Balancing A Phase?

Status
Not open for further replies.
A

arjo_reich

Member
I have a rather esoteric question that may seem rather basic I'm sure, and if there is another resource I can persue for the answer please direct me as such. But I'm very curious....

I know that you calculate a circuit's load.
I know that you calculate a service's load.

However, are you required to calculate and balance a phase's load? Common sense says you would do so anyways for a variety of reasons, but is it required / safe.

I don't have plans with the knowledge per sae, just trying to learn a little.

I know many of the automation manufacturers I research suggest all devices you wish to be controlled need to be on the same phase until you want to buy rather espensive phase bridging devices so that they can communicate with each other...which seems to imply that there is some latitude in how this works.

---
Also, for those that have gotten to know me. If you could refer me to any type of reference / educational books on this type of theory I would be most appreciative. After all, any answer I can read or look up is one less question I'll have to ask whatever poor soul takes me as an apprentice.

Regards,

R. Joe Reich
 
George Stolz

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Re: Balancing A Phase?

However, are you required to calculate and balance a phase's load? Common sense says you would do so anyways for a variety of reasons, but is it required / safe.
Click to expand...
Balancing the phases does a couple things, at least on our side of the meter. For one, the more balanced the phases are at any given moment, the less current the neutral carries.

For another, with balanced phases, there is less likelyhood of having an overcurrent protective device (OCPD) open under load; Imagine a 100 amp 120/240V service, with the entire house on one phase. Imagine 120 amps of 120V line-to-neutral loads being connected to this phase. This house will effectively only allow half of the possible amperage before the main breaker kicks, because one of the poles of the breaker exceeded 100 amps.

Spread it out over both phases, and the main OCPD is fine, because each phase is only drawing around 60 amps.

If the loads were perfectly balanced, the load on the neutral would be zero. :)
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: Balancing A Phase?

how about someone that balances the phases by putting an a/c on one phase , and an electric space heater on another? the potential loads are balanced but it is very unlikely that the loads will ever be on simultaneously.

I am just not concerned about balancing. I make sure the feeds can handle all the potential loads that can be on at the same time and leave it at that.

And yes, I allow for space heaters and air conditioenrs to run simultaneously when i figure things.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: Balancing A Phase?

I have had it happen more than once that someone adjusted the the thermostats in a control panel so that the heater and a/c are running all the time. I don't want to trip anything. Its more important to keep the panel up than anything else.

Once I had a panel that was in an unheated space with a fan for bringing in outside air for cooling and a heater to prevent condensation. The plant electrician set the thermostats for both devices to 75 degrees because someone told him that was the ideal temperature to operate electronics at. You can imagine what happened when it got cold and the little 100 watt strip heater tried to keep the panel above freezing with a fan bringing in outside air continuously.
 
R

rcwilson

Senior Member
Location
Redmond, WA
Re: Balancing A Phase?

Be careful how you divide/combine heating and cooling loads.

At the end of a power plant project in the Southeast, we put the first floor unit heaters and the boiler building roof-mounted cooling fans on the only MCC with some space left, physical and electrical. We assumed cooling fans and heat would not be on at the same time and sized the feeders accordingly.(using engineering judgement, load diversity, etc.)

On Christmas Eve, the plant operators got cold because the roof fans were pulling in cool air through grade level vent openings. They turned on the first floor heaters which never turned off because the fans sucked the heat away. The MCC feeder breaker held, but the upstream breaker tripped and took down the plant.

My boss had me on the 1:00 pm cross-country flight on Christmas Day to investigate. At least it wasn't crowded.

Moral: Either plan for HVAC and heat to be on at same time, or set up the controls and interlocks so they can't.

(To correct the problem, I added parallel feeder cables and increased the breaker size, in addition to thermostat interlocking.)
 
Status
Not open for further replies.
Top