Design current and circuit breakers

Alessandro · Feb 22, 2016

Hello,

I have a problem with design current (Ib) and selection of the circuit breaker (In).
If I have a circuit of 7A and I apply no diversity, then Ib = 7A and I select a 10A circuit breaker.
But if I apply diversity and I get, say, 5A, is 5A now the design current Ib? So can I select a 6A circuit breaker?

Another problem: On-Site Guide says that for domestic cooking appliances I should consider the first 10A plus 30% of what's remaining. So if I have a cooker with 35A current demand, after diversity the current demand of the circuit is 10A + 0.3 * 25A = 17.5A. If this is the design current, then I select a 20A circuit breaker. But now this half of the full load current demand (35A), and if someone uses just 60% of the current demand the circuit breaker will trip.

Last question: if I have discharge lighting, let's say 5A, then I should probably use a type C 6A circuit breaker. But if I multiply by the factor of 1.8, 5A * 1.8 = 9A. Should I use a type B 10A circuit breaker?
Are both valid options?

Thank you.

telectrix · Feb 23, 2016

taking the cooker example. if your design current is 17.5A after applying diversity, you could use 2.5mm cable., but only if not derated. then you would select the OCPD rating In such that Ib<In<Iz. however to allow for derating and possible future proofing against a bigger cooker being installed, you'd would go up a size to 4mm or even 6mm and select OCPD accordingly. then consider volt drop and Zs. these would both factor into your calculations.

Marvo · Feb 23, 2016

Alessandro said:
............Last question: if I have discharge lighting, let's say 5A, then I should probably use a type C 6A circuit breaker. But if I multiply by the factor of 1.8, 5A * 1.8 = 9A. Should I use a type B 10A circuit breaker?
Are both valid options?

Thank you.

With discharge lighting you can have a high starting current for as long as 15 minutes depending on lamp and ballast type so whatever OCPD you chose it should be able to accomodate 1.8 times the run current for that length of time. Whether you could use a C curve MCB or not would depend on your Zs and whether required disconnect times would be met under fault conditions.

happysteve · Mar 2, 2016

Alessandro said:
Hello

Hello

Alessandro said:
I have a problem with design current (Ib) and selection of the circuit breaker (In).
If I have a circuit of 7A and I apply no diversity, then Ib = 7A and I select a 10A circuit breaker.
But if I apply diversity and I get, say, 5A, is 5A now the design current Ib? So can I select a 6A circuit breaker?

Yes (5A is now the design current) and yes, you can select a 6A circuit breaker.

Alessandro said:
Another problem: On-Site Guide says that for domestic cooking appliances I should consider the first 10A plus 30% of what's remaining. So if I have a cooker with 35A current demand, after diversity the current demand of the circuit is 10A + 0.3 * 25A = 17.5A. If this is the design current, then I select a 20A circuit breaker. But now this half of the full load current demand (35A), and if someone uses just 60% of the current demand the circuit breaker will trip.

If you've got a copy of the regs, have a look at figure 3A4 on page 325 (appendix 3). This shows the time-current characteristics of type B circuit breakers. For a 20A breaker, the trip time for and overload current of 35A is about 400 seconds (6m40s). By that point, at least one of the thermostats, probably more than one, will have kicked in, reducing your load and increasing your trip time. So in practice, unlikely to be a problem.

Alessandro said:
Last question: if I have discharge lighting, let's say 5A, then I should probably use a type C 6A circuit breaker. But if I multiply by the factor of 1.8, 5A * 1.8 = 9A. Should I use a type B 10A circuit breaker?
Are both valid options?

See Marvo's post above.