Question on MCB Loading

[self builder 123]

Hi

We have a workshop which is being supplied from the house via a 40A Type B MCB. In the workshop we have a dedicated consumer unit.

In the workshop we have 2 x 3kw fan heaters to be controlled via a room stat / contactor ( relay ).

We also have a 3kW vented water heater and a 3hp ( 2.2kw ) air compressor.

It is not possible to change the cable running from the house.

In the winter I expect that both fan heaters will be running, albeit via the room stat ( the workshop is well insulated ), the water heater will be on, albeit controlled via the thermostat on the unit.

Now when the air compressor kicks in to charge the cylinder I calculate that the total loading being drawn at that time will be :

( 3kW x 2 ) plus 3W plus 2.2kw = 11.2kw albeit only while the air compressor is running.

Based on 230Vac I calculate the theortical current drawn will be 48.7A.

Will the 40A MCB take the loading for a period based on the MCB time current characteristics or will it trip immediately ?

Cheers

 

[flameport]

48A will not trip a 40A MCB. It is highly unlikely that current would be drawn for any length of time anyway as all of the heaters will have thermostats in them.

On that basis, what you have will be fine.

However, a 3hp motor is a pretty substantial thing, if that starts when one or more of the other items are on, that might trip the breaker.
Does it have a soft start or is it just on/off?

The rating of 2.2kw would imply the motor was 100% efficient, which it clearly won't be, so the running/starting power could easily be more than that.

 

[mikhailfaradayski]

realistically i think you design demand would be about 39.6A

Very close, but it will probably hold.

 

[Steve]

How big is your workshop to require 6kw of heat?

 

[mikhailfaradayski]

re the 2.2kW comp. The duty on this would depend on how big the res is and what you are using it for.

 

[self builder 123]

Hi

Thanks for all your feedback.

1. The workshop is 7.1m x 7.1m x 2.5m tall. The 6kW requirement came from a heat loss calculation which I ran.

2. I don't think that compressor has a soft start - it will have a Type C MCB in the consumer unit to prevent a localised trip in the workshop.

3. Could you please advise how you arrived at 39.6A ?

Many thanks again.

 

[flameport]

Before changing to a type C breaker, some other information is needed:

What size and type of cable supplies the workshop?
How long is it (from house consumer unit to workshop)?
What breaker(s) are going in the workshop consumer unit?
Are there any lights in this workshop?

A picture of the meter / incoming mains area / house consumer unit will also help.

 

[mikhailfaradayski]

Just making an allowance for diversity. from OSG

 

[NotHimAgain]

BS 60898 mcbs must work within certain performance requirements.

One of these is 'conventional non-tripping current' - this specifies the multiple of the rated current that it must carry without tripping during the period of 'conventional time'

For a 40A mcb the multiple is 1.13, and conventional time is 1 hour. The unit must trip at a current exceeding 'conventional non-tripping current', but not exceeding conventional tripping current, within 1 hour - this multiple is 1.45 for a 40A device. Tests assume an ambient temperature of 30 degrees.

So your mcb may operate anywhere between just over 45.2A and just under 58A, at a time between 0 and 1 hour Good init.

These figures specify overload protection performance - but we must also considered fault current protection performance.

Your compressor might cause the fault current detection within the mcb to operate when it starts up. This would be most likely when you first fire up your workshop if you switch on all loads at the same time.

A type B mcb must trip instantaneously (which we take to mean with 0.1 seconds) at between 3 and 5 times the rated current. Your compressor might just trigger this if all other loads are also on.

A type C mcb is less sensitive (5 to 10 times rated current). However, I could not recommend that you just change the mcb as the circuit design (cable size etc) would have to be re-evaluated.

 

[self builder 123]

The workshop is supplied, I believe, via a 6 sq.mm SWA cable which runs under the ground from the house to the workshop.

I would estimate that the cable from the house consumer unit to the workshop consumer unit is approx. 30m.

All the other breakers will be MK Sentry Type B MCBs.

There will be 3 lighting circuits - one with 4 twin 1500mm internal fluorescent lights - one with 3 single 1500m internal fluorescent lights - one with 3 typical outside lights.

There is also a 32A ring main circuit plus two radial 16A circuits ( one for a fridge freezer ).

In the house we have an MK Sentry 21W insulated consumer unit which is where the cable is fed from - sorry no pictures.

 

[flameport]

30m of 6mm cable is almost certainly too long for a 40A type C.

 

[mikhailfaradayski]

but actually, i think i was wrong, my next offer is either

40.4A ( @240V)

or

42.2A ( @230V)

 

[mikhailfaradayski]

aahhh, in light of further info, i say this install will be dodgy.

I suggest getting a professional in to do an acurate assessment.

 

[AndyPRK]

I thought the OP has a Type C MCB in the worshop CU,
not the 40A house CU.

 

[sparkyspike]

That's quite a lot of volt drop too. Depending on how long the cable actually is, the volt drop could be too high for a 40A load. You could be losing 10 volts or so.

If the loop impedance is also too high, then you're really in trouble. You're only allowed 1.15 ohms impedance on a type B.