Refrigeration Load

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laketime

laketime

Senior Member
We are wiring a new grocery store and they have a refrigeration (cold case) unit that has a label with this info:
115v
1 phase
13.0 refrig cycle
locked rotor 46.0

I have never had to deal with locked rotor rating and does this play in at all on conductor/ocp sizing. Right now we are feeding this unit with #12's and a 20 amp breaker with meets the requirements I have read in the code. Any input or are we doing this right?
 
laketime said:
We are wiring a new grocery store and they have a refrigeration (cold case) unit that has a label with this info:
115v
1 phase
13.0 refrig cycle
locked rotor 46.0

I have never had to deal with locked rotor rating and does this play in at all on conductor/ocp sizing. Right now we are feeding this unit with #12's and a 20 amp breaker with meets the requirements I have read in the code. Any input or are we doing this right?
Click to expand...

Since the description does not qualify the 13.0 as Full Load Amps, FLA, you may be justified in treating it as that and going from there in the rest of your calculations.
The Locked Rotor Amps, LRA, is most important for evaluating starting surges, and hopefully will be short in duration. The motor's own overload, and a compressor should have one, will trip eventually at LRA, and your upstream OCPD should allow the LRA for a short period of time on its I vs. T curve. If the refrigeration unit is disconnected hill running and then reconnected without time for bleed-down, you will probably find the current at or near 46A until the internal overload trips out.
The conductor sizing will be based on FLA, and you would only have to worry about LRA specifically if there is a very long wire run and you were worried about voltage drop when starting under load.

Or, to sum it up, looks good to me.
 
GoldDigger said:
you would only have to worry about LRA specifically if there is a very long wire run and you were worried about voltage drop when starting under load.
Click to expand...
In which case the size of the wire need be increased. (I think there is a formula for that.)
 
The unit should have electrical specs's that tell you the Maximum Overcurrent Protection (MOCP). I use that to size the breaker then size my wire to the breaker size.
 
dav5y said:
I use that to size the breaker then size my wire to the breaker size.
Click to expand...
Sizing the wire to the breaker (Max OCPD) is conservative, but not necessarily required. MH and others feel that sizing the wire to the FLA, even though it is lower than the breaker size is appropriate, since non-short-circuit overload protection is provided by the motor overload at the other end of the wire.
If it is a short run or you just feel more comfortable with it, upsizing the wire to match the breaker will not cause anyone to write you up. :)
 
dav5y said:
The unit should have electrical specs's that tell you the Maximum Overcurrent Protection (MOCP). I use that to size the breaker then size my wire to the breaker size.
Click to expand...
The unit also had a spec that tells you the Minimum Circuit Ampacity (MCA). I use that to size my wire and I connect that size wire to a breaker that matches the MCOP.
 
jumper said:
I agree with David. How is it a 250.122 violation?
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It's not.
If you look at table 250.122 you would think being a 25 amp breaker it requires a #10 EGC.
but section 250. 122 (A) states size the EGC to table 250.122 but it also says "in no case shall they be required to be larger than the circuit conductors supplying the equipment.
So it is not a violation.
 
shortcircuit2 said:
250.122 says with a 25amp OCP, the EGC must be #10AWG...
Click to expand...

Try again........:)

The circuit conductors David proposed were #14.

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case shall
they be required to be larger than the circuit conductors supplying
the equipment. Where a cable tray, a raceway, or a
cable armor or sheath is used as the equipment grounding
conductor, as provided in 250.118 and 250.134(A), it shall
comply with 250.4(A)(5) or (B)(4).
 
Seems like a conflict of 2 parts of code.

So your saying that one sentence trumps the what table 250.122 lays out for sizing an equipment grounding conductor for a specific overcurrent device?

Right now I'm not convinced this is the correct way to look at this.
 
shortcircuit2 said:
Seems like a conflict of 2 parts of code.

So your saying that one sentence trumps the what table 250.122 lays out for sizing an equipment grounding conductor for a specific overcurrent device?

Right now I'm not convinced this is the correct way to look at this.
Click to expand...

You seem to be looking at the Table (T250.122) but ignoring the text of Section 250.122(A).

Read the section that Derek posted above. It is the text of Section 250.122(A) that directs you to use the Table 250.122, and it is the VERY SAME sentence that tells you that the EGC is not required to be larger than the circuit conductors. There is no conflict when you read the text.
 
shortcircuit2 said:
Seems like a conflict of 2 parts of code.

So your saying that one sentence trumps the what table 250.122 lays out for sizing an equipment grounding conductor for a specific overcurrent device?

Right now I'm not convinced this is the correct way to look at this.
Click to expand...

Well, since the sentence specifically mentions the table and then overrides the result of using the table, it seems pretty clear that is what is intended. :)
The fact that the table provides different answers for different situations does not affect the fact that the whole table process is being overridden IF the result from the table is larger than the rest of the circuit conductors.
Note that if the circuit conductors include a reduced size neutral, this does not allow you to just use the size of the neutral! The limit would be the largest of the circuit conductors.
Now hold tight as I stretch the rubber band even farther:
If the circuit is a 3 wire 120/240 or three phase wye in which you have provided an oversized neutral for reasons of non-linear loads, then you will be required to make the EGC at least as large as that neutral when the table requires that size or larger.
 
GoldDigger said:
Well, since the sentence specifically mentions the table and then overrides the result of using the table, it seems pretty clear that is what is intended. :)
The fact that the table provides different answers for different situations does not affect the fact that the whole table process is being overridden IF the result from the table is larger than the rest of the circuit conductors.
Note that if the circuit conductors include a reduced size neutral, this does not allow you to just use the size of the neutral! The limit would be the largest of the circuit conductors.
Now hold tight as I stretch the rubber band even farther:
If the circuit is a 3 wire 120/240 or three phase wye in which you have provided an oversized neutral for reasons of non-linear loads, then you will be required to make the EGC at least as large as that neutral when the table requires that size or larger.
Click to expand...

Now my head hurts........
 
GoldDigger said:
As intended. :)

Just another illustration that the workings of the NEC are convoluted, and any simplified rewording is likely to be wrong.
Click to expand...

"Well, since the sentence specifically mentions the table and then overrides the result of using the table, it seems pretty clear that is what is intended."

It does not override the table it merely states that the EGC in no case is required to be larger than the ungrounded conductors supplying the equipment. It can be larger but would serve no advantage as far as a ground fault.
If a ground fault where to occur there would never be more current returning on the EGC than the current limit of the circuit.
 
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