3 phase kW

[menoknow]

Hoping someone can help me understand what I'm not understanding. So let me start by giving a bit of backround. I am about to install some 3 phase 480v pieces of manufacturing equipment, but there have been some questions as to whether or not the utility owned transformer could safely handle the new load. I figured one piece of equipment at (44a x 480v x 1.73 = 36,537W = 36.5kW). installing 10 of these so (36.5kW x 10 = 365kW). So to sum things up 480v 440a 365kW of new load.

However when I gave these figures to the utility engineer he said that I wasn't calculating the kW correctly and that all I needed to do was volts times amps. so just 480v x 44a for total of 21.12kW for one unit. He then told me that I only have to throw in the 1.73 if I'm doing the calculation from 277 volts. However that doesn't really make sense to me since 277v out of a 480/277v system is inherently single phase since as soon as you add another leg it becomes 480v.

Suffice it to say I didn't really argue with him since both his math and my math give me the answer I want which is that there is plenty of room on the transformer for this new installation. However when I did try to dig in a little with him he told me he doesn't actually have an engineering backround, but this is how they all do it in his office. Am I missing something here? I've been browsing the interwebs trying to make sure I'm not wrong in my understanding, and yes I am aware I am leaving out power factor, however I fail to see what I'm missing here. Can somebody possibly help me see what I'm missing here?

 

[Jraef]

Your utility person is incorrect. They have it exactly backward.

 

[ruxton.stanislaw]

It's 3 instead of 1.73 if you are calculating for 277 x 3 legs. Maybe he thinks you are connecting single phase devices.

Show him like this, to help him understand: 277 V x 3 legs x 44 A x 10 units?

If it sounds like the transformer has enough room, just go with it. If it blows up, it's his fault. You have checked with the utility and done your due diligence.

 

[charlie b]

What is the existing service transformer rating and the existing load in the facility? I am wondering how you came to the conclusion that there is spare capacity available.

BTW, I also agree with your math.

 

[ptonsparky]

I don't know if the POCO ever supplies a transformer sized to equal our NEC calculations. That is their problem, as long as you have given them what you know. CYA with paperwork.

 

[ruxton.stanislaw]

Often, those transformers have no issue running at 150% capacity for a few hours, especially in colder climates. E.g. think of Thanksgiving day when the entire neighbourhood is using their oven, stoves and hot water at the same time to prepare turkey dinner. It is less expensive for the power company to replace the few transformers that explode, rather than over-provision everyone!

 

[jim dungar]

Your utility person is incorrect. They have it exactly backward.

Yep.

You were probably talking to a person with the title of engineer, not someone trained as an engineer. This happens often.

Just tell them you need to calculate loading per the NEC, and the utility can use them anyway that want to.

 

[ggunn]

Yep.

You were probably talking to a person with the title of engineer, not someone trained as an engineer. This happens often.

I have a nephew who has "engineer" in his title working for Tesla but he doesn't have an engineering degree, or any college degree for that matter. He thinks he is an engineer.

 

[menoknow]

What is the existing service transformer rating and the existing load in the facility? I am wondering how you came to the conclusion that there is spare capacity available.

BTW, I also agree with your math.

Thank you all for confirming that 5 years of apprenticeship weren't a complete waste. I thought I might have been going crazy for a minute
The transformer is a 1000kVA. The utility has verbally informed me that the facilities peak usage is around 600a. Since they are giving me an amp value I decided to convert the transformer secondary to amps so (1000kVA divided by (480 x 1.73) ) gives me just about 1,204 amps. So new load plus existing load (440a + 600a = 1040a). Transformer 100% load minus existing and new load = 164a of unused transformer ampacity.

The utility hasn't been forthcoming in telling me how much their transformers can be loaded up under their policies 80%, 90%, 100%, etc. I am aware the FLA doesn't exactly translate to demand. As I know the customer is going to be using these machines at the lowest possible settings that these machines are capable of due to the specific product they are using in them. They are essentially large resistive heaters with a couple of very small motor loads for blower fans.

