Elementary question on sub panel ratings

[Srv52761]

I am not an electrician, I monitor electrical use for a school district and try to figure out ways to reduce consumption and/or cost.
These two are not always the same because of how the utility bills us.

So given: an auxiliary building with say a 240v sub panel fed by a 60 amp breaker in the main distribution panel. We will be adding some equipment in that building.
The discussion is whether to upgrade the sub panel. An upgrade would entail a separate meter and hence an additional monthly meter charge and demand charge.
The building is almost exclusively 120volt circuits.

I contend that that 60 amp breaker in the main gives 60 amps per leg. We know which things will be on at what times.
By judicious use of balancing the legs I contend we could theoretically be running a total of 120 amps worth of equipment at one time.
(We don’t need that much, only about 60-65 but we want some slack)
My supervisor says we are limited to 60 amps.
Which is it?

 

[roger]

You are correct, each leg can carry 60 amps or serve 7200 Watts at 120 V, using 240 you can serve 14,400 watts.

Roger

 

[growler]

So given: an auxiliary building with say a 240v sub panel fed by a 60 amp breaker in the main distribution panel. We will be adding some equipment in that building.

The building is almost exclusively 120volt circuits.

I contend that that 60 amp breaker in the main gives 60 amps per leg. We know which things will be on at what times.
By judicious use of balancing the legs I contend we could theoretically be running a total of 120 amps worth of equipment at one time.

You do get 60 amps per leg.

What you or an engineer or electrician needs to do is a load calculation for the building. Once the total calculated load is known (measured in volt amps) it will be divided buy 240V.

 

[powerpete69]

60 amps total. 60 amps thru each leg only counts as 60 amps. We will assume you are using a 240V, double pole 60 amp breaker to feed sub panel. There will likely be a neutral and ground conductor as well. As said above, you will have 14,400 watts available with the 240V feed. P=IV. 14,400 watts = 240V X 60A. We typically design up to 80% of the rating. That being said, a full connected load can go way beyond 60 amps or 14,400 watts simply because things are almost never turned on at the same time, this is referred to demand load or diversity. So we typically design the demand load up to 80% of the 60 amp rating. It is my opinion that 95% of the electrical panels on this earth are overbuilt and never come close to reaching their full capacity. Which is a good thing.

Whether or not you want a meter on sub panel is up to you. Does the person or company using the 60 amp sub panel need to be charged separately for their electrical use? If so, please install a meter on the load side of the 60 amp breaker and charge them monthly per that meter reading.

 

[charlie b]

I contend that that 60 amp breaker in the main gives 60 amps per leg. . . . My supervisor says we are limited to 60 amps. Which is it?

In a sense, you are both right. However, you are both using terms that do not correctly describe what is happening. For starters, I strongly suggest that you forever drop the notion of "amps per leg" out of your vocabulary.

The two legs have their AC waveforms in opposite directions. When Phase A is rising towards its positive peak, Phase B is at the same moment falling towards its negative peak. At that moment, current is leaving the upstream source along Phase A, and it will return to the upstream source along Phase B. IT IS THE SAME CURRENT! So yes, you can load Phase A up to 60 amps of 120 volt components, and at the same time load Phase B up to 60 amps of 120 volt components. You could say (and I don't like to express it in the fashion) that you have 60 amps plus 60 amps for a "total" of 60 amps. It is in this sense that your supervisor is right. You are limited to 60 amps that will flow from the upstream source and return to the upstream source. But when one set of loads is seeing current in the positive half cycle, the other set of loads is seeing current in the negative half cycle. So you can feed 60 amps to each set of loads, and the system will still see 60 amps flowing to your panel.

 

[Srv52761]

....... Whether or not you want a meter on sub panel is up to you. Does the person or company using the 60 amp sub panel need to be charged separately for their electrical use? If so, please install a meter on the load side of the 60 amp breaker and charge them monthly per that meter reading.

It is all ours. When we have all the scheduled equipment running, it is likely we would have equipment with a total of about 60-65 amps @ 120 volts on at the same time.

The present set up was built about 70 years ago. The building is about 500 feet from our main building.
To upgrade from our main panel, we would be looking at adding conduit within the main building then laying cable under a parking lot and around a football/soccer field to get to the auxiliary building.
In the last 70 years our power company’s grid has expanded, so there are now overhead lines about 20 feet from the building. If we upgrade our supervisor would have the power company add a new service at the building, which they would do for free. But, then we pay the additional monthly fees.... forever, the exact opposite of what I was hired to do.

 

[roger]

But, then we pay the additional monthly fees.... forever, the exact opposite of what I was hired to do.

Why do you think it would cost more? In theory it would be a wash, you will be using the same amount of electricity one way or the other.

Roger

 

[powerpete69]

It is all ours. When we have all the scheduled equipment running, it is likely we would have equipment with a total of about 60-65 amps @ 120 volts on at the same time.

