# AC condensing unit question



## SparkYZ (Jan 20, 2010)

Working on a TI. We pulled in a 30A single phase 208 circuit for a Liebert air conditioning condeser.unit shows up, nameplate says minimum circuit size is 25 amps, max fuse size 40 amps, and that the system actually uses 21 amps. Is my 30A circuit and 30A fused disconnect ok? It doesn't sound 100% right to me. Could someone cite a code article that would help?


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## NolaTigaBait (Oct 19, 2008)

SparkYZ said:


> Working on a TI. We pulled in a 30A single phase 208 circuit for a Liebert air conditioning condeser.unit shows up, nameplate says minimum circuit size is 25 amps, max fuse size 40 amps, and that the system actually uses 21 amps. Is my 30A circuit and 30A fused disconnect ok? It doesn't sound 100% right to me. Could someone cite a code article that would help?


Read 440. Is it ok?...Probably...I wire to the minimum and fuse to the max..THe mca has the 125% already built into fyi ...


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## BBQ (Nov 16, 2010)

SparkYZ said:


> Working on a TI. We pulled in a 30A single phase 208 circuit for a Liebert air conditioning condeser.unit shows up, nameplate says minimum circuit size is 25 amps, max fuse size 40 amps, and that the system actually uses 21 amps. Is my 30A circuit and 30A fused disconnect ok? It doesn't sound 100% right to me. Could someone cite a code article that would help?


If it only gives a fuse size you are required to use a fuse. 

You can use a 40 amp fuse with the 10 AWGs, that would be allowed for this application by 240.4(G).


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## rnr electric (Jan 29, 2010)

SparkYZ said:


> Working on a TI. We pulled in a 30A single phase 208 circuit for a Liebert air conditioning condeser.unit shows up, nameplate says minimum circuit size is 25 amps, max fuse size 40 amps, and that the system actually uses 21 amps. Is my 30A circuit and 30A fused disconnect ok? It doesn't sound 100% right to me. Could someone cite a code article that would help?


 i have had this discussion with the inspectors here many times. this is what they have told me. if it says max 40 and min ckt size 25 and runs at 21, put it at 30, i would run one size lrger wire in case you have to upgrade. i believe the 40 amp is for startup, the running amp is 21 and should be sized at 25 amps.. accordingly


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## NolaTigaBait (Oct 19, 2008)

rnr electric said:


> i have had this discussion with the inspectors here many times. this is what they have told me. if it says max 40 and min ckt size 25 and runs at 21, put it at 30, i would run one size lrger wire in case you have to upgrade. i believe the 40 amp is for startup, the running amp is 21 and should be sized at 25 amps.. accordingly


You could run #12 and fuse it at 40.


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## rnr electric (Jan 29, 2010)

NolaTigaBait said:


> You could run #12 and fuse it at 40.


 tried to "thanks" you but would not let me.. you are right..:thumbsup:


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## jhall.sparky (Jun 14, 2011)

rnr electric said:


> i have had this discussion with the inspectors here many times. this is what they have told me. if it says max 40 and min ckt size 25 and runs at 21, put it at 30, i would run one size lrger wire in case you have to upgrade. i believe the 40 amp is for startup, the running amp is 21 and should be sized at 25 amps.. accordingly


if the compressor fails it will trip the breaker 25a or 40a.
if the odfm(out door fan motor) fails it too will trip the breaker.

the min is 25a and add too that your 125% lra increase and ill say 30a is fine.:thumbsup:


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## NolaTigaBait (Oct 19, 2008)

jhall.sparky said:


> if the compressor fails it will trip the breaker 25a or 40a.
> if the odfm(out door fan motor) fails it too will trip the breaker.
> 
> the min is 25a and add too that your 125% lra increase and ill say 30a is fine.:thumbsup:


It has integral OCP. The breaker is for ground faults and shorts...You don;t have to add the 125% to the mca, its already built in.


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## jhall.sparky (Jun 14, 2011)

NolaTigaBait said:


> It has integral OCP. The breaker is for ground faults and shorts...You don;t have to add the 125% to the mca, its already built in.


what has "IT" already built in?


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## BBQ (Nov 16, 2010)

jhall.sparky said:


> the min is 25a and add too that your 125% lra increase and ill say 30a is fine.:thumbsup:


30 would likely work, or if it trips you are out a service call.

