Starting Current of Air Conditioner & CB Size

[Nedhal Al-Faraj]

Hi

I have read in NEC article 440 that overcurrent protection device of AC should be sized based on NAMEPLATE of AC. Also, I have read in EC&M Web magzine in article wrritten by Mr. Milk Holt (Jul 23, 2004) that the sizing of AC CB must carry the starting current.

I have AC unit of room ( Window Type) has Nameplate date shows that the rating current is 15 Amp and the starting current is 59 Amp. The CB used in our service panel for this branch circuit is 30 Amp breker which is not carrying the starting current !

Can you explain this for me ?
Is it a reason of TIme Delay , i.e, the breaker used in AC is Time Delay Breaker so taht it trips the circuit if the current is more than 30 Amp for period more than starting period ?

 

[suemarkp]

Most modern HVAC units have fixed this problem by having meaningful labeling on the nameplate. Today, they have Minimum Circuit Ampacity (MCA) and Maximum Breaker Size. This tell you what size wire to run (MCA) and how large you can go on the breaker.

What number is on the nameplate -- compressor amps, fan amps, electronics amps, all of it combined? Are any 125% factors incldued? To calculate MCA, you take motor and compressor nameplate amps, add 25%, and then add in the control amps. Asking an electrician to do this is stupid and may be impossible if there are no subsystem labels. To calculate max breaker size is complicated because you have the compressor and fan together on the same circuit so you have to merge the rules in the HVAC and motor section to calculate a value. Again, that is why the factory should do this, because they should know what works (and the equations allow even larger breakers if the system won't start...).

Generally, from the HVAC units I've seen, an HACR breaker will work if it is sized about double the compressor amps. Yours appears to be that size, but if just the compressor nameplate is 15A, then you're right on the edge. If the whole combined unit draws 15A, then a 30A should be plenty. You could have a defective breaker or an air conditioner that is failing.

Most circuit breakers are inverse time type. They have a thermal element that allows more than the rated current for a period of time. How much time depends on how much over the rating you go. A 30A breaker will go 35A for quite some time, 60A for maybe a minute. But there is also a magnetic trip section that quickly trips the breaker on short circuits. This way you don't have to wait to heat things up when the currents are huge. Somewhere around 150A, the magnetic section will begin to kick in on a 30A breaker.

 

[infinity]

Since this is a window unit AC I'm wondering what kind of receptacle you have with a 30 amp CB? With a 15 FLA does the unit have a 20 amp attachment plug?

 

[LarryFine]

I've never seen a 15- or 20-amp plug-equipped window AC trip its breaker upon start-up, unless the AC was in need of repair. I submit that the window unit is bad; likely the compressor.

Is this a new unit, or conversely, is it old?

 

[infinity]

I've never seen a 15- or 20-amp plug-equipped window AC trip its breaker upon start-up, unless the AC was in need of repair. I submit that the window unit is bad; likely the compressor.

Is this a new unit, or conversely, is it old?

My thoughts too Larry. I'm just curious as to the plug/receptacle/OCPD set up for this unit.

 

[Nedhal Al-Faraj]

Dear suemarkp

Thank you for your response, you gave me very useful information.

My question was : Why the CB isn't sized based on Starting current?

the portion of your answer

Most circuit breakers are inverse time type. They have a thermal element that allows more than the rated current for a period of time. How much time depends on how much over the rating you go. A 30A breaker will go 35A for quite some time, 60A for maybe a minute. But there is also a magnetic trip section that quickly trips the breaker on short circuits. This way you don't have to wait to heat things up when the currents are huge. Somewhere around 150A, the magnetic section will begin to kick in on a 30A breaker.

This imply that the 30 Amp CB of my unit is covering the startup current but itsn't trip because the period of startup current is very short and less that the triping time and for sure it will trip if the startup cureent reaches the triping time.




Thank you guys also for your help .

 

[infinity]

Dear suemarkp

Thank you for your response, you gave me very useful information.

My question was : Why the CB isn't sized based on Starting current?

the portion of your answer

This imply that the 30 Amp CB of my unit is covering the startup current but itsn't trip because the period of startup current is very short and less that the triping time and for sure it will trip if the startup cureent reaches the triping time.




Thank you guys also for your help .

But why do you have a 30 amp CB in the first place? Is this plugged into a 30 amp receptacle? Does the AC have a 30 amp plug?

 

[suemarkp]

My question was : Why the CB isn't sized based on Starting current?

Because there may be no standard for "starting current". Is that locked rotor current (LRC), or some other modified value that is smaller? The NEC section on motors is rather long, and it may take LRC into account when sizing breakers, but I don't remember. But this should come from the HVAC section and I don't believe LRC is used for breaker selection.

As others are saying, I think you have a problem with your AC. Did it come with a power plug and if so what amp rating was it for?

 

[dlhoule]

Quote:
Originally Posted by Nedhal Al-Faraj
My question was : Why the CB isn't sized based on Starting current?


I'm thinking he couldn't understand why breaker wasn't tripping.