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#209812 - 05/01/13 08:12 AM Refrigerant motor and defrost circuit
sparkync Offline

Registered: 10/17/02
Posts: 811
Loc: NC
When figuring the branch circuit size of a Cooler unit, do you add the motor circuit amps with the defrost circuit amps? On the nameplate it has them separate, but no total amperage or maximum fuse size. It only has the maximum fuse size for the motor circuit. The motor circuit is 6.88 amps with max. fuse size as 20 amps. The defrost circuit has 26.09 min. circuit amps with no max fuse size. Thanks.

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#209813 - 05/01/13 08:44 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
twh Offline

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
Maybe the defrost heaters share the compressor circuit.

#209814 - 05/01/13 08:57 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
sparkync Offline

Registered: 10/17/02
Posts: 811
Loc: NC
If I'm reading the code right ( which I'm probably not), I'm to add the two loads and add 25% to get the branch circuit size. Still figuring though...

#209815 - 05/01/13 09:38 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
gfretwell Offline


Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
It sounds like they want 2 circuits. What do the connecting lugs look like?

If you have 26.09 (call it 30) for the defroster, you can't get in under the 20a max O/C device for the motor unless the fuse is on the load side in the cooler.
When in doubt, ask the manufacturer what they are trying to tell you here.
Greg Fretwell

#209816 - 05/01/13 09:49 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
sparkync Offline

Registered: 10/17/02
Posts: 811
Loc: NC
All I got now is just the picture of the name plate. I don't access to the unit. Power of email smile

#209817 - 05/01/13 11:57 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
HotLine1 Offline


Registered: 04/03/02
Posts: 6785
Loc: Brick, NJ USA
If you have the mfg name you can get the wiring info via the internet. I recently found the info on a bunch of 'pre-owned' 5 deck cases for a plan review job. The EE had a 20 amp duplex on the plans!!! (Wrong)

Back many years, we ran a circuit for the compressor, and a circuit for the defrost. 12s on a 20 & 10s on a 30.

IMHO, go to the mfg website, and if the units are 'old', talk to the mfg tech guys.

#209825 - 05/02/13 08:35 PM Re: Refrigerant motor and defrost circuit [Re: sparkync]
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
When an Evaporator goes into Defrost Mode, the most common operation for an Evaporator with _Electric Defrost_ will have the Fans turned OFF as the Defrost Heaters are ON.

Air Defrost, and sometimes Hot Gas Defrost Modes will keep the Fans ON while Defrost Mode is active.

In your case, it is safe to Ass-ume the Fans will be shut down during Defrost Mode.

The Design Maximum Load for the Evaporator in question will be 26.09A - the Defrost Heater load.

There _SHOULD BE_ Two (2) individual Circuits feeding the Evaporator, which are typically derived from the Condensing Unit:
a.: Fan Circuit
b.: Defrost Circuit.

Per standard installations, Two Safety Switches (Disconnects) are mounted to the Evaporator;
One for the Fan Motors,
One for the Defrost Heaters.

Fan Disconnect should be a Fusible type, with (3) RK5 20 Amp Fuses (240V or 600V, as applicable).
The Defrost Heaters do not require Fuses at the Unit, just a local Disconnecting means, so a Branch Circuit OCPD of 30 - 35 Amps is compliant.

1.: In Defrost Mode, each Evaporator should draw the rated 26.09A steadily for a minimum 15 Minutes to a maximum 60 Minutes.

2.: Use NEMA 3R Enclosures as minimum. We use NEMA 4X commonly.
Drop from the Lid with LFMC (Sealtite); Use "Sealed Penetrations".

3.: If the Target Temperature is at, or below 34F (2C), use Conductors with XHHW Insulation.

4.: For extremely cold Environments, consider using "FREP" Cable.

5.: Verify if the Scope of Work includes Control Wiring (drop an additional Sealtite for this), along with Terminations (Defrost).

6.: The Fans and Defrost Heaters may derive from a Panelboard, in lieu of the Condensing Unit.
In this case, One 30 Amp Branch Circuit may be used for the Fans and Defrost.

A lot of this depends on what Spec's have been placed on the Proposed Installation.

If you have additional questions, feel free to contact me directly.

--Scott (EE)
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#209833 - 05/03/13 08:26 AM Re: Refrigerant motor and defrost circuit [Re: sparkync]
sparkync Offline

Registered: 10/17/02
Posts: 811
Loc: NC
Scott, thanks for the info. It helps me out greatly. I also contacted the manufacturer and am waiting for their input.
This is going to be a pretty major job if I get it. Just working on an estimate now. They are replacing a small walk in cooler with a much bigger cooler and freezer unit. It is requiring several units on top of roof area. Some are 3 phase and some single phase, 240 volts and one is 115 volts. A lot of circuits involved. Thanks again


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