An engineering project I am involved in requires the connection of a Glycol Chiller system to a power
source. The glycol chiller consists of two compressors, two circulating pumps, and one filter pump.
The electrical connection is "single point.” All of the pumps are fed from a control panel and
all of the equipment is located on a manufactured platform or skid. Originally, it was requested that
the chiller system be put on optional stand-by generator power. There is not enough capacity on the
generator to support the entire system. So the client ask that we feed all but one circulation pump
with normal power and feed the circulation pump with stand-by power so that, in the event of a power
outage, the circulation pump will operate on stand-by power while the other motors will not have power.
My concern is safety and liability. Is there anything in the National Electrical Code that prohibits
me from disassembling a manufactured system and supplying it with part normal power and part optional
stand-by generator power? Are there any procedures to follow? Signage, etc.
Alfa Tech Consulting Engineers
(408) 436-8300 Telephone
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I have done some checking, and have not been able to
find any code issues with refeeding the single motor as you indicated. However, I can think of several
reasons why it would not be a good idea.
- When you modify an engineered control panel, you set yourself up to take on all liability and
warranty concerning the control panel.
- When you change an engineered control package, you usually void all manufacturers’ warranties
and you could be liable for the entire cooling system.
- There could be dangerous consequences to having two different power sources to one piece of equipment,
and it would be extremely difficult to label where one ends and the other begins.
If the owners are set on modifying the
panel, contact the engineer that designed the equipment and get his/her help in the redesign process
so that you don't end up accepting the liability when things go wrong. I suggest getting a different
control package for the system that already has the motor separated for individual feed.
From: Dela, Sonny, GLENDALE Engineering
One standard I know that covers this kind
of installation is NFPA 79- Electrical Standard for Industrial Machinery. Section 7.5 - Allows that
packaged unit be supplied with more than one source. Section 4.5 - Requires a warning marking that
the main disconnect does not de-energize all source. Section 16.1.1 - Requires a yellow color code
for all wiring that may remain energized when the main disconnect is in off position. Please check
other related section of this standard.
Note: You might need to check how this will affect listing certification of the unit.
From: Alford, Scott
I have recently had a dilemma similar to this. The feeds should be separated by some kind of metal
barrier. The simplest way may be to remove the starter from the main control panel and mount it in
its own disconnect starter enclosure. This is then fed with 3-phase power from the stand-by generator.
The control circuit that runs the original starter can then be extended to the new enclosure to run
it. Specify with a warning tag that the disconnect enclosure is fed from multiple sources for safety.
The control voltage does not fall under the same rules for isolation as the 3-phase feed does.
Mike, we use standby power systems all
the time in our operation here at the gas company where I work. The main thing is not to backfed to
the other power source, in most states, there is a law against doing this, and it is dangerous to
line men working on outside lines. We do this all the time in our factuality by installing manual
or auto transfer switch's. The other thing that needs to be considered is if the system is UL listed
if this would void its listing in any way, and the safety of the system if it will operate in this
manner without damaging the rest of the system. This would be a hard question to answer without looking
at the prints and sequence of the system and the total operation of it.
From: Alexander I. Orloff, P.E.,
As I see from your letter, your concern
is the rearrangement/modification of MCC to accommodate standby power option. Assuming everything
else is reviewed and found to be normal for the proposed operation. The rearrangement of the manufacturers
protective components does create a problem and may void any warranties provided.
The solution to this problem can only be
resolved with the manufacturer/assembler of the skid-mounted equipment. The process of negotiating
an amendment to the warranty should be in writing. The NEC does not cover this problem as it is of
a legal nature. The equipment protection and wiring must follow the NEC code and UL label for intended
use. The concern for safety was not clearly described so there is not much that can be said about
that, however it is paramount in any installation or operation.
From: Mark Smith
Couldn't you simply install a separate transfer switch between the control panel and the standby circ.
From: Jody Wages
I understand the National Electric Code,
Article 110-3(b) to require that the manufactures instructions be used in the installation of listed
and labeled equipment. I would contact the manufacturer and tell them what you want to do and get
their blessing. They should be able to answer any other questions that you have concerning the breakout
of this motor and connection to an alternate power source.
From: Carl Brakob
I am working on a very similar application.
We manufacture chiller systems like you described above. A project we currently are involved with
has similar stand-by power requirements. The approach we arrived at was a little different though.
Instead of routing a separate stand-by power source to the chiller, the main power supply to the chiller
is switched through a transfer switch, and during stand-by operation a set of auxiliary contacts disables
the control circuit to all but one compressor motor, one circulating pump, and the condenser associated
with this package.
From: Bryan Shaffer
I don't think that you need to disassemble
the entire control panel to accomplish your goal. How about a manual transfer switch located at the
control panel to switch between the utility power and the generator. Then as the utility power went
down, a maintenance person following SOP could go to the control panel and turn off breakers that
were not required to run minimal systems. Then Flip the transfer switch to the generator and you are
up and running.
From: Charles Cobb
I would recommend you connect the entire unit to the emergency
branch and through relay control, drop out the undesired loads during emergency operation. This
may increase the "normal" load on the emergency system but it will not impact the size of
the emergency power source for the facility. This is a similar operation to locking out strip heat
and compressors of packaged air handling units during emergency power operation. Relays installed
in your ATS or MTS will provide a contact point for the control circuit. Specify that the chiller
manufacturer conform to the control sequence, this retains the warranty of the manufacturer and the
UL label of the equipment (if it has one).
Mike Holt’s Comment:
This is not specifically addressed in the NEC. I would work the equipment manufacture, install a transfer
switch where necessary, and insure the equipment complies with NFPA 79- Electrical Standard for Industrial