Sizing ocpd for outside Condensing unit

Status
Not open for further replies.
L

Low-impendence2017

Member
Location
Tallahassee Florida
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
 
Low-impendence2017 said:
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
Click to expand...

if it says max ocpd is 15A, then that is the max ocpd you can use.

if you upsize the ocpd you will have to upsize wiring and it may well make the light dimming worse.

the problem is likely to be that your service is not able to supply the inrush current.
 
Max is max, you cannot increase the OCPD size. You could increase the conductors size if you want.
 
amp clamp the ckt, what are the actual inrush and running amps ??
 
Low-impendence2017 said:
In 240 or 440 where is the code art that states size ocpd at 100%. Service is plentyful for loads! Thanks for the reply
Click to expand...
'The code allows the overcurrent protective device to be rated at 175% of the fla of the motor however if the nameplate states less then that is what is needed. If the nameplate says 15 amps is that the minimum circuit ampacity (mca)? If so then that usually has the 175% built in. Art. 440.22 states that you can be 175% of the motors rated load current.
 
Low-impendence2017 said:
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
Click to expand...
The rating of the overcurrent protective device has nothing to do with the lights dimming when the unit starts. Many times, this dimming is caused by the voltage drop in the utility transformer and the service drop.
 
Low-impendence2017 said:
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
Click to expand...

Inrush always causes lights to dim. You really can't do much about it.
 
I put a ocpd at the unit in a disconnect sized for the unit. If the conductors are larger I can put a larger ocpd in the main box. That is so you do not have to remove it on a retrofit. New units draw much less than 20 year old ones. I do not know if this is allowed though even though I have done it before.

I just figured I would try to answer a question since I just asked one :) That does not mean I am correct per the current NEC however!
 
Low-impendence2017 said:
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
Click to expand...

Is the breaker tripping? If not then what do you think a bigger breaker will do?

My guess is that there is a loose connection some place. Could be where a breaker plugged onto the buss. Main breaker connections. Meter connections. Weatherhead connections or even a poor connect at the transformer.

Have the power company put a load with the beast and see what you are getting. Could even be a bad splice in the overhead service cable.

I'm not kidding when I have these problems I normally find a bad connection some place.
 
peter d said:
Inrush always causes lights to dim. You really can't do much about it.
Click to expand...

It doesn't "always" cause it. It is pretty common though where the utility supply is not stiff enough to supply the extra current for that short period of time while the motor comes up to speed.

Keep in mind a 15 FLA motor probably takes no more than 60 A to start. If the service cannot supply 60 A without the lights dimming the problem lies with the service.

No amount of making the OCPD rating larger or increasing the conductor size is going to fix that problem.
 
Is this a new installation? A consideration maybe that the start capacitor is either not sized correctly or not functioning causing the "dimming: of the lights. I agree connections do cause these types of conditions as well. Thermal image or use a non contact IR thermometer and shoot the OCD.
 
petersonra said:
Keep in mind a 15 FLA motor probably takes no more than 60 A to start. If the service cannot supply 60 A without the lights dimming the problem lies with the service.
Click to expand...

The OP doesn't give any real information here that would help to figure out the problem such as service size. Is this a new 200 amp service or larger or is this an old house where the original service was probably 60 amps. If it's an overhead service then how far is it to the transformer, because an extremely long distance and cause a large voltage drop. How many houses are connected to this transformer?

I have one customer where they have a similar problem but the distance to the tranformer is probably 300 ft. overhead. Power company admits there is a voltage drop but they insist that it's within tolerance.

The only way to know if there is a problem with the service is to get the POCO out there and let them check it out.
 
Some people here will disagree with what I am about to post, that's fine.

My experience is that I used to work for an HVAC company as an electrician, and I was the service and warranty guy for the electrical department. In other words I had to solve hundreds of light flicker problems.

I followed 3 techniques with a fair amount of success when a light flicker problem occurred.

  1. Upsize the branch circuit wiring.
  2. Relocate the A/C breaker as close to the main breaker as possible.
  3. Torque all connections between the internal condenser leads and the main breaker as possible.

Using these techniques, you will mitigate the problem, not eliminate it. If your customer still complains, send it to the utility and have them verify their wiring and connections.
 
In the morning hours, put your recording meter on the line side of the main breaker, have the customer turn the A/C on and see what your voltage is. Record it and and try it 3 or 4 times and see if you get the same result.

Do the above test during the afternoon and record the results.
Do the above test during the late afternoon and record the results.

If you get consistent voltage drop then report it to the POCO, it is out of your hands. Causes of the flickering will depend on the time of the day as well. But loose connection on the POCO side, not properly sized service conductors, bad transformer connection, transformer is not sized properly.

However, if the line side measurement is within spec but the load side measurement is where you see the voltage drop then it is your job to troubleshoot the issue.

Could be a loose connection within the main breaker, loose connection between the service conductors and the main breaker (depending on how the panel is setup) it could be a loose connection on the meter side, loose connection on the meter jaws,
 
Low-impendence2017 said:
I think the inrush from the motor is causing the lights to dim.
Click to expand...

Nuber said:
Some people here will disagree with what I am about to post, that's fine.


I followed 3 techniques with a fair amount of success when a light flicker problem occurred.

  1. Upsize the branch circuit wiring.
  2. Relocate the A/C breaker as close to the main breaker as possible.
  3. Torque all connections between the internal condenser leads and the main breaker as possible.

Using these techniques, you will mitigate the problem, not eliminate it. If your customer still complains, send it to the utility and have them verify their wiring and connections.
Click to expand...

The OP used the word "Dim" to describe the problem.

There is a difference between a slight flicker at time of inrush current and a case of lights dimming.

Another thing to consider is when the problem started. Did it start at the time of AC install or was it working fine and then the lights started dimming when the AC unit turns on.
 
Low-impendence2017 said:
Im trying to make sure a breaker is sized correctly on an a/c unit. Nameplate says max 15 amp. Branch circuit ocpd is 15. Reason being is I think the inrush from the motor is causing the lights to dim. All feeder connections are tight. System is 240volt single phase. Would this be 1.25% x 15 = 18.75 ocpd next size up at 20 amp?
Click to expand...

If nameplate says to use a 15 amp max OCPD, that means they have already factored some extra into that for allowing the unit to start. This compressor maybe only has a full load rating of 7 or 8 amps, and maybe only draws 40-50 amps for a very brief moment when first energized.

If POCO has a small transformer and/or you have a long run of small service drop conductor, nothing you do on your end of the service aside from some kind of "soft starting method" will prevent the voltage drop that occurs when first energizing this compressor - and it is by no means a big compressor either, pretty typical to see units that require anywhere from 25-35 amps max OCPD as being average on a home.
 
Status
Not open for further replies.
Top