The present set up was built about 70 years ago. The building is about 500 feet from our main building.
To upgrade from our main panel, we would be looking at adding conduit within the main building then laying cable under a parking lot and around a football/soccer field to get to the auxiliary building.
In the last 70 years our power company’s grid has expanded, so there are now overhead lines about 20 feet from the building. If we upgrade our supervisor would have the power company add a new service at the building, which they would do for free. But, then we pay the additional monthly fees.... forever, the exact opposite of what I was hired to do.

If we understand you correctly, you need new electrical power to this little building. You can either get a new service from nearby overhead wires for a "small upgrade in price" according to you or you can run 500 ft of wire underground in conduit which is likely going to be fairly expensive. You will need to upsize your #6 wire over that great of distance.

Depending on the "small upgrade in service" cost compared to the cost of running 500 ft of conduit underground, it's very likely that a new 100 amp service from nearby wires might be cheaper.

Find out the exact costs of each and compare.

 

[LarryFine]

Find out the exact costs of each and compare.

Exactamundo!

 

[charlie b]

When we have all the scheduled equipment running, it is likely we would have equipment with a total of about 60-65 amps @ 120 volts on at the same time.

In case I was not clear enough in my earlier post, YOU HAVE NOTHING TO DO! There is not a problem that is in need of a resolution. It's OK as is.

A 2-pole, single phase, 240 volt, 60 amp panel has the ability to supply a total of 240 times 60, or 14,400 VA of load. 65 amps of load that is single phase 120 volts will use up 120 times 65, or 7,800 VA of load. You would be just over 50% of the panel's load capacity. I agree with your stated intention of balancing the loads on the two phases.

 

[Srv52761]

If we need to upgrade then the choice is clear, a new service is cheaper.
It is my contention to my supervisor that we can run the additional equipment without the upgrade as long as we make sure the equipment is split evenly between the two legs.

As to the cost...
Our bill, as almost all electric bills, is split into sections.
There is a charge for the actual electrons used, the kwh. The amount of energy would be the same, as Roger stated. But the cost would not. Our poco charges commercial customers a variable rate. The first 200kwh/kwdemand is at the rate of about 9¢/kWh. After that it drops. If we are a sub panel and just adding that building’s usage to the main building’s, that building would always pay the lower rate. If it is it’s own meter it would always pay the higher rate.
We will also incur a monthly meter charge on this new meter.
Then our power company has what is known in the industry as a ratcheting demand charge. A residential customer has a “facility charge”, it is their share of the power company’s infrastructure and overhead cost -a flat rate of $19/month. They will pay this regardless of the energy use,... every month. For a commercial customer $19 is not close. For us the poco monitors our use and takes the greatest amount of kilowatt usage in any 15 minute period and that becomes our 12 month demand lease. 65 amps @120v is about 8kw. The demand charge rate is about $9/kw. This space will be used for storage. During the summer we will have surplus furniture and paper products that must be protected from mold, several commercial dehumidifiers will be running. We will have various liquids in several small rooms. During the winter we will run small electric heaters to keep the liquids from freezing. We can schedule these at night or after school hours. Since the main building uses its greatest demand when the cooks are here, the usage of the electricity in the auxiliary building after hours would not exceed the reduction from the cooks and lights and things, so it can use the main building’s demand. If it has its own meter it can’t share demand and would have to pay its own.
My spread sheet estimates about $1500/year more if we upgrade.

If our present service is sufficient, it solves our problem.

 

[PaulMmn]

For us the poco monitors our use and takes the greatest amount of kilowatt usage in any 15 minute period and that becomes our 12 month demand lease.

In the 1970s IBM had a 'mini' computer designed for power control. It would 'load shed' within a user's facility to keep that KW usage below the threshold the company set. Interface relays were controlled by the computer to cut power to loads that could be shed without issue (at least in the short term). Typically, A/Cs, fans, heaters, water heaters &c were the loads that were 'easy' to shed; production line items had to be carefully analyzed to see what kinds of things could be shed-- great big heated mixer tanks-- can we cut off the heat for a few minutes?

IIRC, it was the IBM System/7

 

[Srv52761]

In the 1970s IBM had a 'mini' computer designed for power control. It would 'load shed' within a user's facility to keep that KW usage below the threshold the company set.....

We looked at load shedding programs. It was difficult. We have already gone to LEDs in our gyms and auditoriums. We are using the 25w t-8s in the classrooms. We have eliminated hot water in the body of the building and put point source units in the kitchen. As you mentioned HVAC is the big dog. But in a school setting that is difficult to load shed. By ASHRAE guidelines we must supply fresh air at the rate of 15cu.ft/min/student, about 400cu.ft./min per classroom. That means we are exchanging all the air in the building about every 16-20 minutes. There is never a time we can let it coast.
Even if we could it can cost us more in the end. Our demand charge is determined not during a specific time, but by our maximum consumption during ANY time. With 30 rooftop air handlers at our elementary schools to 190 at our high school there is a randomness to the compressors running. Seldom will they all be running at the same time. This helps keep our demand a little lower. If we load shed and let them coast for awhile, then eventually when the load shed time is over, all the air handlers will call for service at the same time, creating the very maximum demand we were trying to avoid (I discovered this was happening anytime a building held a fire drill in warm weather). Trying to conserve in a school setting is interesting.

