MCA to kW

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Npstewart

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This may seem like kind of a basic question but it is kind of critical.

I have an HVAC unit selection. According to the selection the unit has a MCA of 61 amps, MOCP of 70A. The voltage to the panel supplying this unit is 240-1. I need to turn this into kW for my panel schedules and load calcs.

Question: They list the MCA as 61.0 amps, the voltage listed on the selection is 230/1. When they figured the MCA of 61.0 amps, do you think they used 240 for a voltage or 230?

example:

240 * 61 = 14,640 watts

230 * 61 = 14,030 watts

Thats a difference of 610 watts. The panel is existing and is getting close to going over so I am counting every amp right now.

Thanks

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I'm inclined to say that they probably used 230V in their calculation, but that's really just a WAG. You could try calling the manufacturer, but your odds of getting to talk to somebody who understands enough about electricity to be able to answer your question (rather than just looking at the same spec sheet you already have and saying, "Yup, it says 230.") are extremely low.

Now let's throw NEC 220.5(A) into the mix:
220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
Click to expand...
Now the real question becomes, what did the CMP mean by "Unless other voltages are specified"? Specified on the equipment, or specified by the distribution system? Does it mean that you can use 230V instead of 240V because it's "specified" on the spec sheet or nameplate? Or does it mean that you have to use 240V because it will be connected to a 240V nominal system, but if you were connecting something to, say, a 300V nominal system, then you'd be allowed to use 300V?

I don't know the answer to that, and I've come across Engineers from both schools of thought on this issue. As I see it, you have two options:

  1. Use 230V for the calculation and hope the Plan Checker and/or Inspector doesn't disagree with your interpretation.
  2. Use 240V for the calculation. If that puts you over your limit, either tell the person in charge of the project that you don't have enough power available (I'm now having mental images of Scotty: "We don't have the power, Captain! I'm giving you all she's got!"), or revert to Option #1.
I've used both of these options in the past, and for me it has a lot to do with how I "feel" about the job. If it's a facility that I'm familiar with and I know the actual load on this panel is never going to get close to the calculated load, I'm fine with using Option #1 to make the numbers work out if I have to. If there are a lot of unknowns, I'm not willing to stick my neck out and say it will work, because I'm not 100% sure it will.

My default response, though, is to always calculate the worst case.
 
And in the case of a motor load, without knowing the details of the application you cannot be sure that the actual Full Load Amps of the motor will not be lower when the motor is fed from a higher voltage.
But since all you have to go by is the MCA figure on the nameplate that does not do you any good.
Secondly, if you are concerned about going over the rating of the panel, what will actually count is the current it will be carrying not the wattage it will be supplying.
So, wattage aside, I would say that you need to feed 61 extra amps from the panel, regardless of whether you feed the motor 230 volts or 240 volts. :)
 
Thanks guys. This comes up frequently for me and its not really a big issue but ive always wondered. I will probably go with 240v so there isnt any question but may have to resort to 240. Mainly the loads on the panel consist of lighting and general receptacle/show window load most of which will not ever be running.

Much appreciated!
 
How about this?

Table 430.248 Full-Load Currents in Amperes, Single-Phase
Alternating-Current Motors
The following values of full-load currents are for motors running
at usual speeds and motors with normal torque characteristics.
The voltages listed are rated motor voltages. The currents
listed shall be permitted for system voltage ranges of 110 to 120
and 220 to 240 volts.
Click to expand...
 
One way to look at it perhaps, is the MCA includes an extra 25% for the largest motor of the unit. If the motor/motor compressor is not the largest supplied by the panel, then I'd feel safe calculating it at 230V.


And if it is the largest, remember the MCA includes the 25%... ;)
 
Hello,

Out of curiosity, what is the actual voltage at the breaker?


PJHolguin :cool:

Npstewart said:
Thanks guys. This comes up frequently for me and its not really a big issue but ive always wondered. I will probably go with 240v so there isnt any question but may have to resort to 240. Mainly the loads on the panel consist of lighting and general receptacle/show window load most of which will not ever be running.

Much appreciated!
Click to expand...
 
It is base on the nameplate voltage of 230 the nominal system voltage of 240 can and will very.
 
Perhaps this got skipped over if I posted the same time as someone else!:)


Little Bill said:
How about this?
Click to expand...

Table 430.248 Full-Load Currents in Amperes, Single-Phase
Alternating-Current Motors
The following values of full-load currents are for motors running
at usual speeds and motors with normal torque characteristics.
The voltages listed are rated motor voltages. The currents
listed shall be permitted for system voltage ranges of 110 to 120
and 220 to 240 volts.
Click to expand...
 
GoldDigger said:
And in the case of a motor load, without knowing the details of the application you cannot be sure that the actual Full Load Amps of the motor will not be lower when the motor is fed from a higher voltage.
But since all you have to go by is the MCA figure on the nameplate that does not do you any good.
Secondly, if you are concerned about going over the rating of the panel, what will actually count is the current it will be carrying not the wattage it will be supplying.
So, wattage aside, I would say that you need to feed 61 extra amps from the panel, regardless of whether you feed the motor 230 volts or 240 volts. :)
Click to expand...
GoldDigger makes a very good point here. Since panel calculations are typically done in VA or kVA and then the total is converted to amps at the very end, that tells me you ought to use 240V. When you convert from VA to amps to show that your total for the board does not exceed the OCPD, you'll divide your VA by 240V, not 230V. So, if you're using 240V to go from VA to amps, you should also use 240V when going from amps to VA.
 
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