Why two circuits?

zepfan

I am curious why ever geo manufacture on their equipment requires two electrical circuits? One specifically for the compressor, and one that does the controls, fan motor, and heat kit if there is one. It seems that it would make more sense to make the unit a single point connection. I used to think this was done by just one manufacturer, but every geothermal unit I see is made this way

109A_5

Are you referring to the building's electrical supply wiring or inside the equipment ? Ultimately both ?

Many reasons, (probably forgetting some);

NEC rules (maybe) or influence and/or other rules from another authority.

Building's electrical supply wiring;

Construction cost, larger wires and wiring methods needed.

Smaller more manageable building wire.

Better protection of the equipment (better application of over-current devices).

The loads may be in different locations.

Minimizing Arc Flash exposure hazard (personal and to the equipment).

Less cost of the components inside the equipment. Higher current components get bigger and cost more.

Compressors can be from 120 VAC, 240 VAC single phase to 480 VAC 3 Phase depending on the application. The control and lower power loads may only need 120 VAC single phase.

Troubleshooting benefits.

Inside the equipment;

Independent control of otherwise unique functionality.

A Relay, Contactor, SCR, TRIAC, IGBT or other switching device may be less expensive for a lower current device.

Circuit board and equipment real estate.

Wiring cost and management. Larger wires may cost more, termination methods and their cost is higher and harder to deal with.

Kaos

You can't connect a 25A compressor directly to a 50A circuit. You now need to add in power distribution and fusing. That cost money which either increases your price or reduces your profit, manufacturers don't like either.

Also not all people need resistance heat backup, so for them installing a single larger circuit would just add cost.

Jamie Hall

@109A_5 pretty well covered the basics — but I'll add or emphasize two. First, there is — inevitably — a voltage dip when any large motor starts (such as a compressor). Unless the wiring from the main switchboard to the unit is made considerably larger than needed for the running load, that voltage dip my be enough to upset even moderately stable relays, never mind electronic controls. A separate feed powering the controls eliminates this problem. The other factor I would emphasize is actually related — the controls, especially newer electronic ones — should be protected by fast blow fuses sized close to the circuit involved, and the circuit feeding them should be protected by either fast blow fuses (my preference) or fast acting circuit breakers, also sized to the circuit. A large motor, on the other hand, needs protection by motor starting fuses or circuit breakers. You can split the feed inside the unit — and many units do put fast blow fuses on the control circuits anyway — but there is much to be said for independent feeds.

mattmia2

the control will need its own built in overcurrent protection for any meaningful protection. Even a 15a 120v circuit will deliver something like 40x the normal power draw of the control circuit indefinitely without triggering its overcurrent protection.

109A_5

Thank you @Jamie Hall, I knew I was forgetting some stuff. Power quality for the control equipment, probably should have been first on the list.

Also in larger facilities with multiple units, power up and re-start sequencing. If the electric utility drops out when it is restored you don't want everything cumming back up at once. Also the restoration from a fire alarm system that shuts down HVAC equipment.

Depending on how they interconnect the systems for sequential restarting after a facility wide shut down event, having the control power independent may make the sequential power up implementation easier and less expensive.

EBEBRATT-Ed

That usually happens when you have resistance heat.

zepfan

thanks to all that responded. I was just curious because most all water source heat pumps (glycol or water) are single point, single circuit feed

It’s only geothermal where there is two circuits

109A_5

You would have to compare the equipment example you cited. I suspect the reason (possibly size or topology) would be obvious. Other larger commercial / industrial equipment often has some sort of dual power feed.