HVAC Replacement Circuit

[mkgrady]

I'm looking at a cut sheet for a 10 ton A/C condensing unit. It operates on 208/230-3-60 and the MCA is listed as 40.6 amps. The existing unit has a #8 wire and a 40 amp CB feeding it. 40.6 being more than the 40 amp wire can handle tells me I need to upgrade the wire size to #6. What I am confused about is that the sheet states that the data is based on a system of 230-3-60. I will be connecting 208 not 230 volts. How does that affect the MCA? Does it make it larger or smaller or stay the same? If I could get the mca below 40 amps I won't have to change the wires.

 

[SIRSPARKSALOT]

Someone please correct me if I am wrong, but I see it like this:

MCA @ 230 = 40.6
40.6x230x1.732=16,173.4kW
[16,173.4/(208x1.732)]/3=44.9A

Since you are reducing the voltage you in turn increase your amperage. Now, for the wire selection, I will assume you have THHN. Therefore, the ampacity of you conductor is rated at 55A prior to any adjustments. If ALL of your equipment in the circuit were to have 75 degree terminations you would need to base your ampacity off the 75 degree column. So, you have an ampacity of 50A to work with not 40A.

Next, since you MCA is 44.9A @ 208V we need to compensate for the fact that it would be considered a continous load. Now your 44.9 becomes 56.125A (125%).

Your conductor at 75 degrees (if all of your terminations are rated as such) needs to carry a minimum of 56.125A.

 

[infinity]

Someone please correct me if I am wrong, but I see it like this:

MCA @ 230 = 40.6
40.6x230x1.732=16,173.4kW
[16,173.4/(208x1.732)]/3=44.9A

Since you are reducing the voltage you in turn increase your amperage. Now, for the wire selection, I will assume you have THHN. Therefore, the ampacity of you conductor is rated at 55A prior to any adjustments. If ALL of your equipment in the circuit were to have 75 degree terminations you would need to base your ampacity off the 75 degree column. So, you have an ampacity of 50A to work with not 40A.

Next, since you MCA is 44.9A @ 208V we need to compensate for the fact that it would be considered a continous load. Now your 44.9 becomes 56.125A (125%).

Your conductor at 75 degrees (if all of your terminations are rated as such) needs to carry a minimum of 56.125A.

If the unit says 208/230 and MCA 40.6 amps, you would use the 40.6 amps for sizing the conductors. Where does it say that you have to calculate the MCA higher if the voltage is 208 or add 25% for a continuous load?

 

[Dennis Alwon]

Usually a unit with dual voltage will have the dual amperage shown also

 

[Sierrasparky]

If the unit says 208/230 and MCA 40.6 amps, you would use the 40.6 amps for sizing the conductors. Where does it say that you have to calculate the MCA higher if the voltage is 208 or add 25% for a continuous load?

I would agree that you do not add for the continous load as that would be calculated by the manufacture. I would not think it was rated for 208 otherwise it would have a label as such.

 

[mkgrady]

Someone please correct me if I am wrong, but I see it like this:

MCA @ 230 = 40.6
40.6x230x1.732=16,173.4kW
[16,173.4/(208x1.732)]/3=44.9A

Since you are reducing the voltage you in turn increase your amperage. Now, for the wire selection, I will assume you have THHN. Therefore, the ampacity of you conductor is rated at 55A prior to any adjustments. If ALL of your equipment in the circuit were to have 75 degree terminations you would need to base your ampacity off the 75 degree column. So, you have an ampacity of 50A to work with not 40A.

Next, since you MCA is 44.9A @ 208V we need to compensate for the fact that it would be considered a continous load. Now your 44.9 becomes 56.125A (125%).

Your conductor at 75 degrees (if all of your terminations are rated as such) needs to carry a minimum of 56.125A.

The existing cable is 20 year old 8-3 romex so I assume it is only good for 40 amps.

I believe when the manufacturer provides the min circ ampacity (MCA) there is no other calculation required for continous load or because it is a motor load. All that is done for us by providing the MCA. I'm just not sure what I am supposed to do with the fact that the data is based upon a 230 volt system whereas I am connecting to a 208 system. How does that afffect what I do?

 

[mkgrady]

Usually a unit with dual voltage will have the dual amperage shown also

I agree that when they provide dual voltage they usually provide the dual amps but I have found that when they state the MCA it is typically just one answer. The only reason I am wondering at all it to see what I can do about being .6 amps over the wire size limit

 

[Dennis Alwon]

I agree that when they provide dual voltage they usually provide the dual amps but I have found that when they state the MCA it is typically just one answer. The only reason I am wondering at all it to see what I can do about being .6 amps over the wire size limit

In reality I doubt the unit really draws 40.6. Unfortunately 220.5(B) allows you to go lower when you are under .4 amps. I would run it by the inspector. That 40.6 has the 1.25 built in so I doubt there would ever be an issue.

 

[Dennis Alwon]

The mca may be the same because it calculates the same for 208 and 240???? I think

 

[infinity]

The existing cable is 20 year old 8-3 romex so I assume it is only good for 40 amps.

I'm just not sure what I am supposed to do with the fact that the data is based upon a 230 volt system whereas I am connecting to a 208 system. How does that afffect what I do?

Does the nameplate say 208/230 with an MCA of 40.6? If so then the voltage (either 208 or 230) is irrelevant. Since it's the MCA the manufacturer likely used the 208 volts to calculate the MinimumCA. Running on 230 volts just makes the current a little lower but doesn't affect the minimum conductor size. Here's an example of a 208/230 nameplate:

 

[mkgrady]

In reality I doubt the unit really draws 40.6. Unfortunately 220.5(B) allows you to go lower when you are under .4 amps. I would run it by the inspector. That 40.6 has the 1.25 built in so I doubt there would ever be an issue.

Good suggestion Dennis. Thanks, I will ask the inspector

 

[jxofaltrds]

Are we on 440 here?

 

[Sierrasparky]

Since when does one ask the inspector as to how the manufacture wants the part installed. We have to install the product as listed. If the unit is not listed for 208 then you are not supposed to install it that way. That is why they make buck boost transformers.

 

[mkgrady]

Since when does one ask the inspector as to how the manufacture wants the part installed. We have to install the product as listed. If the unit is not listed for 208 then you are not supposed to install it that way. That is why they make buck boost transformers.

I have already said the unit is rated 208/230. I was agreeing with Dennis that being .2 amps over the limit of what #8 wire could handle might be acceptable to the inspector. The unit will never draw the full 40.6 amps.

 

[mkgrady]

Are we on 440 here?

Yes. It is a condensing unit for a split system AC

 

[Sierrasparky]

Got it , Sorry, I retract my comment.