breaker type?

hello,
i recently replaced an old failed condensing unit with the same type, on starting the unit 230v it tripped the 16a b type breaker, i checked the start amps and it pulled 54a then ran at 9.4a (max run 11.5a on the unit plate) i advised the customer that a c type 20a breaker was required, the customer got an electrician out who said the breaker was ok for the appliction and the fault lay with my unit, was i right ? i went back today and checked it over again all ok except the breaker trips on 50% of starts.

please advise

thanks

I'd be inclined to fit a MOTOR rated type 3 (D) breaker to anything with a motor.
B curve is not designed for high inrush applications.

How long does the comp draw 54A?

Needs to be a 'C' or 'D' type to run a motor. 'B' types are for resistive loads only.
Sound like the sparky is used to doing domestic work only and not motor circuits.
If the max run is 11.8A then a 16 C/D type would do.

Jon

thanks for the reply boys, just wanted to make sure i wasnt talking bull!!!!!:D

oh the 54 amp is just the start amps only showing when the amp meter is on peak hold and only for 1/2 a second or less

Quote:

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Originally Posted by tait71

thanks for the reply boys, just wanted to make sure i wasnt talking bull!!!!!:D

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Starting current of normal compressor is 6 times grater then running current (which is exactly your case). Sparky need few days of additional education!

The tripping curve on a 'C' type mcb is rated for motor inrush currents and is the correct application for full speed motors.

The tripping curve on a 'D' type mcb is designed for Transformer circuits as these tend to have a higher and longer inrush current.

You may find that you could end up with damage to a compressor if you use a D type mcb as these will allow the motor to operate under fault conditions longer than a C type mcb.

Type 'B' mcb's are suitable for inverter driven motors as these tend not to have large inrush currents

From what I remember without getting the regs book out is that for instant tripping 0.1 secs
a type b needs 5 x rating
a type c needs 10 x rating
a type d needs 20 x rating of the breaker

so the 16a type b would be 16 x 5 = 80 amp would make it trip in 0.1 secs
a type c would need 16 x 10 = 160 amps for instant trip and a 16 amp type d would need 16 x 20 = 320 amps to flow before it tripped instantly.
So the 16 a type b is not in the the instant tripping part of the curve but in a near tripping area.
I would say the 16 c sounds best.

Would fit type D breaker for AC unit

just an update, the sparkie has fitted a c16a breaker and the plant is now running fine not trip for 24 hours, thanks for all the replys.

cheers

Some very useful info Guys!
Thanks Grizzly

Would always go for a C first, if there is still a neusance trip issue then fit D type.

Bumping this one again.
I've got an LG 7kW wall split that have nuisance trips.
It's wired with 2.5mm cable and a 16A c/b (c type)

The system FLA is 13A, it's a recip compressor with soft start PCB and extra capacitor.
When starting, it easily pulls 16A measured with a clamp meter.

Worth trying a 16A D type? I know the MCBs trip point is to do with way higher in rush current that I couldn't measure, it's the 16A I could see that's buggin me.
I'd rather not use a 20A due to the unknown longish cable run (unknown at this stage where the cable runs exactly through the roof and walls, ventilation, insulation etc)

I mean I guess I know the answer myself, try the 16A D curve, and if still a problem, run 4mm cable and a 20A MCB. For such installs that as this (7kw conventional recip split system with a over 15m cable length that I have done myself), I've usually got a sparky to use 4mm. But this system is over 5 years old and I don't know the history of the install, nuisanse trips, and neither does the new owner.

I'd try a D type as well, breakers can also get tired if they have been reset a lot in the past.

Had a customer with a 3ph breaker that would trip a few times every milking and ignored our advice to get it changed till the day it blew up as he reset it, think it needed a new board once the smoke had cleared :D

D type are a bit of a nail, if you dont have a option it looks like a C 20 to me, 2.5mm cable and its only start current so why not. wait for the rush of why nots ! :D

Quote:

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Originally Posted by monkey spanners

I'd try a D type as well, breakers can also get tired if they have been reset a lot in the past.

Had a customer with a 3ph breaker that would trip a few times every milking and ignored our advice to get it changed till the day it blew up as he reset it, think it needed a new board once the smoke had cleared :D

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Yeah the circuit breaker in this is new.
A electrician was called first. They replaced the probably good C16 with some cheap chinese C16 and told the customer that if it trips again the a/c is faulty and get someone to look at it :rolleyes:

edit:
If electricians of that calibre are the type to wire these installs, I don't trust their cabling enough to just go up a size to 20A :p
What I really mean is 16A is probably the right size c/b for the cable, and they got that part right, problem is they used the wrong size cable.