 

[suemarkp]

As Charlie said, you most likely have no problem. But even if the calculated load ends up being close to or just over 60A at 240V, what is your wire and breaker size? It sounds like this is a long feeder, so things were probably increased for voltage drop so you could perhaps increase the breaker one notch or two. Code doesn't mandate voltage drop limits. Will anything in the classroom care if the voltage drop is on the high side? Computers will take anything from 100-250V typically. Resitive loads won't care. Are there any motors or compressors being fed from that panel?

 

[hbiss]

Will anything in the classroom care if the voltage drop is on the high side? Computers will take anything from 100-250V typically. Resitive loads won't care. Are there any motors or compressors being fed from that panel?

This space will be used for storage. During the summer we will have surplus furniture and paper products that must be protected from mold, several commercial dehumidifiers will be running. We will have various liquids in several small rooms. During the winter we will run small electric heaters to keep the liquids from freezing.

-Hal

 

[PaulMmn]

We looked at load shedding programs. It was difficult. We have already gone to LEDs in our gyms and auditoriums. We are using the 25w t-8s in the classrooms. We have eliminated hot water in the body of the building and put point source units in the kitchen. As you mentioned HVAC is the big dog. But in a school setting that is difficult to load shed. By ASHRAE guidelines we must supply fresh air at the rate of 15cu.ft/min/student, about 400cu.ft./min per classroom. That means we are exchanging all the air in the building about every 16-20 minutes. There is never a time we can let it coast.
Even if we could it can cost us more in the end. Our demand charge is determined not during a specific time, but by our maximum consumption during ANY time. With 30 rooftop air handlers at our elementary schools to 190 at our high school there is a randomness to the compressors running. Seldom will they all be running at the same time. This helps keep our demand a little lower. If we load shed and let them coast for awhile, then eventually when the load shed time is over, all the air handlers will call for service at the same time, creating the very maximum demand we were trying to avoid (I discovered this was happening anytime a building held a fire drill in warm weather). Trying to conserve in a school setting is interesting.

The load shedding program should take massive demands (like all the air handlers at once) into account-- and only start up some of them, and swap them around so every room gets some air.

What if you cut the A/C but let the fans continue to run? That might take some control rewiring to separate the 2 functions, but could save money.

Does your complete air exchange every 16-20 minutes continue on days when outside air is considered too polluted to breathe? What do you do then? Or are your air handlers equipped with filters that can purify the air you breathe?

 

[Srv52761]

We have done something similar in one of our all electric buildings, manually.
Our peak demand was being set between 8:30 and 9:30. Cooks were making breakfast, computer labs ramping up, all the lights coming on and HVAC wise, all the classroom doors were being opened and exterior entrance doors opening and closing. Only thing we could play with was HVAC. We split the building into three zones and started at 7:00 am, using a time of day schedule. From 7:00 to about 7:30 all the classrooms would get heat, set to 72° (our normal set point was 69°). When the cooks turned on their equipment we rotated, 2 out of the 3 zones would get heat whether they needed it or not. As the hallway lights and other stuff came on at 8:20 we switched to rotating 1 out of 3 zones. At 10:00 when the cooks were switching to warm mode, the outside temp was higher and we had fewer exterior doors opening and closing we switched back to letting the thermostat control the service. It saved about 85 kw of demand charges, but the temperature swings, depending on the outside temp, cloud cover and wind, were noticeable. The CO2 level occasionally went above 1350. This is not an optimum condition for a classroom environment, especially with students with various special ed problems.

Pollution in rural Iowa was not much of an issue. We had one building about 800 yards due west of a grain processing plant, but with the prevailing Westerlies even that little bit was enough to show a significant reduction of airborne particulates.

 

[Srv52761]

In case I was not clear enough in my earlier post, YOU HAVE NOTHING TO DO! There is not a problem that is in need of a resolution. It's OK as is.

A 2-pole, single phase, 240 volt, 60 amp panel has the ability to supply a total of 240 times 60, or 14,400 VA of load. 65 amps of load that is single phase 120 volts will use up 120 times 65, or 7,800 VA of load. You would be just over 50% of the panel's load capacity. I agree with your stated intention of balancing the loads on the two phases.

Charlie, thank you.
We will use the building as is.

As we anticipate using this storage building for a number of years, you have saved our district thousands of dollars.

 

[Srv52761]

Post script....
Thanks to all that responded. The sum total of the replies was probably needed to convince my supervisor our current set up could handle the equipment.