I would also like to mention that I myself am not an engineer, just an electrician that tried my best to pay attention when going through transformer calcs in the apprenticeship. If anyone has any tips to help improve my understanding of transformer calcs I am all ears.

 

[charlie b]

A formal calculation would require more precise input from the utility. 220.87 has the rules.

But working with what you were given, you need to add 25% to the existing load before adding in the new load (again, per 220.87). So 600 times 1.25 plus 440 gives you 1190. That is within the transformer's rated current, so you are still OK. But you might even have more room left. If each machine does not use every bit of 44 amps, or not all are running at the same time, you might be able to calculate a "new load" of something less than 440 amps.

By the way, I suggest getting into the habit of doing all calculations in units of power, and only converting to amps atvte very end.

 

[charlie b]

I would also like to mention that I myself am not an engineer, just an electrician that tried . . . .

Being an electrician deserves more than "just." I have a great deal of respect for electricians, as well as an appreciation for the time and effort needed to learn how to work safely in your profession.

 

[ggunn]

Being an electrician deserves more than "just." I have a great deal of respect for electricians, as well as an appreciation for the time and effort needed to learn how to work safely in your profession.

I totally agree. I was in semiconductors for 20+ years before I made the change to solar; without good electricians around me when I started dealing with AC systems I would have been lost, or more lost than I was, anyway.

 

[tortuga]

Yep.

You were probably talking to a person with the title of engineer, not someone trained as an engineer. This happens often.

They better not be doing that in Washington State there was a multi year court case about that recently.

 

[ptonsparky]

They better not be doing that in Washington State there was a multi year court case about that recently.

We have Staking Engineers here. Just means they've advanced from Lineman.

 

[ggunn]

In another life when dinosaurs roamed the earth I was an oilfield Mud Engineer.

 

[jim dungar]

They better not be doing that in Washington State there was a multi year court case about that recently.

Every company can give their employees a title containing the word engineer. The laws are to prevent them from selling engineering services outside of their company.

 

[TX+ MASTER#4544]

Thank you all for confirming that 5 years of apprenticeship weren't a complete waste. I thought I might have been going crazy for a minute
The transformer is a 1000kVA. The utility has verbally informed me that the facilities peak usage is around 600a. Since they are giving me an amp value I decided to convert the transformer secondary to amps so (1000kVA divided by (480 x 1.73) ) gives me just about 1,204 amps. So new load plus existing load (440a + 600a = 1040a). Transformer 100% load minus existing and new load = 164a of unused transformer ampacity.

The utility hasn't been forthcoming in telling me how much their transformers can be loaded up under their policies 80%, 90%, 100%, etc. I am aware the FLA doesn't exactly translate to demand. As I know the customer is going to be using these machines at the lowest possible settings that these machines are capable of due to the specific product they are using in them. They are essentially large resistive heaters with a couple of very small motor loads for blower fans.

I would also like to mention that I myself am not an engineer, just an electrician that tried my best to pay attention when going through transformer calcs in the apprenticeship. If anyone has any tips to help improve my understanding of transformer calcs I am all ears.

Hello, menoknow,

Yes, listen to what charlie b says.

You should be able to determine the plants service load from the power company representative to determine their actual load for existing installations for a one year period as per section 220.87, 2023 NEC.

Use VA when calculating your loads and don't forget to use any demand factors if any or increase your continuous motor loads by 1.25%.
Most motors are considered to be continuous duty, but not all. Do you have any other than continuous duty motors?
Will all of the motors run at the same time?

Will all of your motors be located together in a particular area? Do you have available breaker spaces for the three phase motors plus any other circuits that may be needed?

If you don't, you need to consider sizing a motor feeder to feed the motors if you don't have available space in the panelboard for all 10 motors.
This might prevent having to upgrade the service to make room for all 10 motors. I can show you how to do that.

See T.430.22 (E) which is for motor duty cycle service and they have various nameplate rating percentages. See Part II Motor Circuit Conductors section 430.22 (E) about that table. It has four different types of classification of service and minute ratings.

After calculating all of your motor loads using VA, then obey Ohm's law and convert it to amps for sizing conductors, OCPDs and other disconnects. Because you are using VA there's no need to consider the power factor.

