Determining amp ratings when measured voltage is between the nameplate voltages?

[Jon456]

I'm wiring a 3-phase air conditioning unit with an auxiliary heating element. The nameplate ratings are as follows:

Volts: 208/230
Min Ckt Amps: 57.8/61.9
Max OCPD: 60/70
Minimum Unit Disconnect: 33/36 (FLA) | 112/112 (LRA)

When I measure the voltage at the subpanel, it's 220V across each of the three phases. I believe the wire should be rated at 60A. But I'm not sure about the HACR type 3-pole breaker: do I rate it at 60A or 70A?

Also, I'm not sure about the rating of the disconnect switch which is to mounted at the compressor unit. I was given a 3-pole non-fused disconnect that is rated at 30A. However, the disconnect's nameplate states: "Suitable for use on a circuit capable of delivering not more than 10,000 RMS symmetrical amperes, 240 volts maximum, when protected by any other overcurrent device rated 30 amperes maximum." (Emphasis mine.)

 

[GoldDigger]

Also, I'm not sure about the rating of the disconnect switch which is to mounted at the compressor unit. I was given a 3-pole non-fused disconnect that is rated at 30A. However, the disconnect's nameplate states: "Suitable for use on a circuit capable of delivering not more than 10,000 RMS symmetrical amperes, 240 volts maximum, when protected by any other overcurrent device rated 30 amperes maximum." (Emphasis mine.)

Under the manufacturer's labeling, if the device is connected to a single set of breaker poles which feeds the compressor and therefore is 60 or 70 amps, you cannot used that disconnect. I don't think that any overload, as opposed to overcurrent, protection on the compressor itself will count.
And the wires from the panel to the disconnect and from the disconnect to the motor would also have to be large enough to be protected by the 70 amp breaker.

 

[Jon456]

And the wires from the panel to the disconnect and from the disconnect to the motor would also have to be large enough to be protected by the 70 amp breaker.

Since this is an application with a motor, it is my understanding that the wire need only be sized to the minimum circuit amps rating (60A in this case), and the breaker is sized to the maximum OCPD rating (60A or 70A depending on the supply voltage). The reason for the OCPD being higher than the minimum circuit amps rating is to allow for the inrush current (LRA) while the compressor motor starts.

I'm still not sure if I should be using 60A for the OCPD (based on a supply voltage of 208V) or 70A (based on a supply voltage of 230V), since my measured supply voltage is 220V.

 

[Gregg Harris]

Since this is an application with a motor, it is my understanding that the wire need only be sized to the minimum circuit amps rating (60A in this case), and the breaker is sized to the maximum OCPD rating (60A or 70A depending on the supply voltage). The reason for the OCPD being higher than the minimum circuit amps rating is to allow for the inrush current (LRA) while the compressor motor starts.

I'm still not sure if I should be using 60A for the OCPD (based on a supply voltage of 208V) or 70A (based on a supply voltage of 230V), since my measured supply voltage is 220V.

Your will use the second column for 230 at 70 and you #6 is good for 80 feet

 

[GoldDigger]

Your will use the second column for 230 at 70 and you #6 is good for 80 feet

Measured voltage at 220 should correspond to nominal system voltage of 240.
Similarly motor rating at 230 corresponds to use with system voltage 240.
Simple isn't it.

Interesting that the FLA is higher for higher voltage, indicating that the HP the motor is producing will actually be higher at the higher voltage. But the LRA figures are identical. Part of that may be related to the purely resistive aux heater?

 

[Jon456]

Interesting that the FLA is higher for higher voltage, indicating that the HP the motor is producing will actually be higher at the higher voltage. But the LRA figures are identical. Part of that may be related to the purely resistive aux heater?

These are the ratings with no auxiliary heater:

Volts: 208/230
Min Ckt Amps: 31.8
Max OCPD: 45
Minimum Unit Disconnect: 31 (FLA) | 112 (LRA)

So the difference in supply voltage only affects the pure resistive load.

Thanks to GoldDigger and Gregg for your assistance!

 

[Jon456]

Ok, so the wire will be rated to 60A (#6 for less than 80 ft) and the OCPD will be rated to 70A. Now about that safety disconnect switch...

I still don't understand the nameplate rating for Minimum Unit Disconnect. For my application, the numbers would be: 36 (FLA) & 112 (LRA). But how does that translate into a disconnect switch rating? Based on what I've read about disconnect switches for motor loads it seems that the switch should be rated the same as the branch circuit wiring, which would be 60A. But in light of the nameplate data on the 30A switch that I have here, does the disconnect need to be rated no less than the OCPD? And how do the 36 (FLA) & 112 (LRA) numbers figure in?

The switches I'm looking at are 60A and 100A. Would I be able to use a 60A switch? If so, and if it's fused, would I use 60A TD fuses?

Or do I have to use a 100A switch? If so, and if it's a fused switch, would I use 70 TD fuses to match the OCPD?

 

[Jon456]

Bump. I still need some assistance figuring out the disconnect switch ratings please.