Demand Question

[Grouch1980]

Hey guys,
I came across this statement: "no more than 400 amps per phase per half hour demand".

Per half hour demand: does this mean for every 30 minutes, the usage cannot exceed an average of 400 amps?

 

[Dennis Alwon]

I asked this question to AI

ChatGPT
The statement "no more than 400 amps per phase per half hour demand" typically refers to an electrical power supply context, often related to industrial or large-scale commercial electricity usage. Here’s what it generally means:

1. Amps per Phase: This indicates the amount of electrical current (in amperes) flowing through each phase of a three-phase electrical system. In this case, it specifies that the maximum allowable current per phase should not exceed 400 amps.
2. Per Half Hour Demand: This part refers to the time period over which the current demand is measured. It suggests that the maximum current of 400 amps per phase is allowed as an average over a half-hour period. This is important in scenarios where electrical loads can vary significantly over time (e.g., in factories or large buildings).

In practical terms, this statement implies that the electrical system or the equipment connected to it should not draw more than 400 amps per phase continuously over a half-hour period. Exceeding this limit could lead to overheating of wiring, equipment failures, or potentially tripping of protective devices like circuit breakers or fuses, which are designed to prevent damage from excessive current.
Therefore, adhering to this limit ensures safe and efficient operation of the electrical distribution system, preventing overload conditions that could compromise safety and reliability.

 

[Grouch1980]

Thanks Dennis.
Since it's an average... is it safe to say it's not an issue if the current draw exceeds 400 amps at any point in time? As long as the average doesn't go above 400.

 

[ggunn]

Thanks Dennis.
Since it's an average... is it safe to say it's not an issue if the current draw exceeds 400 amps at any point in time? As long as the average doesn't go above 400.

That's what the statement says.

 

[Carultch]

Thanks Dennis.
Since it's an average... is it safe to say it's not an issue if the current draw exceeds 400 amps at any point in time? As long as the average doesn't go above 400.

Since it takes time for heat to build up in infrastructure, brief loads that spike in excess of 400A are not as much of a problem as a continuous load exceeding 400A on any given phase. I'm surprised to see a half hour as the averaging time interval, since usually 15 minutes is the industry standard for measuring demand. Spikes in load can be a problem for other reasons, like causing transient voltage surges, but not for reasons due to components that typically fail by heat accumulation (e.g. wires, terminals, transformers, fuses and breakers).

Note, that this is the RMS value of current that is averaged over this interval, and not the peak current. Each cycle (1/60 seconds long) would have its RMS value taken, and then we'd average the cycle-by-cycle RMS values

 

[Grouch1980]

I'm surprised to see a half hour as the averaging time interval, since usually 15 minutes is the industry standard for measuring demand.

Agreed. I always see and hear about 15 minutes.

Great! Thanks guys for the help.

 

[Jraef]

The Pacific NW uses a 30min. demand window, or at least used to (I've been gone from there for over 25 years now).

Since it takes time for heat to build up in infrastructure, brief loads that spike in excess of 400A are not as much of a problem as a continuous load exceeding 400A on any given phase.

To this point, back in the days before everything became digital, the guts of a Demand Meter were essentially a heater that moved a needle (on a ratchet so that it only went one way). It took the time of that demand window for it to affect movement in the needle. So a brief moment of really high current, such as a motor starting, has little effect on that, so long as the motor gets to it's running speed quickly. Digital demand meters have just emulated that action.

This by the way was/is contrary to what a lot of people think, especially people selling soft starts and VFDs. Reducing the starting current has little to no real effect on the demand charges that you get assessed each month. There are a FEW utilities that have "instantaneous" demand metering, but they are few and far between. In 40+ years in this business, I have encountered that exactly once, it was a small irrigation district who was buying and reselling power from another provider. Yet I have seen lots of brochures and sales literature stating that "You can save on peak demand charges with our device!", it's all "marketing speak", similar to what comes out of the south end of a bull heading north...

 

[Carultch]

The Pacific NW uses a 30min. demand window, or at least used to (I've been gone from there for over 25 years now).


To this point, back in the days before everything became digital, the guts of a Demand Meter were essentially a heater that moved a needle (on a ratchet so that it only went one way). It took the time of that demand window for it to affect movement in the needle. So a brief moment of really high current, such as a motor starting, has little effect on that, so long as the motor gets to it's running speed quickly. Digital demand meters have just emulated that action.

This by the way was/is contrary to what a lot of people think, especially people selling soft starts and VFDs. Reducing the starting current has little to no real effect on the demand charges that you get assessed each month. There are a FEW utilities that have "instantaneous" demand metering, but they are few and far between. In 40+ years in this business, I have encountered that exactly once, it was a small irrigation district who was buying and reselling power from another provider. Yet I have seen lots of brochures and sales literature stating that "You can save on peak demand charges with our device!", it's all "marketing speak", similar to what comes out of the south end of a bull heading north...

Thank you for sharing that detail of history. So this also would mean that on a legacy demand meter, that "negative demand" could be a serious concern if you add on-site generation with an "instantaneous" net export that significantly exceeds your history of demand. And it meters in absolute value, so the meter is blind to whether the demand event happened on import or export. Are modern digital demand meters meant to register demand events due to negative demand? Or is it policy to ignore such demand events, if the meter can tell the difference?

I do think that soft starts and VFD's have their purpose, even if mitigating peak demand is an unrealistic goal to expect. One thing it does, is it minimizes surges to other loads in your system, and to the equipment itself. It allows motor loads to run at more nuanced speeds for their application, and avoid losses that come with thermostatically-controlled abrupt stop/starts.