heat pump electrical

[arthropod98]

ok, so we got our new heat pump and air handler installed, and i've got a wiring question.

currently, we've got 10 gauge wire proving the power, and i noticed they replaced the 30 amp breaker with a 40.  i talked with one of the dudes, and got them to go back to the 30.  i don't know tons, but i do know the wire size / breaker basics.

now, i was doing some reading, and from what i'm seeing, there are exemptions for a few things  --  heat pumps, welders, and maybe a few other things on a dedicated line.

so, the question is, is a heat pump on 10 gauge wire with a 40 amp breaker acceptable??

onto the specs of our heat pump (trane 4 ton 18 seer), which i think i understand:

Min Cir Ampacity =28
Max Fuse Size (Amps) =45

right now, we're still on the 30 amp breaker, and the heat pump's been functioning fine.

thanks for any info!  

[Herb G]

You are not counting start up amperage. It can be as much as 1.75 times the running current. I'd go with a 40 A breaker and #8 copper wire at least.

But that's just me. You did not mention if you have aluminum or copper wire.
It makes a big difference between the two.

[TDKPE]

Yes, the conductor ampacities everyone 'knows' to be the max are more complicated than the simple rules suggest. In the case of hermetic refrigeration, induction motors, transformers, welders, and some other loads, things other than the breaker or fuse limit the operating current, and the breaker or fuse is only there for short-circuit and ground-fault protection. You can have very small conductors with very large fuse protection under certain circumstances.

So it's entirely possible that the 40A breaker on 10 gauge wiring is code compliant and safe. That's without cracking open the NEC to check. But it is very common to see an 'oversized' breaker on HVAC equipment, since the internal controls limit the operating current. The breaker is 'oversized' to prevent it from tripping on a hard start or short-cycle slugging (don't know if that's even possible with modern equipment, though), and the internal protection trips then eventually resets. It would be bad for refrigeration equipment to trip the breaker with no possibility of resetting on its own. For instance.

I should add that if it works without tripping the breaker, and continues to do so in all conditions, then there's no reason to replace it.

[arthropod98]

thanks, herb . . . oh, and it's copper wire.  
 

thanks for the replies, fellas.  
 

i do understand about the startup, but doesn't a standard breaker have a bit of tolerance for that, or am i mistaken??  
 

thanks, tom . . . i was just really surprised when i was reading some of that stuff.  i think i did even see some of those NEC references  --  good grief.  

[TDKPE]

Like a 50A (primary current) arc welder on 10 gauge copper conductors (like mine
).  Has to do with duty-cycle, and number of welding machines on the circuit.

Or an inverse-time breaker sized up to 250% of the motor's tabulated full-load current (from an NEC table), with conductors sized to 125% of FLC.  Not something normally done with the little motors we deal with, but it starts to matter when the conductors get big, and are run long distances.  'Oversizing' is for transient absorption during startup, and overcurrent (for the conductors) and overload (for the motor) is via the motor control.  The breaker is for short-circuit and ground-fault protection of the conductors and disconnects.

[MKepke]

You are not counting start up amperage. It can be as much as 1.75 times the running current. I'd go with a 40 A breaker and #8 copper wire at least.

But that's just me. You did not mention if you have aluminum or copper wire.
It makes a big difference between the two.

Herb has the right idea (start up) but the wrong conclusion (install bigger wire).

The NEC allows certain HVAC equipment to use a larger breaker on a smaller wire-size than a person might expect because the NEC thinks nuisance trips due to the starting current are worse than the alternative AND the equipment itself has several internal overload controls.  See NEC 240.4(g)

Heck, the equipment manufacturer may be dictating the exact combination of wire size and breaker size...

Naturally the OP's building inspector may have other opinions..the NEC is not the supreme law of the land.

-Mark

[arthropod98]

yeah, i was reading some REAL interesting setups for welders!  


it's just something i wasn't familiar with, and i'm actually kinda surprised the installer dudes didn't bring it up.  
 

mark, i kinda started getting that the "min circuit ampacity" dictated wire size, and "max fuse size" dictated breaker size.

over 40-something years old and i learnt something new . . . wonder what i'll forget now though.  

[daddo]

With all good and clean connections, the 30 amp is fine.