load calc for main panel

[rllevine]

It has been way too long since school. I don't know how to figure this out.

Here is what is in this panel:
2-pole 100 amp for a sub-panel
2-pole 30 amp for a dryer
2-pole 60 amp for a range
2-pole 50 amp for a hot tub
2-pole 30 amp for A/C
2-pole 20 amp for an EV charger
two single pole 20 amp - unlabelled.

what size main breaker does this panel need?

 

[tom baker]

Load calcs are done in KW or KVA. The breaker size is always larger than the connected load. Exmaple a water heater has a 30 amp 2 pole but the KW load is 4500 watts.
go to the examples in annex D for typical load calcs.

 

[rllevine]

I'll ask another question. In addition to the breakers listed above, this panel used to have another 2-pole 100 amp breaker for a different subpanel and a 2-pole 40 amp breaker for a compressor. The main disconnect on the panel was 200 amps. Now that those are gone, can I downgrade the main disconnect to 125 amps?

 

[retirede]

I'll ask another question. In addition to the breakers listed above, this panel used to have another 2-pole 100 amp breaker for a different subpanel and a 2-pole 40 amp breaker for a compressor. The main disconnect on the panel was 200 amps. Now that those are gone, can I downgrade the main disconnect to 125 amps?

As stated in post 2, you need to do a load calc. Existing breaker count / sizes mean nothing.

 

[KdogSparky]

I am also going thru a load calc for a 4 family. Using the NEC example D4(b).
Electric waters for each unit, Electric Ranges for each unit.
My stuck point is I can't get into the individual units to determine kw rating of each range (coronovirus concerns).
Example D4(b) shows 8000 VA for a range. I called Home D$show to get a number for an typical 30" range. Found out Self cleaners have a higher rating = One example I called about was a GE self-cleaning range, nameplate rating of 12.1 KW
so, 12.1 kw = 12,100 VA. Funny at 240 V that would draw 50.41 amps, yet the Manufacturer only requires a 40 amp breaker. - different issue, I know

General Question: without looking at the units themselves, what is a approx. KW rating of a "standard" range rating you would use in determining a load calc.

 

[KdogSparky]

And in answer to the 1st post, I too haven't done a load calc in a while. Get in the NEC and use those examples in the back of book.
1) find out which example fits your needs. Annex D: Examples
2) get sq. footage
3) get what numbers you need just as the examples show
4) following along with each step, it takes the guess work out. (Unless you can't find KW rating of Ranges when you can't get in the units due to coronavirus)
5) Then jump on here again. We're all here to help. Good Luck and God Bless.

 

[david luchini]

One example I called about was a GE self-cleaning range, nameplate rating of 12.1 KW
so, 12.1 kw = 12,100 VA. Funny at 240 V that would draw 50.41 amps, yet the Manufacturer only requires a 40 amp breaker. - different issue, I know

See T220.55, note 4.

 

[FaradayFF]

Shouldn't the main breaker in the panel be based on the panel bus ampacity?

 

[WA_Sparky]

It has been way too long since school. I don't know how to figure this out.

Here is what is in this panel:
2-pole 100 amp for a sub-panel
2-pole 30 amp for a dryer
2-pole 60 amp for a range
2-pole 50 amp for a hot tub
2-pole 30 amp for A/C
2-pole 20 amp for an EV charger
two single pole 20 amp - unlabelled.

what size main breaker does this panel need?

I'll ask another question. In addition to the breakers listed above, this panel used to have another 2-pole 100 amp breaker for a different subpanel and a 2-pole 40 amp breaker for a compressor. The main disconnect on the panel was 200 amps. Now that those are gone, can I downgrade the main disconnect to 125 amps?

I would not go down to 125A. Assuming 80% of all loads shown above and assuming that 100A sub had 0A load you're still above 125A demand. My final demand loads on typical homes/condos are around 100A-120A not including hot tubs or sub panels.

 

[WA_Sparky]

Shouldn't the main breaker in the panel be based on the panel bus ampacity?

In my experience its most common to do so but not always the case. You just need breaker and feeder to be rated above demand amperage. I've had a 300A MCB in 400A Panel before (hospital) for selective coordination reasons. I would suggest rating MCB to panel amperage in the event of future additions so you don't need to change feeder down the road.

 

[jimport]

No point in downgrading the service and losing capacity.

 

[mlnk]

I use 12,000 watts for electric range load calc. I install a 50/50 breaker with #6 wire for range plug. Homeowner could replace range with a heavy duty model in future.