13 A for a 7 kW LG :eek:
Speaking of a very low COP.
There's something wrong with your unit or your clampmeter.

Ha, no, that is what is printed on the label of the A/C too.

We may replace in Belgium the breaker with a motor protector, of course setted to the propers AMPs of the motor and not exceeding max AMP for the cable.

A good point there Peter.
The MCB is installed to protect the cable, not the motor. You can increase the rating of the MCB providing you do not exceed the cable rating for the installation, taking into account proper cable sizing calcs including all derating factors.
To protect the motor, you need to install a motor overload, set approximately 15 to 20% above FLC

As a sparky all I can say is that we generally use 'C' curve breakers for everything in Australia (resi+commercial+industrial) as that is what the wholesaler stocks! I have seen 'D' curves in industrial apps. I have never seen a 'B' curve in service. I'm sure they're about.

For instant trip:
Type B = 4x rated current
Type C=7.5x rated current
Type D=12.5x rated current
(AS3000 Appendix B4.5)

Paul H
2.5mm2 is rated to:
27A spaced from surface
26A touching surface
20A exposed to sun
23A enclosed in air
20A partially surrounded by insulation (but laying on a surface)
19A partially surrounded in an enclosure clipped to a surface
13A completely surrounded by insulation
12A in an enclosure completely surrounded by insulation, 30A buried direct
30A buried in enclosure.
(AS3008 Table 10)

Max cct length using 80/20 rule for 2.5mm2:
(These figures DO NOT take volt drop into account, only fault loop impedance, frequently this distance is halved for volt drop. These numbers can be stretched with calculation and checking the upstream impedance)

Type B 16A MCB = 160M
Type C 16A MCB = 85M
Type D 16A MCB = 51M

Type B 20A MCB = 128M
Type C 20A MCB = 68M
Type D 20A MCB = 41M
(AS3000 Table B1)

Pheew!

Go the D curve 16A or try a 20A C curve. I don't think you'll burn the house down with a 20A breaker in the middle of winter. If that doesn't solve it then the AC is stuffed. It is an LG after all!!! Are the caps dodgy and causing a higher inrush current than necessary? Sell them a new install and fit an MHI 7.1kW. I have one with an 10M run on a 16A C curve and no dramas at all, its max current is like 10A on the clamp. Even an MHI 8kW will work on a 16A 2.5mm run, we got confirmation from MHI and did it on a 50M+ run no worries at a school using the old RAC supply. Then you are the hero for saving them a power upgrade and another $250.

For the last company I worked for I said stuff it and always used 4mm2 + 20A CB for 7kW units, especially fujitsu (warranty), LG and mideas. I wasn't paying for cable but all the other sparkies used 2.5mm and 16A because of stupidity/being tight for the boss.

Yeah it's all different for inverters. Even conventional scrolls are normally fine.
It's the big old 7kw recips that tend to draw higher current on start up. Normally need a 20A c/b on them.
Problem was I don't chuck a 20a c/b on an install that someone decided on a 16A c/b, because it might have been for a good reason, ie heavily surrounded by insulation.

How hard is it to trace the cable? In a single story house often its just run in the eaves or on the ceiling lying across the batts near the walls unclipped and you can spend 10 minutes making sure it isn't completely surrounded by insulation. If its run in the wall or a 2 story between floor/ceiling I would just leave it with a 16A CB. The 20A CB can be a quick test to see if a D curve 16A will do the job. Actually the D curve 16A will allow more startup current than the 20A C curve but a 20A C curve is a lot easier to come by cheap.

I'm going to try a d curve 16A, as I can get them cheap from a refrigeration wholesaler anyway.

Acutally a couple of months ago I had a similar problem when someone replaced a HRC fuse with a c type 20A on a recip. Heaps of nusiance trips. No electrical wholesalers even had a 20a D type or could even get one! Best they could do was 25A.

I went to a refrigeration wholesaler this time for this LG, and they said no probs, can get 16A and 20A d type easily.

Which wholesaler would that be Paul? Handy for me to know!

Actrol, they have them in their warehouse, had to order it in to the branch, but at least they can get them. after I asked for a couple, they said they'd get a few extra and carry them on the shelf at that branch.
L&H told me they couldn't even get a 20A D as it's not in their system and their suppliers don't carry them. Advanced electrical and rexel couldn't get them/didn't have them either