I recently had a home inspection in preparation for selling my house.
The inspector informed the buyer that I need to downgrade the circuit
breaker (from 40amp to 15amp) for new hvac? He said he sees this
problem a lot. My father, who does a lot of DIY including some
electrical, says that if we change to 15amp when the hvac switches to
aux. heat it will blow the breaker?
I need to find out if this really needs to be changed, the inspector
warned the hvac unit could be damaged if this isn't changed. But I
don't want to have the change done if it will cause problems when the
unit goes to aux heat.
I'd call the hvac installer, but, clearly, they didn't think it was
necessary to change the breaker & I'd really like an independent
opinion on this.
ALSO, is changing a breaker best left to an electrician or might this
be something we could do ourselves?
Thanks in advance for any input.
If the unit has an electric heating element in it that requires a 40 amp
feeder, you don't want to change it. It may be possible that two feeds are
required, one for the blower and a separate one for the heater and your
father installed an improper jumper to feed both. I would call a local
electrician to give a look
Thank you for your responses.
We didn't do any of the wiring ourselves, the breaker is the same from
the prior HVAC unit (ca. 1987), I'm fairly certain the contractor who
installed the hvac a few months ago didn't make any change to the
breaker(s). He seemed quite competent and earnest, so I'm a little
reluctant to accept the inspector's word as gospel. Either way, I'm
concerned about a liability situation down the road, so I'm trying to
research this prior to having someone in to make a change.
The inspector stood in front of the breaker box pointing to the 40amp
breaker when he made the statement, so I'm pretty certain he's
referring to the breaker. The buyer read aloud a liability disclaimer
based upon the inspector's report.
I read all of the literature that came with the unit - an electric
heat pump/air conditioning unit w/air handler.
I did find this within the paperwork for the new unit (not sure if
this is relative):
This unit is designed for single-phase electrical supply. DO NOT
OPERATE ON A THREE-PHASE POWER SUPPLY. Measure the power supply to
the unit. The supply voltage must be in agreement with the unit
nameplate power requirements and within the range shown in:
Nominal Input 208/240
Minimum Voltage 187
Maximum Voltage 253
I don't mind saying this is pretty much greek to me, but does this
mean if I check the plate on the unit I'll be able to tell what kind
of breaker is best (based upon the range info above)?
I plan on having someone come in to make the change, if it's
necessary, but I'd like to have an idea of whether it needs to be done
before 'going there.'
Again, thank you so much for any input.
Yes, but what Bill Allerman posted is important as well. The feeder cable
must be sized for a 40 amp breaker, which would be at least #8. Look on the
unit's name plate for an amperage rating or even a recommended circuit
breaker size. The full load amperage will determine the size wire and
On Sun, 24 Jun 2007 11:36:02 -0400, "RBM" <rbm2(remove
Nope, not on an HVAC condenser. Read the label on the condenser and
size the breaker/wire accordingly
You should see miminum circuit ampacity, that is the wire size based
on 301.16 (not 240.4(D) so 12ga is good for 25a, 14ga 20a.
Then look at maximum branch circuit over protection size and that is
the breaker you can use.
It will NOT be what you would think. This is for short circuit
protection, not overload protection, that is internal in the
compressor as a general rule.
You can very well see a 40a breaker on 14 ga wire.
All that said you still have to wire and protect the heater strips in
the air handler like a regular circuit, using the 310.16 rating on the
wire. Again that will be in the installation instructions as to what
wire and breaker combo you need for each heater option size.
For motors the rules change. The breaker protects the wire from short
circuits, not overloads, as gfretwell said. Rather arcane rules for
motors are among the reasons why electricians have licenses.
It is not possible to give an answer to the OP's question without
nameplate information for the equipment connected and wire size. I
wouldn't necessarily trust a home inspector apply the rules. I also
wouldn't necessarily trust an installer to do the electrical right -
particularly if the electrical was not done by an electrician.
Exactly right Bud.
In fact there is a question on every inspector exam I have taken about
a 115v 1HP motor with internal overload protection (16a FLA) and the
correct answer is "40a breaker, 14 ga wire".
I understand this is an anomally using all the exceptions and "round
up" rules but it is NEC compliant.
That's plain nuts!
Your run of the mill CB takes seconds to trip just from FLA currents.
There just isn't any need to put a 40 amp breaker on a motor circuit.
If there is something about the motor the keeps it from coming up to speed
in a second or two then fix that. But don't put a 40 amp breaker on #14
I suppose I could walk you through it all but this is what the code
The short answer is
16a FLA from the nameplate is 20a,
Article 430 (motors) is one of the exceptions to 240.4(D) (the 14a
rule) in 240.4(G)
310.16 (the wire ampacity table) lists 14 ga copper at 20a in the 60c
If you have internal overload protection you can use table 430.52 to
size the branch circuit O/C device. If that is an inverse time breaker
it can be 250% of FLA (16 x 2,5 @a)
FLA (full load amps) are not the problem. Starting current/LRA (locked
rotor amps) are. I presume this was a typo on your part.
