 46A