The breaker is a problem if units label only lists a fuse size and not a 'maximum over current protection' size.


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## NolaTigaBait (Oct 19, 2008)

jhall.sparky said:


> what has "IT" already built in?


The minimum circuit ampacity has the 125% built into to the equation. No need to add 125% to the mca. Unless I misunderstood your post.


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## rnr electric (Jan 29, 2010)

NolaTigaBait said:


> It has integral OCP. The breaker is for ground faults and shorts...You don;t have to add the 125% to the mca, its already built in.


this was also my interpretation.. conductor protection, not equipment specifically


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## JmanAllen (Aug 3, 2011)

Not always true. I do some hvac also and changed some compressors that were bad but nothing had tripped and I've changed fan motors that were bad but no trip or blown fuses either way

Sent from my iPhone using ET Forum


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## BBQ (Nov 16, 2010)

JmanAllen said:


> Not always true. I do some hvac also and changed some compressors that were bad but nothing had tripped


With most hermetic compressors they have an internal thermal protector that self resets until it finally breaks.

A compressor that is locked up should not trip the breaker it should go out on the internal OL protection.

If a compressor shorts to ground the breaker should trip. 

YMMV


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## jhall.sparky (Jun 14, 2011)

JmanAllen said:


> Not always true. I do some hvac also and changed some compressors that were bad but nothing had tripped and I've changed fan motors that were bad but no trip or blown fuses either way
> 
> Sent from my iPhone using ET Forum


 
not sure what your asking/saying ........bad as in short to ground or bad as in open winding (with no short occurring) . either im mis understanding or your confusing me either way by bad i meant "short to.. " where as an uncontrolled ammount of amps will be present- in turn tripping the olpd or breaker or what ever you are getting at , im not being argumentitive im just not sure what your asking/saying is untrue?:thumbsup:


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## SparkYZ (Jan 20, 2010)

So, as a rule of thumb, could I size the branch circuit conductors off of the minimum circuit ampacity, and size the ocpd off of the "max fuse size" ? 

We pulled in this circuit per plan. The HVAC contractor is screwed up. We already have the disconnect, and fuses, 30amp each, respectively. 
The circuits are fed from a 30A 2P breaker.

So I could legally put a 40A breaker on these, and use a 40A disconnect? If so, please explain. I know you can oversize OCPD for motors and such, but could never figure out where or how in the NEC it says it.

_440.6 (A) Exception 1: Where so marked, the branch circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the disconnecting means, the branch circuit conductors, the controller, and the branch circuit short circuit and ground fault protection_

Is that what allows me to use a 40A breaker on 10 AWG?


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## SparkYZ (Jan 20, 2010)

Anyone?


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## Roadhouse (Oct 16, 2010)

I set a 3 ton Nordyne condenser just now, minimum 23 circuit onto an existing max 30 amp breaker, number 10. I have a Goodman 4 ton with a similar max circuit rating that I'm installing first thing in the morning. What is existing is a 40 amp and not certain the wire size but it'll just have to do, and it will work regarldess.

After over a gazillion condensers set I can say it's not going to matter and that you're overthinking this, it's only a breaker. 30 or 40, just do it and be done.


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## Roadhouse (Oct 16, 2010)

I thougth I read something about start up. Incorrect. Start up is included. You can use a 25 amp on that 21 and it will hold for that milli-split second at over three times the amp draw, designed to be able to handle the what is known as "in rush current", aka start up.

Minimum is just that, start up included.


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## BBQ (Nov 16, 2010)

Roadhouse said:


> After over a gazillion condensers set I can say it's not going to matter and that you're overthinking this, *it's only a breaker. 30 or 40, just do it and be done.*


Says the non electrician .............:laughing::laughing:

Typical DIY hack.


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## BBQ (Nov 16, 2010)

Roadhouse said:


> I thougth I read something about start up. Incorrect. Start up is included. You can use a 25 amp on that 21 and it will hold for that milli-split second at over three times the amp draw, designed to be able to handle the what is known as "in rush current", aka start up.
> 
> Minimum is just that, start up included.


For what it is worth typical motor inrush is about _six_ times the marked running load. (LRA vs RLA)

Just because you don't see that on your amp clamp does not mean it is not there.


