Calculating Main Disconnect

[profT]

Hi guys, newb here though I've always used this forum to help my answers to my questions.

I hired an electrician to install a main disconnect with meter sockets on a residential apartment building (all electric, heating, boilers, etc.)

My electrician calculated the the entire load as 387,000 vA = 1612 amps. He then said he uses the 32% rule and it becomes 516amps which derates the main breaker to 600 amp fused.

Anyways heres the question:

For 1612 total amperage I feel like a 600 amp main breaker would not be enough but he seemed to be sure about the calculating. I definitely want to avoid nuisance tripping but these main disconnects are very expensive to upgrade to the next size up (800 amp).

Is there anything my electrician is missing in his calculation? He calculated first 10k at 100% and remainder at 40% (leaving electric heat separate and full amperage) to get this amperage.

Any thoughts/advice is welcomed.

 

[Dennis Alwon]

I have no idea where the 32% rule comes from. It doesn't exist. If he calculated according to the nec then that is what you need to use. Lot missing from the info so I would do what the electrician stated-- I assume he has to present the info to the power company and/or inspections

 

[profT]

[FONT=&quot]He just told me that the main disconnect is calculated by code at 32% of total subpanel(s) rating. Will ask him where he got it from.[/FONT]

 

[Smart $]

He just told me that the main disconnect is calculated by code at 32% of total subpanel(s) rating. Will ask him where he got it from.

As Dennis said, there is no such Code.

 

[profT]

Would his calculation for the main disconnect of 600 ampsbe within the a close/accurate range of 1600 amps for derating for residential use?

 

[david luchini]

Would his calculation for the main disconnect of 600 ampsbe within the a close/accurate range of 1600 amps for derating for residential use?

32% is the optional method demand factor for 31 Dwelling Units. How many apartments are in your building?

 

[Smart $]

32% is the optional method demand factor for 31 Dwelling Units. How many apartments are in your building?

Now I have to retract my statement because of this specific 31-unit case.

 

[profT]

20 units!

What would be the optimal demand factor for 20 units?

 

[profT]

Now I have to retract my statement because of this specific 31-unit case.

We all miss some!:thumbsup:

 

[david luchini]

20 units!

What would be the optimal demand factor for 20 units?

The permitted demand for 20 units is 38%.

 

[JFletcher]

If he left electric heat at 100% for all units, continuous, that is probably the largest demand of the year. Are they PTAC units? A 208V PTAC has ~ 14-17A draw on its own with electric heat, and all 20 can and will be running simultaneously for hours during the dead of winter. Dunno about the calcs, just some "real world" experience.

David, what table is that from?

 

[Dennis Alwon]

If he left electric heat at 100% for all units, continuous, that is probably the largest demand of the year. Are they PTAC units? A 208V PTAC has ~ 14-17A draw on its own with electric heat, and all 20 can and will be running simultaneously for hours during the dead of winter. Dunno about the calcs, just some "real world" experience.

David, what table is that from?

Table 220.84

 

[profT]

Thanks for the information everyone.

They are not PTAC units but are in-closet units with ductwork for the units.

Looks like I need a more experienced electrician. Mine hasn't dealt with multifamily before but I assumed that wouldn't be an issue which it clearly is. Can anyone refer me to someone who can calculate the electrical load for a fee?

Side note - Fused main breakers are able to go to the full load ex. 600 amp main disconnect wouldnt trip until 600 is surpassed VS a breaker main disconnect which would trip at about 80% of load correct?

Thanks for all the help.

 

[Smart $]

...
Side note - Fused main breakers are able to go to the full load ex. 600 amp main disconnect wouldnt trip until 600 is surpassed VS a breaker main disconnect which would trip at about 80% of load correct?

There are fuses. There are circuit breakers. Usually, one or the other is used. Quite rare for both. No such thing as a single unit "fused main breakers".

Breakers tripping at 80% load is a false myth.

Both fuses and breakers operate on a curve, i.e. current vs. time. A graph is known as its time-current curve (TCC)

Example TCC of a fuse family: http://ep-us.mersen.com/fileadmin/c...ormation-How-To-Read-A-Time-Current-Curve.pdf

Example of breaker TCC (pgs. 2, 3): http://static.schneider-electric.us...100-400 A Frame FA-LA/FA-FC-FH/0600DB0105.pdf

 

[Dennis Alwon]

Thanks for the information everyone.

Looks like I need a more experienced electrician. Mine hasn't dealt with multifamily before but I assumed that wouldn't be an issue which it clearly is. Can anyone refer me to someone who can calculate the electrical load for a fee?

Get an EE... I am surprised there isn't a requirement to have an EE's stamp

Don't give up on the electrician - looks like he did pretty good- ask him where the 32% came from. You will be hell to work for if you micro manage the job. Things will not turn out well.

 

[wbdvt]

in the Low Voltage Circuit Breaker world there are breakers that have fuses installed in them to increase short circuit rating so there are fused main breakers. One example would be a GE Type AKU breaker

 

[Smart $]

in the Low Voltage Circuit Breaker world there are breakers that have fuses installed in them to increase short circuit rating so there are fused main breakers. One example would be a GE Type AKU breaker

Seldom seen anywhere but industrial applications. Do they even make any nowadays?

 

[wbdvt]

Seldom seen anywhere but industrial applications. Do they even make any nowadays?

True but they do exist and a blanket statement about no fused circuit breakers is mis-leading. There are many industrial places that still use these breakers and companies that maintain and test them. They are also for sale on the used equipment market place.

 

[profT]

Get an EE... I am surprised there isn't a requirement to have an EE's stamp

Don't give up on the electrician - looks like he did pretty good- ask him where the 32% came from. You will be hell to work for if you micro manage the job. Things will not turn out well.

I haven't gotten involved in anything else other than the main disconnect, 600 amps just seemed too little for 20 all electric units. With the chart 220.8 we see its actually 38% which pushes us to 612 amps. That would've been a huge and costly mistake to replace an entire main disconnect. I will find out where he got the 32% from and show him the chart.

Thanks everyone for the help will report back.

 

[Smart $]

True but they do exist and a blanket statement about no fused circuit breakers is mis-leading. There are many industrial places that still use these breakers and companies that maintain and test them. They are also for sale on the used equipment market place.

Yeah... but no one is going to buy a used or reconditioned unit for a new installation in residential, which was the intended target of the blanket statement.