Adding Circuits to Subpanel

[JMToth]

Hello, I am looking for some guidance on adding circuits to a subpanel. I will try to add as much detail that I think is relevant but happy to add something if its an important detail I missed.

I have a 200 amp main panel, and 100 amp sub panel. Main panel is full. Currently the wire feeding the sub panel is 8 gauge, from a 40 amp breaker. Subpanel currently only has one double slot circuit, 40 amps, feeding my main bath which has normal bathroom electrical finishes plus a heated floor. My understanding is, I can add a total of 40 amps additionally to this sub panel.

My problem is that I want to add 60 more amps to the subpanel, all feeding my basement. I want two 20 amp circuits, for two sump pumps, if one pump would fail, then hopefully second could continue to work. I also want two more 20 amp circuits to power a large number of fish tanks.

I figured out that I would need to upgrade the wire feeding the subpanel to a 6 gauge wire. But what I haven't been able to determine is, is this too many amps for the subpanel to have? Or is a 60 amp breaker on the main panel which would feed the sub panel, too much? Thanks.

 

[Afjes]

Subpanel currently only has one double slot circuit, 40 amps, feeding my main bath which has normal bathroom electrical finishes plus a heated floor.

This 40amp circuit feeding the bathroom heated floor does it also feed the/a receptacle and lighting in the bathroom without going to a sub panel at or near the bathroom first and then individual circuits feeding the heater floor, receptacles/light/s? If there is not sub panel at the bathroom or near it other than the one sub panel you first describe then you may have a problem. You can't feed general purpose receptacles off of a 40amp circuit.
Please explain further what you have in the setup so we can advise further.

Currently the wire feeding the sub panel is 8 gauge, from a 40 amp breaker.

If you want to increase the ampacity of the sub panel:
#1 - What is the sub panel rated at; not what is presently feeding it?
#2 - If the sub panel is rated high enough to increase then the wiring and breaker from the panel feeding it must be upgraded to meet your needed increase.

My problem is that I want to add 60 more amps to the subpanel,

Before you just add circuits to the sub panel you have to determine what you load will now be on the proposed sub panel.
You mentioned sump pumps. Need to know the power consumption on these two pumps. Large fish tanks (we don't know what power is needed) but I can't see it being a lot except if you have heaters in the tanks and we would not know the requirements of the heaters for the tanks.
The proper way to determine what the needs would be would be to do a "load calculation". There are many of them on the internet available for free. This would give you a good idea whether your proposed needs would be ok for the sub panel, meaning your power needs do not outweigh the sub panel ability in the way of amps.

Telling us you will have 20 amp circuits for receptacles and how many receptacles you will have on a circuit is meaningless only because receptacles do not consume power on their own, only the devices/appliances plugged into them consume power which dictates whether a circuit will handle the devices/appliances needs.

After reading what I have posted try and give a bit more detail where needed and we can guide you further.

 

[Sponsor]

 

[JMToth]

Thank you for all those details and questions.

The main panel, 200 amp, breaker feeding the subpanel is 40 amp, the sub panel is 100 amp rated, I learned yesterday that the breaker in the subpanel is only servicing the heater floor in the bathroom, double 20 amp breaker. All other bathroom electrical needs are running from the main panel. We had this bathroom redone last year and added the heated floors but all other electricals stayed the same.

I understand that if I want more amp in the subpanel, I would need to upgrade feeder breaker from main and feeding wire as well to the appropriate size. I get that load calculations need to be considered. But the crux of my concern is, can I feed 100 amp to my subpanel from a 200 amp main panel? This would assume a 100 amp feeder breaker and appropriately sized feeder wire, and 100 amp rated subpanel.

I can look tonight for the Watts on the 2 sump pumps, and expected fish tanks needs. This seems like the simplest calculation I found, Watts= Amps x Volts. Is this a good measure to go from? Thanks.

 

[Afjes]

This seems like the simplest calculation I found, Watts= Amps x Volts. Is this a good measure to go from?

@JMToth - Yes, it is one of basic Ohm's Law formulas which electricians go by.
If a device only shows Watts and Volts on it's sticker/panel as example: You would take the watts and divide it by volts and you would get your amps. (1500 watts / 120 volts = 12.5 amps) If it only showed volts and amps then simply 120v x 12.5amps = 1,500 watts

 

[Reach4]

#2 - If the sub panel is rated high enough to increase then the wiring and breaker from the panel feeding it must be upgraded to meet your needed increase.

But the crux of my concern is, can I feed 100 amp to my subpanel from a 200 amp main panel? This would assume a 100 amp feeder breaker and appropriately sized feeder wire, and 100 amp rated subpanel.

That Afjes post looks like a yes to your later question, to me.