Condenser Question

[JohnDS]

1) I was told that when it comes to A/C condensers the 125% motor calculation does not apply. Instead you should just go by it's max ampere rating. So if it's 35a max load, you should size the circuit for 40a. If it's a 40amp max load, you should size the circuit for 40a also. Is this correct?
- if yes, is the continuous load calculation already taken into consideration with these numbers?


2) On the subject, i'm a little bit cloudy on the motor calculations in general. If you have an air compressor rated at 37a:

37a x 1.25 = 46.25a

So, size the circuit for 50a. Is this correct?

- and if it will be running for 3 hours or more what is the continuous load calculation and how does it apply in this example?


3)Is this the proper way to size circuits for all motors other than A/C condensers?
- what is the max allowable voltage drop on a motor circuit?

Thanks guys.

 

[kbsparky]

1) I was told that when it comes to A/C condensers the 125% motor calculation does not apply. Instead you should just go by it's max ampere rating. So if it's 35a max load, you should size the circuit for 40a. If it's a 40amp max load, you should size the circuit for 40a also. Is this correct?
- if yes, is the continuous load calculation already taken into consideration with these numbers?...

In dealing with A/C units, you use the information on the nameplate. You are looking for 2 different pieces of information:

1) Minimum circuit ampacity. This is what determines the wire size you use for your branch circuit.

2) Max overcurrent protection. This determines the circuit breaker (or fuse) size.

If the nameplate says max overcurrent protection =35 Amp, then you use a 35 Amp breaker. IF that same unit had a minimum circuit ampacity rating of 22 Amps, then you could utilize a #12 conductor for the branch circuit, connected to that 35 Amp breaker.

 

[stickboy1375]

Simple example...

 

[brantmacga]

for motors, you first find the FLC (table 430.248)

-Size the motor overload protection according to 430.32 (will answer your question on continuous duty)
-Size the branch circuit conductors @ FLCx125%
-Size the branch circuit overcurrent protection from 430.52 (you are permiited to use the next higher standard size if standard breaker sizes do not match your overcurrent protection needed.)


VD- 3% or nameplate will give safe operating voltage range.

 

[Chamuit]

I was taught by an engineer that the MCA (Minimum Circuit Ampacity) included the 125%. I do predominantly residential A/C's (circuits) for a larger A/C company.

So, using kbsparky's criteria:

If the nameplate says max overcurrent protection =35 Amp, then you use a 35 Amp breaker. IF that same unit had a minimum circuit ampacity rating of 22 Amps, then you could utilize a #12 conductor for the branch circuit, connected to that 35 Amp breaker.

We would pull 8-2 NMB from the panel to the A/C. Breaker it @ 35A. If you were in a pinch, you could legally pull a 10-2 NMB and breaker it at 30A because the MCA doesn't exceed (24A) 80%.

One more thing to add, we almost always have a disconnect installed on the A/C unit with fuses as well.

 

[brantmacga]

I was taught by an engineer that the MCA (Minimum Circuit Ampacity) included the 125%. I do predominantly residential A/C's (circuits) for a larger A/C company.

So, using kbsparky's criteria:



We would pull 8-2 NMB from the panel to the A/C. Breaker it @ 35A. If you were in a pinch, you could legally pull a 10-2 NMB and breaker it at 30A because the MCA doesn't exceed (24A) 80%.

One more thing to add, we almost always have a disconnect installed on the A/C unit with fuses as well.

you could legally pull a #12 and protect it w/ 35A; we're always in a pinch, why waste copper?

 

[Chamuit]

brantmacga

you could legally pull a #12 and protect it w/ 35A; we're always in a pinch, why waste copper?

I'm not sure which table you are using. Have I missed something?

I'm thinking 240.4 (D):-?

 

[brantmacga]

I'm not sure which table you are using. Have I missed something?

I'm thinking 240.4 (D):-?

Read the first line in 240.4(D). :wink:

Then look at Table 240.4 (G)

 

[Chamuit]

Okay, I don't get where you arrive at using #12 and breakering at 35A.

 

[infinity]

Okay, I don't get where you arrive at using #12 and breakering at 35A.

If you look at the graphic you'll see that the MCA is 23 amps. Go to 310.16 and select a conductor, #12 THHN @ 75 degrees C=25 amps. So #12 THHN is good. Next look at the Max OCPD in the graphic, 35 amps, this is the maximum sized OCPD you can use for short circuit and ground fault protection.

