Adding Load to Existing House

[Will Wire]

I want to add load to an existing house. The service is rated at 125 amps. I did the NEC 220.83 load calculation. The existing load is 83 amps.

With 42 amps available on the breaker, what would be an acceptable load addition.

I am in California under the 2020 code.

Thank you,

Trent

 

[retirede]

I would repeat the load calculation adding in the proposed new loads.

 

[Will Wire]

What my customer wants will exceed the Main Breaker rating. I want to add two 20 amp circuits to a new detached office. I would let him know that he will have to rework his office plan within the confines of the two circuits.

I know that load should never exceed the rating of Main, I was just curious if it would be unacceptable to intend to have a 123 amp load on a 125 amp main.

 

[rc/retired]

I would not hesitate to add 2 20 amp circuits for an office.

Ron

 

[ptonsparky]

I would not hesitate to add 2 20 amp circuits for an office.

Ron

Nor would I.

Those 20 amp circuits will never be loaded to that extent, unless someone is trying.
The existing load calculation of 83 amps will never be seen by the 125 amp breaker, unless some one is trying very hard.
If they are trying, you can't fix stupid.

I want to add load to an existing house. The service is rated at 125 amps. I did the NEC 220.83 load calculation. The existing load is 83 amps.

With 42 amps available on the breaker, what would be an acceptable load addition.

I am in California under the 2020 code.

Thank you,

Trent

Do the load calculation for your own home. Put an amp clamp on the incoming. Turn everything on in your house that you would typically use.

 

[infinity]

I know that load should never exceed the rating of Main, I was just curious if it would be unacceptable to intend to have a 123 amp load on a 125 amp main.

No it's perfectly fine. The NEC load calculations are very well padded with the actual loads being much less.

Also 2-20 amp, 120 volt circuits would only add 20 amps of load if put one on each leg.

 

[garbo]

I always liked to look at a few of the customers electric bills ( preferably two winter & 2 summer bills ) to see what the average daily load is. If they have all natural gas for everything have found all but the summer load to average under 2KW a hour. When I replaced a 60 amp service years ago they had an electric range and 4 window air conditioners. ( rest of home ran off natural gas )Was surprised they only had 50 amp fuses in the main pull out fuse block. Fuses appeared to be over 50 years old. Now with LED & even large screen TV'S loads have decreased. Adding two 20 amp circuit should not cause any problems.Would ask them if they intend to purchase one or two EV in the near future that could require a new upgraded service.

 

[wwhitney]

I want to add load to an existing house. The service is rated at 125 amps. I did the NEC 220.83 load calculation. The existing load is 83 amps.

With 42 amps available on the breaker, what would be an acceptable load addition.

If the load added does not include HVAC, then it will be factored at 40% in the updated 220.83 calculation. So in that case the answer is 42A / 40% = 105A before the 40% factor (but after applying a 125% factor to any continuous loads).

Cheers, Wayne

 

[ptonsparky]

If the load added does not include HVAC, then it will be factored at 40% in the updated 220.83 calculation. So in that case the answer is 42A / 40% = 105A before the 40% factor (but after applying a 125% factor to any continuous loads).

Cheers, Wayne

And adding two 20 amp branch circuits will still not be an issue.

 

[wwhitney]

And adding two 20 amp branch circuits will still not be an issue.

Was not suggesting otherwise.

But the question of two 20 amp circuits arose in post #3, which states "What my customer wants will exceed the Main Breaker rating. I want to add two 20 amp circuits . . " Which suggests the OP is perhaps being overly conservative, since the available headroom is 105A unfactored, not 42A.

Cheers, Wayne

 

[Will Wire]

Thank you everyone for your input. I truly appreciate it.

Trent

 

[suemarkp]

If you are not adding square footage and these circuits are not for fastened in place items, then they don't change the load calc at all. They would be included in the 3VA/sq ft allowance for misc stuff. Even if you were adding a kitchen or laundry 20A appliance circuit, those only go in at 1500VA each and not 20A.

 

[Will Wire]

In my original load calculation I only applied the demand factors for the existing house, and not the additional load of the detached office. After being reminded of this I redid the load calculation combining the existing house and the additional load of the office and the new load is 116 amps. It turns out that my customer will be able to have what he wants. I now plan to install a small sub-panel at the detached office.

Thanks again,

Trent