Example for calculating your motor load:
Since you have motors they will be calculated at 1.25 of the NEC tables found in Tables 430.247,430.248 and 430.250 for ampacities, this is found in section 430.6 (A) (1),do not use nameplate ratings. Nameplate ratings are to be used to determine the sizing of the motor overload devices, (heaters).

Now you used 44 amps, where did you get that number? You're suppose to use the tables that I wrote about for sizing the branch circuit conductors. See 430.6 (A) and not nameplate ratings. Name plate ratings are for sizing the motor overloads (heaters). Is your motors marked with 'EP' as per 430.32 (A) (3) (2), Thermal protector or electronically protected.

44 amps x 480 volt x 1.732 or 480 x 1.732 = 831.36 44 amps x 831.36 = 36,564 VA per motor.
36,564 VA x 10 motors = 365,640 VA 365,640 / 1,000 = 365.64 kVA or 366 kVA 365,640 / 831.36 = 440 amps

Do not use the motor name plate unless it's a torque motor per 430.6 (B) for branch circuit sizing.

I have to sign off now but will get back with you tomorrow, Friday , March 15, 2024.
There's little more you need to know. And this very lengthy, typical for motors.

Thanks for reading.
Comments accepted.
TX+MASTER#4544

 

[ggunn]

Every company can give their employees a title containing the word engineer. The laws are to prevent them from selling engineering services outside of their company.

The laws in Texas about this are a bit convoluted; there are regulations for when and where you can call yourself an engineer if you are not degreed in engineering. Years ago when I worked for Motorola there were job titles that contained the word "engineer" that people who did not have an engineering degree could have, but the company would not put the word "engineer" on your business card unless you were a P.E.

 

[tortuga]

Every company can give their employees a title containing the word engineer.

They used to but not in Oregon or Washington anymore, they are all 'technicians' now, the case in WA was a complaint that the state of WA was giving state employees the job title “Engineer” improperly even though they were not licensed or registered as engineers. Nothing to do with selling services.
Now all the business cards say 'staking' (or network or whatever job) technician instead of a 'staking' engineer.

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[topgone]

Thank you all for confirming that 5 years of apprenticeship weren't a complete waste. I thought I might have been going crazy for a minute
The transformer is a 1000kVA. The utility has verbally informed me that the facilities peak usage is around 600a. Since they are giving me an amp value I decided to convert the transformer secondary to amps so (1000kVA divided by (480 x 1.73) ) gives me just about 1,204 amps. So new load plus existing load (440a + 600a = 1040a). Transformer 100% load minus existing and new load = 164a of unused transformer ampacity.

The utility hasn't been forthcoming in telling me how much their transformers can be loaded up under their policies 80%, 90%, 100%, etc. I am aware the FLA doesn't exactly translate to demand. As I know the customer is going to be using these machines at the lowest possible settings that these machines are capable of due to the specific product they are using in them. They are essentially large resistive heaters with a couple of very small motor loads for blower fans.

I would also like to mention that I myself am not an engineer, just an electrician that tried my best to pay attention when going through transformer calcs in the apprenticeship. If anyone has any tips to help improve my understanding of transformer calcs I am all ears.

Both of you were talking about the same subject in two different languages. You, in kW and the engineer in kVA. When sizing transformers, my preference is to talk in kVAs. It would be best to express your requirements in kVA, the maximum capacity a transformer provides. That said and done, you only need to sum up the figures to arrive at the total electrical requirement. But, if you have not-so-balanced 1-phase loading on a 3-phase system, the best way is to use the highest loaded phase and multiply by 3 to come up with a safe estimate of your electrical load. Doing that will be an overestimate but that's for good measure, IMO.
BTW, utilities do not provide transformers sized according to the figures we get using NEC calculations. They have their own sizing based on the so many variables; demand factors, coincidence, hourly load variations in a day, etc. Experience-wise, I've never seen a feeder transformer that were sized to the maximum demand, they were always lower. Utlities have their own simulations on how to size transformers that will give them the highest revenues during the transformers' usefull life.