As a rule of thumb a motor draws a current (LRA) of about 6x its FLA
when it starts.
Looking at a SquareD current - trip-time curve, at 6x the rated breaker
current a breaker may be in it's "instantaneous" trip region - no time
If the motor was rated 80% of the breaker rating (more likely) the
starting current is (0.8)x(6) = 5.4x the breaker rating. The trip time
can be 0.3 to 2.5 seconds - not reliable.
Obviously the motor current goes down as the motor accelerates. But that
doesn't help in the instantaneous trip region. And in the 0.3 to 2.5 sec
range there is no guarantee the motor will start. If turn up a
thermostat then turn it down a refrigeration compressor it will likely
not restart immediately. That causes the LRA until the motor protector
opens the circuit. You would generally not want the circuit breaker to
open. And high inertia motor loads increase the time to start.
The NEC is really quite pragmatic. Rules like this come from field
experience as well as engineering considerations. Not only can a circuit
breaker be 250% of FLA, as in gfretwell's example - if the breaker does
not permit the motor to start the breaker can be increased to 300 or
400% of the FLA.
And as has been a couple times, the breaker provides short circuit
protection for the wire, not overload protection.
I have one issue with your otherwise excellent post -- this last sentence:
The breaker provides *overcurrent* protection for the wire -- IOW, the breaker
is sized to open if more current is flowing through the wire than it can
safely handle. This does not occur solely through short circuits, but can
result from overload as well (e.g. fifty 60W incandescent light bulbs on a
single 15A circuit is supposed to pop the breaker, even though there is no
Doug Miller (alphageek at milmac dot com)
Note that gfretwell and I agree on this.
This is a motor circuit, not light bulbs. The rules are in article 430.
The circuit breaker provides *short circuit* protection - 430 part 5.
*Overload protection* is typically provided by thermal overload units
in, or attached to, the motor. - or motor starter (contactor) overload
units - 430 part 4. (Note that the overload protection is near the
motor, not the panel.)
Wire size is 430 part 2.
gfretwell's nice 'worst case' example gave code citations for his
calculations - would be a good place to start.
IMHO article 430 is of similar difficulty to article 230 - grounding.
Article 230 is the most difficult of the commonly used articles. As I
wrote earlier, there is a reason why electricians are required to have
licenses (in many jurisdictions).
If you (and John) are appalled by the 'oversizing' of the breakers for
motors make sure you don't read article 630 - welders. Particularly for
low duty cycle welders.
Right, I know -- I'm objecting to what appeared, at least, to be a general
statement that breakers provide only short-circuit protection. In the specific
cases you cited, that's correct, but as a general rule, it's not.
Doug Miller (alphageek at milmac dot com)
On Wed, 27 Jun 2007 15:43:25 GMT, firstname.lastname@example.org (Doug Miller)
Doug I think the point was that in the case of motors on dedicated
circuits the rules are different. You are right that in general
lighting circuits the breaker provides overload protection as well as
overcurrent protection. The installer has no control over what the
user will plug in.
That is the reason why we have atrticle 240.4(D) that gives us the
familiar rule of thumb 14gaa, 12ga a, 10ga0a
This includes the other general rule of thumb that you shouldn't load
a circuit more than 80%. The 80% is built into this limit when you
look at what 310.16 says the wire is really good for. (14ga a)
NFPA knows the typical user will keep loading a circuit until the
breaker trips and then unplug the clock if that is all it takes to
make it hold.
Right, I know that. I just wanted to make sure it was understood that as a
_general_ rule, breakers are there to provide overcurrent protection
generally, which is not the same as short circuit protection.
Doug Miller (alphageek at milmac dot com)
Read article 440 and 430, then get back to me.
You are citing 240.4(D) and is says
(D) Small Conductors. Unless specifically permitted in 240.4(E)
through (G), the overcurrent protection shall not exceed 15 amperes
for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper;
or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and
copper-clad aluminum after any correction factors for ambient
temperature and number of conductors have been applied.
HVAC units are one of the exceptions in (G)
Then change the breaker and add into the purchase agreement a disclaimer for
the inadequate breaker and lay hte blame on future breaker trips on the
incompetent inspector. Include power requirements (amp draw) of the HVAC
unit. Your attorney should know how to word it and lay the blame and
liability squarely upon the inspector's report. Be absolutely certain to
include the SIGNED (the inspector and buyer's signatures) inspection report
in the closing of the house. Have fun finding a 15 amp 240 breaker! 20 is
about the smallest I have seen and if you do find one, the wires might be
too big to fit them into the breaker! All you need to do to replace it is
to shut if off, take off the breaker box cover, unscrew the wires, pull out
the breaker, put the new one in, connect the wires, replace the cover, and
turn it back on. Simple job!
I can guarantee that it draws MUCH more than 15 amps unless it is a micro
.25 ton unit! My 2 ton requires a 25 amp breaker and it also needs the
furnace! An electric heat pump plus aux heat requires even more amperage!
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