 

[Minuteman]

Let me put it in other words. Wire for the minimum, breaker for the maximum.

If you amprobe a unit that is clean and has the right amount of refrigerant, you will notice that it runs at or below the minimum amperage.

 

[Chamuit]

I was talking NM-B. Still, in my code classes, #12 is limited to 20amp breaker. You definitely couldn't get that to fly with an inspector here.

And, why doesn't 240.4 apply with the smaller sizes. I must have missed this.

 

[stickboy1375]

I was talking NM-B. Still, in my code classes, #12 is limited to 20amp breaker. You definitely couldn't get that to fly with an inspector here.

Why not? .......... You have to remember the AC Unit is providing the overload protection, the breaker is providing the short circuit and ground fault protection...

 

[iwire]

I was talking NM-B. Still, in my code classes, #12 is limited to 20amp breaker.

That is not true for all circuits. For typical branch circuits yes, 240.4(D) will limit 14, 12 and 10 to 15, 20 and 30 amp breakers.

However look at the first words of 240.4(D)

'Unless specifically permitted in 2404(E) or 240.4(G)'

Well 240.4(G) specifically allows 12 AWG on larger then 20 amp breakers for certain loads, such as motors, HVAC equipment, welders etc.

Take a look at Table 240.4(G).

You definitely couldn't get that to fly with an inspector here.

Unless you have a local amendment the inspector has to accept it.

 

[augie47]

That is not true for all circuits. For typical branch circuits yes, 240.4(D) will limit 14, 12 and 10 to 15, 20 and 30 amp breakers.

However look at the first words of 240.4(D)

'Unless specifically permitted in 2404(E) or 240.4(G)'

Well 240.4(G) specifically allows 12 AWG on larger then 20 amp breakers for certain loads, such as motors, HVAC equipment, welders etc.

Take a look at Table 240.4(G).



Unless you have a local amendment the inspector has to accept it.

Bob,
I agree, however, I would be willing to bet at least 1/3 of the inspectors
are not aware of 240.4(D) and would initally reject it.
Not saying, don't do it, just saying be ready to explain why.

 

[stickboy1375]

Bob,
I agree, however, I would be willing to bet at least 1/3 of the inspectors
are not aware of 240.4(D) and would initally reject it.
Not saying, don't do it, just saying be ready to explain why.

I would bet 1/3 of electricians do not know about 240.4(D).

 

[iwire]

be ready to explain why.

I agree. :smile:

I also will admit I do not think I have ever used 240.4(G), 99% of my work is from prints and the EEs seem to always base the wire size off that OCPD size.

They may not be aware of 240.4(G) or they may feel voltage drop is to much of an issue when using conductors based on the MCA.

 

[Minuteman]

I agree. :smile:

I also will admit I do not think I have ever used 240.4(G), 99% of my work is from prints and the EEs seem to always base the wire size off that OCPD size.

They may not be aware of 240.4(G) or they may feel voltage drop is to much of an issue when using conductors based on the MCA.

Give some people a little more credit. Those of us who install A/C units and many inspectors understand 240.4 (G) Sure, we could use bigger wire, but why not use a smaller wire, if it is okay by code?

 

[iwire]

Give some people a little more credit.

I am going by the posts on this forum, many people do not understand or do not know of 210.4(G). I do not think that can be disputed.

Those of us who install A/C units and many inspectors understand 240.4 (G)

Some of those who install A/Cs know that and some do not, I could probably find a hundred threads asking about it.

The fact you know and the inspectors know is great.

Sure, we could use bigger wire, but why not use a smaller wire, if it is okay by code?

I agree but I also posted a couple of reasons why, voltage drop or job specifications. In a home the run may be short, in a retail store the run may be 200' +.

The RTUs I deal with may have a breaker size of 200 and a MCA of 125 but most likely the EE will spec 3/0 or larger CU.

 

[infinity]

Let me put it in other words. Wire for the minimum, breaker for the maximum.

If you amprobe a unit that is clean and has the right amount of refrigerant, you will notice that it runs at or below the minimum amperage.

I should run below the MCA, since the MCA has an additional 25% of ampacity added to the compressor current by the unit manufacturer.