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## jhall.sparky (Jun 14, 2011)

BBQ said:


> For what it is worth typical motor inrush is about _six_ times the marked running load. (LRA vs RLA)
> 
> Just because you don't see that on your amp clamp does not mean it is not there.


is there a chart that you can point me to? not being smart just asking .......... also does the load on the motor at start-up have a coefficient? ............... im not being annoying just dont have any info on this ....... i also am pretty sure LRA isnt the same as inrush current as i start up motors and the inrush spikes the falls out; whereas LRA is going to stand because its a component issue fault , right? again im asking!


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## BBQ (Nov 16, 2010)

jhall.sparky said:


> is there a chart that you can point me to?


No, sorry, I have just read that somewhere and belive it to be true for standard motors



> not being smart just asking ..........


Has anyone accused you of being smart? :laughing::laughing::laughing:





> also does the load on the motor at start-up have a coefficient?


I do not understand.



> ............... im not being annoying


We will be the judge of that. :thumbup:




> i also am pretty sure LRA isnt the same as inrush current as i start up motors and the inrush spikes the falls out; whereas LRA is going to stand because its a component issue fault , right? again im asking!


LRA is 'locked rotor amps' and when a motor is first energized that is what you have. If the driven load has a lot of mass it will remain at LRA for a bit and of course start dropping quickly as the RPM comes up.


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## jwjrw (Jan 14, 2010)

SparkYZ said:


> So, as a rule of thumb, could I size the branch circuit conductors off of the minimum circuit ampacity, and size the ocpd off of the "max fuse size" ?



Yes that is how I always do it.



SparkYZ said:


> We pulled in this circuit per plan. The HVAC contractor is screwed up. We already have the disconnect, and fuses, 30amp each, respectively.
> *The circuits are fed from a 30A 2P breaker.*


*
*

Then you only need a 30 amp disconnect



SparkYZ said:


> So I could legally put a 40A breaker on these, and use a 40A disconnect? If so, please explain. I know you can oversize OCPD for motors and such, but could never figure out where or how in the NEC it says it.




Read section 430 and you could legally put it on a 30 A disconnect. 




SparkYZ said:


> _440.6 (A) Exception 1: Where so marked, the branch circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the disconnecting means, the branch circuit conductors, the controller, and the branch circuit short circuit and ground fault protection_
> 
> Is that what allows me to use a 40A breaker on 10 AWG?



Nope. Read 430


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## mgraw (Jan 14, 2011)

jhall.sparky said:


> is there a chart that you can point me to?


Here is a 2 ton scroll data.


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## madmaxx (Nov 12, 2010)

Overcurrent Protection Size

25 x 1.75 = 43.75A
Next size down 40A

Maximum Protection
Not more than 225%
25 x 2.25 = 56.25A
next Size Down 50A

40 or 50 amp protection for No. 10 is okay. That is based on calculation not on reality such as passing through hot areas that affect ampacities

Branch Current Conductor Size

25 x 1.25 = 31.25A

No. 10 wire

See 110.14(c)(1) requires 60C ampacities to be used for equipment rated 100A or less


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## Roadhouse (Oct 16, 2010)

It was a three.five ton i installed this morning. I have pics but later disconnected circuit amps


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## jhall.sparky (Jun 14, 2011)

mgraw said:


> Here is a 2 ton scroll data.


based on what parameters.............. i appreciate the fact that you took the time to answer at all thx,

J.HALL


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## mgraw (Jan 14, 2011)

Not sure what parameters you are looking for. This is a 2 ton Copeland scroll compressor 208/230V 1ph R410. Just used it as an example.


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## RIVETER (Sep 26, 2009)

SparkYZ said:


> Working on a TI. We pulled in a 30A single phase 208 circuit for a Liebert air conditioning condeser.unit shows up, nameplate says minimum circuit size is 25 amps, max fuse size 40 amps, and that the system actually uses 21 amps. Is my 30A circuit and 30A fused disconnect ok? It doesn't sound 100% right to me. Could someone cite a code article that would help?


You are less than 80% of a 30 amp circuit. I'd say you are okay.


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## jhall.sparky (Jun 14, 2011)

mgraw said:


> Not sure what parameters you are looking for. This is a 2 ton Copeland scroll compressor 208/230V 1ph R410. Just used it as an example.


on start up?>i assume. and im thinking what about the loaded evap and linset lenghts ? just a lot of unknowns rolling around in my head sorry? that is not your problem though ?


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## Service Call (Jul 9, 2011)

Some insp. in our area will require max. ocp. If the wire is pulled and terminated, well it is what it is. Other than that I'd run 8's & 10.


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## mgraw (Jan 14, 2011)

jhall.sparky said:


> on start up?>i assume. and im thinking what about the loaded evap and linset lenghts ? just a lot of unknowns rolling around in my head sorry? that is not your problem though ?


 The highest current on the compressor is when it starts. That is LRA as BBQ stated earlier. Once the compressor starts turning the current drops very quickly. Usually LRA is about 1/5 or 1/6 of a second.


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## jhall.sparky (Jun 14, 2011)

mgraw said:


> The highest current on the compressor is when it starts. That is LRA as BBQ stated earlier. Once the compressor starts turning the current drops very quickly. Usually LRA is about 1/5 or 1/6 of a second.


 
i understand ............ what if the compressor in the 2-ton unit draws 88.5 amps on start-up on a 110* day is it a bad issue with the electrical or even a fault?............ :001_huh:


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## Roadhouse (Oct 16, 2010)

Pulled the old 60 amp breaker disconnect off that was bypassed to begin with.









New 60 amp disconnect.









New 3.5 Goodman with a Copeland Scroll 13 Seer condensing unit. Note that the max fuse is 40 amp and that is what the breaker is in the electrical service panel. I had the 60 amp non fusable disconnect on the truck so that's what was installed, too bad so sad.










And of course to get that 13 Seer efficiency I had to remove the 4 ton piston and install the piston that came with the condenser in the evaporator coil. The evaporator coil luckily was the match to this condenser and already was installed, replaced not too long ago so now the system is matched perfectly. 100 degree ambient at 5 degrees superheat refrigerant r-22 charge. According to the Goodman chart, perfect and supply air temperature even before system stabilized was 63.


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## jhall.sparky (Jun 14, 2011)

Roadhouse said:


> Pulled the old 60 amp breaker disconnect off that was bypassed to begin with.
> 
> 
> 
> ...


good time to know the word NON-fuseable........ good job guy, now they can turn that baby down to 40*f and leave the fridge open:thumbup::laughing:


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## Roadhouse (Oct 16, 2010)

For those who don't know and those who might be wondering, the evaporator coil can change capacity from a 3 ton to a 3.5 ton to a 4 ton, all based upon the piston size that should be the same size as the condenser. The existing condenser was a 3.5 so that is what was installed today. I'm assuming that when the evaporator coil was replaced not too long ago the installer simply left the 4 ton piston in it as that is what came with it already installed so the system was not properly sized, a 3.5 condensr on a 4 ton evaporator. That lowers all efficiency ratings, negates them completely actually and for the most part lowers the rating.

It can actually lower it to below a 10 seer overall system.

Eh.


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## jhall.sparky (Jun 14, 2011)

Roadhouse said:


> For those who don't know and those who might be wondering, the evaporator coil can change capacity from a 3 ton to a 3.5 ton to a 4 ton, all based upon the piston size that should be the same size as the condenser. The existing condenser was a 3.5 so that is what was installed today. I'm assuming that when the evaporator coil was replaced not too long ago the installer simply left the 4 ton piston in it as that is what came with it already installed so the system was not properly sized, a 3.5 condensr on a 4 ton evaporator. That lowers all efficiency ratings, negates them completely actually and for the most part lowers the rating.
> 
> It can actually lower it to below a 10 seer overall system.
> 
> Eh.


at least the installer didnt put the txv in and leave the pistion in had that happen a time or two.


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## Roadhouse (Oct 16, 2010)

jhall.sparky said:


> at least the installer didnt put the txv in and leave the pistion in had that happen a time or two.


 
I have a txv to replace (it's sticking closed, head pressure in low 400's) in a few days that I think has the piston still in the line. I didn't have the time or the want to break open the lines at time of diagnostic.

if that does turn out to be the case, farkin' ****** is all I have to say about that. 

Btw, I got your pm. Thanks jhall.hvac.


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## Roadhouse (Oct 16, 2010)

jhall.sparky said:


> good time to know the word NON-fuseable........ good job guy, now they can turn that baby down to 40*f and leave the fridge open:thumbup::laughing:


 


Good thing about the Goodman sh chart, at 100 ambient the wet bulb doesn't really matter as all across the board it's 5 degrees. After I liquid charged the vacuumed lines, turned her on (he he!, eh.) and watched her stabilize, I started off with 30 superheat and kept charging according to the chart and it's all good.


Good to have guys interested and knowledgeable in hvac around these parts. :thumbsup:


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## jhall.sparky (Jun 14, 2011)

Roadhouse said:


> I have a txv to replace (it's sticking closed, head pressure in low 400's) in a few days that I think has the piston still in the line. I didn't have the time or the want to break open the lines at time of diagnostic.
> 
> if that does turn out to be the case, farkin' ****** is all I have to say about that.
> 
> Btw, I got your pm. Thanks jhall.hvac.


not a problem just a way to help beat the heat when all calms down!

by the way do you really need a f-150 i got a red 4door short bed for sale 2003 118k for $10,467 (KBB) OBO.


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## Roadhouse (Oct 16, 2010)

jhall.sparky said:


> not a problem just a way to help beat the heat when all calms down!
> 
> by the way do you really need a f-150 i got a red 4door short bed for sale 2003 118k for $10,467 (KBB) OBO.


My old faithful has and is serving me well, knock on wood.

Short bed is a definate no no, but thanks. 

Kinda high mileage for that price, bud. Around here we have '06's and '07's under 90,000k for around $7,500. That's about the average.

I'm looking for an extended cab '05 or newer with less than 80,000 with a 7' bed. My boss will buy it for me if you know of anything but again, no more than $7,500 as that's negotiable asking price around here.

Ford _*IS*_ the best in Texas. :thumbsup: They're a dime a dozen.


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## Roadhouse (Oct 16, 2010)

I bought that truck four years ago for $3,200 cash with 92,000 miles. I've replaced the serpentine belt, the battery, front brakes, lupper and lower ball joints, all shocks and tires and that's it. Kept oil changes at the regular 3,000 mile interval or as close to it as possible. She's at 159,000+ now and taking on an average of 100 miles daily.

Good truck.


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## mgraw (Jan 14, 2011)

jhall.sparky said:


> i understand ............ what if the compressor in the 2-ton unit draws 88.5 amps on start-up on a 110* day is it a bad issue with the electrical or even a fault?............ :001_huh:


Generally if the compressor draws more amps than the LRA on startup there is a low voltage problem or the rotor and stator could be touching.


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## NolaTigaBait (Oct 19, 2008)

madmaxx said:


> Overcurrent Protection Size
> 
> 25 x 1.75 = 43.75A
> Next size down 40A
> ...


What the hell are you talking about? The nameplate has all the calculations built in. You don't add 125% to the MCA ...and you don't add 225% to the MCA to get max fuse size. Wire the branch circuit for 25 and and fuse it @40...Also believe they say that it should be no more than 175% of the motor rating and if that isn't sufficient to go to no more than 225%...


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## BBQ (Nov 16, 2010)

NolaTigaBait said:


> The nameplate has all the calculations built in.


I agree, any new units I see provide the information directly.












Keep Your Cool When Working With A/C and Refrigeration Equipment

Air-Conditioning and Refrigeration Equipment


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## jwjrw (Jan 14, 2010)

NolaTigaBait said:


> What the hell are you talking about? The nameplate has all the calculations built in. You don't add 125% to the MCA ...and you don't add 225% to the MCA to get max fuse size. Wire the branch circuit for 25 and and fuse it @40...Also believe they say that it should be no more than 175% of the motor rating and if that isn't sufficient to go to no more than 225%...


If this has already been stated sorry.....
I still run across some old units that don't have the 125% figured in the nameplate. As far as new ones I agree the 125% is figured in already.


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## Gary007 (Aug 14, 2011)

30amp is absolutely fine,you can also fuse @40amp with the existing #10 because the constant running load ampsdoes not exceed 30amp ,the motor will pitch high @initial start up but not long enough to trip the breaker or fuse if you already have a 30amp installed


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