Max Breaker

Status
Not open for further replies.

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
After roughing in a new house and before final, we customarily get what is called a "service release". This is just an inspection with a few circuits connected to allow the POCO to connect power. This is mostly for HVAC to be turned on to acclimate the house for flooring, painting, etc.
When I was preparing the panel for the service release I asked the HVAC installer what size breaker he needed for his unit as it was not on site when I left. He told me a 30A breaker and that's what I put in.

Now I'm back doing trim-out and I just thought I would look at the unit to see what the nameplate said. The "Max Breaker" size was 40A. I know the 30A is legal but I don't want to be called back for nuisance tripping. However, this unit has been running for a couple of months and no reported tripping. Since this house is quite a distance for me to drive, do you think I should swap the breaker out for a 40A?
 

nickelec

Senior Member
Location
US
What is the minimum ckt ampicity? My guess is if it's been running for months your probably good, but as time goes by frequent over heating would likely cause that breaker to trip

Sent from my SM-G965U using Tapatalk
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
What is the minimum ckt ampicity? My guess is if it's been running for months your probably good, but as time goes by frequent over heating would likely cause that breaker to trip

Sent from my SM-G965U using Tapatalk

MCA was a little over 23A. It was 23.xx, I can't recall exactly.
 

kwired

Electron manager
Location
NE Nebraska
If it will hold the starting surge it will likely be fine. If you are running on some temp service and then later switch to permanent service, there is a chance there is less overall circuit impedance in your permanent service and that could lead to higher starting surge from that arrangement - increasing the chance it trips upon starting.
 

kwired

Electron manager
Location
NE Nebraska
I would leave the 30 amp CB. It should never trip under normal conditions.

NEC allows a 35 amp breaker for 10 HP 480 V three phase motor.

Many manufacturer selection charts suggest a 25 amp breaker.

I've seen many connected to a 15 amp breaker and they never trip upon starting, but that don't mean all will hold on a 15 amp breaker either. All depends on starting surge current, which will remain pretty consistent until you make some change to the supply circuit.

Had a big feed mixer motor at cattle feed yard several years ago - maybe 25 or 30 HP 230 volts. It ran for years on a 100 amp breaker no troubles. Then we happened to replace the supply line to that area with larger conductor - 100 amp breaker would randomly trip at startup - new supply conductors with less resistance meant starting surge increased enough to get into the instantaneous trip range on that breaker. Unfortunately it was a QO breaker and they don't make a 125 amp three pole so it took more than simple swap of a breaker to resolve this issue.
 

growler

Senior Member
Location
Atlanta,GA
I know the 30A is legal but I don't want to be called back for nuisance tripping. However, this unit has been running for a couple of months and no reported tripping. Since this house is quite a distance for me to drive, do you think I should swap the breaker out for a 40A?

What's the downside to swapping it out ? you purchase a new 40 amp breaker but you still have the 30 amp breaker that you remove and those get used all over the place. It's not like it goes to waste.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Leave the 30 and if the customer has a call back or something else for you then change it out. I would not make the extra trip unless you were in the area.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Max breaker?
I don't want to break Max.......:D

Sorry mods...........:(
 

growler

Senior Member
Location
Atlanta,GA
Now I'm back doing trim-out .

Leave the 30 and if the customer has a call back or something else for you then change it out. I would not make the extra trip unless you were in the area.

I wouldn't make an extra trip to change that breaker either. That would defeat the purpose of the change (save time and money). But if he is there doing a trim out.

Edit: you can't buy much gas for the price of a two pole 40 amp breaker ( not to mention the drive time).
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Many new units have a voltage range, my current unit has a range of 197-253 volts, MaxOCPD 30 amps, MCA 18.6 amps. I would assume that the 18.6 takes into account the unit running at 197 volts. If it's operating near the top of the range say at 240 volts the current will be much lower than the MCA/125%.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
If it will hold the starting surge it will likely be fine. If you are running on some temp service and then later switch to permanent service, there is a chance there is less overall circuit impedance in your permanent service and that could lead to higher starting surge from that arrangement - increasing the chance it trips upon starting.

It's not a "temp service". That get's replaced by the permanent service when you receive a "service release". As I said earlier, the "service release" just allows the POCO to energizes the service before final is ready. This allows the HVAC to run and also allows finish workers to plug in inside without dragging an extension cord across a finished floor. Also, have to have power of some sort to get a final. So basically, it's the main service without everything hooked up. All that's required for a "service release" inspection is grounds made up in the panel and one 120V GFCI circuit, whether that be one receptacle or several on the same circuit. The impedance doesn't change from "service release" to "final" .
 

kwired

Electron manager
Location
NE Nebraska
Many new units have a voltage range, my current unit has a range of 197-253 volts, MaxOCPD 30 amps, MCA 18.6 amps. I would assume that the 18.6 takes into account the unit running at 197 volts. If it's operating near the top of the range say at 240 volts the current will be much lower than the MCA/125%.
Running current is well under trip range, it is starting current that is going to be a potential nuisance tripping issue. Starting current is going to be impacted by overall circuit impedance during starting conditions. Same HVAC unit may trip the 30 amp breaker in one location because there is a pretty stout source, maybe short service run and short branch circuit length. Another install with same unit may never trip same breaker because the source is higher impedance, maybe has long service run to the source, but same install maybe sees more severe dimming of lights when this unit is starting compared to the first example.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If you Amprobe the unit when it's running the current is probably close to 15 amps @ 240 volts. A 30 amp inverse time circuit breaker sized at near 200% should not trip on a normal start up. I do get that the warm fuzzy feeling when I size the OCPD at the maximum but won't loose any sleep using a 30 amp CB in this application. :cool:
 

kwired

Electron manager
Location
NE Nebraska
I have a 3 ton heat pump in my house, 45A MOCP. I have taken readings when in heating mode and it is only pulling around 6 amps.

I don't know what peak starting current is, but I can tell you it starts up pretty quickly on utility power, if I try to run it on my generator, it really bogs it down trying to start, and won't start it if there is much for other load running at same time. Runs fine once you get it started on that generator.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Why didn't you swap it out when you noticed the issue. It was cheap then.

Don't sweat it now. Blame it on the AC guy.
 
Status
Not open for further replies.
Top