Discuss lighting circuits- Type B or Type C MCB in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

atm84

-
Reaction score
94
What do you guys tend to use Type B MCB for lighting circuits but know that some use Type C even when the loadings are low.

What do others prefer and why?
 
you'd use C type for lights with high start-up currents. bearing in mind that the Zs values would be lower when calculating your cable sizing.
 
I've always used Type B but come across a few installs where type c have been used. This is even with LED lighting. Just wondered if it was common practice years ago and some ppl are just stuck in their ways.
 
I've always used Type B but come across a few installs where type c have been used. This is even with LED lighting. Just wondered if it was common practice years ago and some ppl are just stuck in their ways.

Common practice is, or at least should be, to design the circuit for the load it is intended to supply.
 
I've always used Type B but come across a few installs where type c have been used. This is even with LED lighting. Just wondered if it was common practice years ago and some ppl are just stuck in their ways.

To note the inrush of a LED driver can peak higher than many other inductive lighting methods but as for domestic it would be a rare case that a type C would be needed.

Commercial and Industrial is a whole different ball game ;)
 
Ah thanks Darkwood. These have been for commercial installations. LED loads are generally lower than T5 luminaries. Recently seen 6amp type c mcbs protecting a circuit. Was playing devils advocate and wondered why that hadn't installed 10amp Type B. The runs weren't particularly long, approx 20m
 
The issue with inductive loads is not so much the individual fitting and its inrush (excluding the very large wattage ones).. its the accumulative inrush so say you had 400w metal hallides you may be limited to maybe 2 or 3 for 6amp mcb because you have multiple spikes all together for the mcb to cope with, leds can be the same if you have to many switching on together. Going to a C or D type is a possible solution to resolve tripping issues but to be honest if a C or D is in then it should only be there by design and not because the circuit wasn't designed correctly (Zs allowances in mind if uprating to a C or D)
 
or, perish the thought, the installing spark had used all his stock of type B the day before and only had type C s on the van.
 
on a similar idea. once was subbing for a well known national firm on a hospital refurb. doing the containment for lighting. trunking and galv. conduit. from the site foreman " we got no 20mm conduit, use 25mm, that's all we got". so, we wired all the lighting in trunking and 25mm conduit. following week, we're onto the ring finals for sockets. guess what? "no 25mm conduit, but the 20mm has been delivered. just do it". 6 x 4mm lsf cables down 20mm conduit with bends everywhere. used 20lb of lard from the hospital kitchen. bet is stank a bit on load.
 
The issue with inductive loads is not so much the individual fitting and its inrush (excluding the very large wattage ones).. its the accumulative inrush so say you had 400w metal hallides you may be limited to maybe 2 or 3 for 6amp mcb because you have multiple spikes all together for the mcb to cope with, leds can be the same if you have to many switching on together. Going to a C or D type is a possible solution to resolve tripping issues but to be honest if a C or D is in then it should only be there by design and not because the circuit wasn't designed correctly (Zs allowances in mind if uprating to a C or D)


Thanks for the reply Darkwood my query is if you have a 6 Amp MCB that may trip because of in rush current why wouldn't you uprate the MCB to a 10 Amp, obviously permitting cable sizing etc.

This is a new install with max 18 LED 600 x 600 luminaries on a circuit. (9 luminaires in each room)
 
Last edited:
The 3 MCB types and the speed at which they trip depends upon the level of overload and is usually determined by a thermal device within the MCB. All 3 MCB types use a magnetic fault protection which trips the MCB when the overload reaches a set level.
Type B trips between 3 and 5 time full load current, Type C trips between 5 and 10 times full load current and Type D trips between 10 and 20 times full load current. MCB ratings relate to continuous service although cables can carry higher currents for short periods without causing permanent damage.
 
Have a look at the time/current graphs in the Big Green/yellow/brown/red/ book(s).

Then it will become clear, hopefully.
 
Thanks for the reply Darkwood my query is if you have a 6 Amp MCB that may trip because of in rush current why wouldn't you uprate the MCB to a 10 Amp, obviously permitting cable sizing etc.

This is a new install with max 18 LED 600 x 600 luminaries on a circuit. (9 luminaires in each room)


Because the rating of the mcb isn't the issue here, its the peak inrush trip point that is too low... changing a 6amp(b) to a 10amp(b) only gains you a small margin on the peak trip where as replacing existing rating with a type (c) can gain you a better margin.

What a lot of people dont understand is when picking a type of mcb the tripping curve is the 'range the mcb may trip' due to inrush spikes so say you have a range 5-8 time mcb rating, this means the mcb could react and trip as low as 5x rated current but also it may hold up to 8x rated current on the next occasion, your inrush peak should clear the lower number so in the case of a 10amp rated your peak inrush shouldn't exceed 50amps ... if you peak is 55amps in this example then the mcb may function correctly 90% of the time but trip occasionally because you are very close to the lower limit but the more you exceed the lower peak trip value the more frequent the tripping.

Design wise when stacking inductive loads (all switch on together) you need to consult the manufacturers of the mcb's as to how many of your fitting types can be stacked together before any other calculation is made (this is opposite to standard methods where the mcb rating is usually worked out later), you'll be surprised by the limitations you will find, this is an area often messed up by the domestic squad because they haven't been taught how to design larger installs which is totally different to doing small premises like often found in domestic and small retail.
 

Reply to lighting circuits- Type B or Type C MCB in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

YOUR Unread Posts

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock