Air conditioning unit on a type C mcb

[Davey1988]

Hey guys

The air con unit has a start current of 83amps and a running load of 13amps. It's currently wired in 6mm which takes 47amps which is obviously lower than 83amps, would you say this is OK Aslong as the mcb protects the cable.

Saying that if I used a 20amp type C mcb it would take 160 amp to trip within 5 sec and would take 20 secs with 80amp which should be enough time for the starting current. But after its loaded I'm worried that it will take 60 seconds to trip on 50 amps which is not acceptable for the cable.

Any advice or help on this?

 

[sparks1234]

A lot of these air con units these days are soft start not taking the current like they used to, personally i would have it on a C16

 

[davesparks]

Oh deary deary me, time to revisit basic cable calculations methinks!

 

[Davey1988]

This would still feed 50 amps for 30 seconds. Without disconnection.

In a unlikely event of the overloading to 50 amps Constantly wouldn't this cause a problem. Do u see where I'm coming from. Is this allowed

 

[snowhead]

Slight bit of over-thinking going on here.


A-C running current 13a.
Cable protection 20a.
Cable rating 47a.

 

[Davey1988]

So are you saying I can install this in 2.5 since cable protection is 20 amp.

Seems unsafe and wrong.

Edit. I'm wanting to know of the logics of this that's all.

 

[Guest55]

So are you saying I can install this in 2.5 since cable protection is 20 amp.

Seems unsafe and wrong.

Edit. I'm wanting to know of the logics of this that's all.

you select a cable thats capable of carrying the nominal continuous running current of the equipment , which is 13A , not the short duration of initial over-current , which may be 50A and upwards.

whether that cable is to be 2.5mm will be dependant on circuit length & volt drop amongst other factors.

 

[Davey1988]

Ah I see. Since the length is 15 meters in 2.5 with 13 amps. It's 2.8899 volt drop which is fine and cable factors are fine which does not derate the cable below 20 amps.

Would I do the calcs on the start up current also or is the expected constant current flow is what we work from?

Would you install it in 2.5 going from the figures I've provided.

 

[davesparks]

So are you saying I can install this in 2.5 since cable protection is 20 amp.

Seems unsafe and wrong.

Edit. I'm wanting to know of the logics of this that's all.

Of course you can!

Cable size calculations are based on Ib, not on the peak inrush current!

If we based cable size on inrush current we'd all be wiring LED downlights in 6mm T&E

You are a qualified electrician aren't you?

 

[shanky887614]

Of course you can!

Cable size calculations are based on Ib, not on the peak inrush current!

If we based cable size on inrush current we'd all be wiring LED downlights in 6mm T&E

You are a qualified electrician aren't you?

be nice, his profile says he is a trainee

 

[Davey1988]

Am I right in saying the heat and cooling kw is not load kw on the air conditioning units.

 

[shanky887614]

Am I right in saying the heat and cooling kw is not load kw on the air conditioning units.

View attachment 26832

all you need really is
single phase at 220-240
power input e.g 12 amps

and 81a starting

 

[Davey1988]

One of the reasons I was baffled as it was installed in 6mm. Which is way to big. Either it stated on manufacture instructions or they didn't have a clue

 

[shanky887614]

One of the reasons I was baffled as it was installed in 6mm. Which is way to big. Either it stated on manufacture instructions or they didn't have a clue

im betting they dont have a clue, if i remember right 6mm is good for around 40ish amps

not 81amps if thats what they were looking at

might have been manufacters instructions but i find that a bit strange

 

[davesparks]

Heating and cooling capacity is exactly what it says on the tin.

It's the section labelled power input that concerns the electrical power input, as you can see it is expressed in both KW and Amps.

You'd normally want to see a power factor quoted on there too.

The starting current will help you to select a suitable OCPD for the supply to avoid tripping due to the inrush.

Taking a rough approximation your 20A mcb will see an inrush current 4X it's rating, and a type C I think is rated for inrush up to 6X it's rating.

 

[davesparks]

One of the reasons I was baffled as it was installed in 6mm. Which is way to big. Either it stated on manufacture instructions or they didn't have a clue

Is it an old supply which has been re-used for a new unit at some point in time?

It is quite possible that a previous AC unit needed a 32A supply and when it was replaced they just changed the mcb.

Or the supply was installed for 32A with incorrect information from the AC contractor and had to be down rated to 20A when they had fitted the unit

 

[Davey1988]

Do you wanna even know the worst bit. It had a 32amp type b.

 

[davesparks]

Do you wanna even know the worst bit. It had a 32amp type b.

What's wrong with that? It makes a lot more sense of the 6mm T&E if it has a 32A B type.

The MCB is there to provide fault and overload protection for the cable, it does not provide provide protection for the appliance.

A 32A type B will be fine with that starting current and sounds right for providing protection for 6mm T&E so no problem.

 

[Davey1988]

81amp start current....

Wouldn't it trip immediately

 

[Davey1988]

Just looked on the graph your right lol 50 sec trip time.

 

[davesparks]

81amp start current....

Wouldn't it trip immediately

Type B mcbs are designed to withstand a starting surge between 3 and 5 times their rated current.

81/32=2.53 so it will be fine with that startup surge.

Is this an existing installation or a new installation which has never been switched on? If it is an existing installation and it is working then you don't need any calculations or theory, the mere fact that it is working without the MCB tripping proves that there is no issue with the startup current.

 

[Davey1988]

There's a short in the circuit some where even with the air con disconnected, and the God knows what else it spured off to which it is easy running a new supply to the air con.

20amp type C with 2.5 flex would be fine.

Appreciate the help BTW. I know it's hard to get a bit of advice sometimes from people on here. They think I'm testing their ability or something.

 

[davesparks]

There's a short in the circuit some where even with the air con disconnected, and the God knows what else it spured off to which it is easy running a new supply to the air con.

20amp type C with 2.5 flex would be fine.

Appreciate the help BTW. I know it's hard to get a bit of advice sometimes from people on here. They think I'm testing their ability or something.

What exactly is the point of this thread? what do you want to find out?

If there is a fault on the circuit then find the fault and repair it. If only god knows what else has been spurred off from the circuit then either go and ask him about it or trace the circuit yourself and find what has been spurred from it.

You need to do some proper calculations before just assuming 2.5mm flex and a 20A MCB will be OK, can you meet the disconnection times with this arrangement? Is flex the most suitable cable to install? What containment will it be installed in or how else will it be supported?

Your profile says you are a trainee? is this correct? if it is then your mentor should be looking over your work and you shouldn't have to resort to asking cryptic questions here to find out what you should be doing.

 

[Resu]

We are wiring new heating systems for our local council and the spec is to supply the air source units with 6mm on B32s and the rating of the unit is only 11.5A!

 

[Davey1988]

Wired the air con with 2.5 flex and 20a type C. Works fine.

Who ever does their specs want shooting. Over estimated. Probz because it's not their money it's the tax payers.