Nameplate interpretation

[JFletcher]

The refrigeration contractor installed the units, when I asked, they told me they came from the factory with the plug ends, I don't know if their electrician put them on or not, I was going to call the manufacturer but they have an overseas number, the whole thing seemed weird to me as they were L14s on a 3phase unit,

NEMA 14 series are not for 3 phase wiring, only NEMA 15 are.

...ofc, my question is why the heck they are cord and plug connected anyway, unless wasting a few hundred dollars per unit is what they want.

eta: a buck says they were returned from someone else who mis-specced or damaged them hooking them up to single phase, and the cord ends just came with them that way. I'd bet their (AC) guys did not take to time to install or spend the money on plug ends that would be on you.

 

[fifty60]

Typical condensing unit is a compressor and a blower motor. Both will have motor overload protection.

Should you have resistance heat elements involved - they don't change resistance they can not overload, they can develop faults though, but unless you fault to ground or line to line very near an end of the element, you still won't have overload level (in relation to supply conductor ampacity) of current.

I still have a hard time believing that heating elements cannot overload. I know they can short circuit without a doubt, but why can't they overload. If somebody screws up the grounding on the system, seems like that would be one way to get them to overload.

 

[Jraef]

I still have a hard time believing that heating elements cannot overload. I know they can short circuit without a doubt, but why can't they overload. If somebody screws up the grounding on the system, seems like that would be one way to get them to overload.

Protection values are not based on protecting something from being hooked up wrong. There are infinite ways to do something wrong, rules cannot cover them all (reasonably). Protection is based on things being hooked up correctly, but something GOES wrong, and most of those things can be anticipated. In the case of resistance heaters, there are only a few potential failure modes worth considering, none of which involve long time low level overloads like what you might find with a motor. A resistance heater is basically just a resistor, so think of a current limiting resistor used for other purposes, it has the same effect here. The only thing (not related to a mistake) that would increase the current flow through a resistor is a lowering of the resistance. The only thing that would lower the resistance is a short to ground or a phase to phase short, either of which will be a sharp enough increase to trip the breaker.

 

[GoldDigger]

440.2 Definitions.
Branch-Circuit Selection Current. The value in amperes
to be used instead of the rated-load current in determining
the ratings of motor branch-circuit conductors, disconnecting
means, controllers, and branch-circuit short-circuit and
ground-fault protective devices wherever the running overload
protective device permits a sustained current greater
than the specified percentage of the rated-load current. The
value of branch-circuit selection current will always be
equal to or greater than the marked rated-load current.

So, in the case of this nameplate, the BCSC is slightly greater than the sum of compressor RLA (17A) and fan motor FLA (.8A).
I still do not see in this particular case how the BCSC can override the requirements of the listed MCA of 24A.

 

[kwired]

I still have a hard time believing that heating elements cannot overload. I know they can short circuit without a doubt, but why can't they overload. If somebody screws up the grounding on the system, seems like that would be one way to get them to overload.

Connecting it to wrong voltage is one thing, but that is an installation error that would likely be noticed immediately. They are fixed resistance, so unless you have variable input voltage you have a fixed load that is either on or off.

If you connect 120 volt heater to 240 volts, you get twice the current, but 4 times wattage. If element doesn't burn out, it probably does at least kick out high limit control devices.

If you connect 240 volt heater to 120 volts, you get half the current but 1/4th watts. Not going to overload anything, probably also going to be noticeable reduction in performance.

Ground fault somewhere in middle of an element - like I said earlier, if very close to an end may be a high current that might kick out overcurrent protection, otherwise if far enough from end(s) may never result in enough fault current to open OCPD, but still may be noticeable performance issues, if you really want to respond to this situation there is always GFCI/ GFPE protection devices.

 

[jumper]

So, in the case of this nameplate, the BCSC is slightly greater than the sum of compressor RLA (17A) and fan motor FLA (.8A).
I still do not see in this particular case how the BCSC can override the requirements of the listed MCA of 24A.

The BCSC does not override the MCA of 24A, it overrides the RLA of 17A and .8A.

The value of branch-circuit selection current will always be
equal to or greater than the marked rated-load current.