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My breaker panel is full, but I need to add at least 3 double pole 30 amp breakers.
I believe after I do the math on the room size and what would be needed for space heating baseboards I'm assuming I need 3 30 amp double pole breakers.
I'm not sure if I have to subpanel off of it or run a new panel altogether. It's a challenger 100 amp panel.. no it's not a Stab-Lok.

I know I would need to know if the service line is big enough first to handle the load. I am also not sure how they rate panels and loads and such. I assume that it is under full threshold load capacity before breaking the main, but what confuses me is I've seen so many panels overloaded when going to customers homes and wondered if I add up all the numbers they exceed the main breaker size how is that?

Should I sub or replace with 150 or possibly 200 amp for future addition to the newly added addition the owner decided to do for an extra apt in the attic to extend the second floor apt.. once I know what to look for in the existing service line.

If it's not rated high enough to handle the full load then it's a no-go, right?

Then my other option is to have a licensed electrician install a new line, weather-head and socket, I can do the rest from there. It wouldn't be my first time installing a new panel.

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    Get cold-climate mini-split heatpumps (~300% efficiency based on electrical input) instead of baseboard resistance heat (100%) and you'll need about 1/3 the power supply, and lower power bills to match.
    – Ecnerwal
    Mar 30 at 20:12
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    We would need to know the other loads but adding ~75 amp load to your service load is fairly large (note 30 amp breakers are 24 amp each by code this is where the 75 comes from)
    – Ed Beal
    Mar 30 at 20:14
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    There are probably easier ways to get heat. Most breakers in a home panel are only using a few amps and nowhere near the number of amps listed on them. A 15 amp breaker for lights might only use maybe 5 amps with all the lights on. Most of the larger breakers only use their power of part of the time, a 40 amp stove breaker maybe a couple hours a day. Electric heat has probably the most full time use of your breakers.
    – crip659
    Mar 30 at 20:28

2 Answers 2

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It's a challenger 100 amp panel.. no it's not a Stab-Lok.

There's nothing wrong with Challenger panels. But Challenger breakers were caught in a VW-like cheating scandal; they don't trip when they're supposed to. Only Type C and Type A breakers will fit in a Challenger panel, fortunately Eaton BR breakers are cross-listed Type C, and BR tandems are cross-listed Type A. So all the Challenger breakers need to go and be replaced by Eaton BR/C/A. BR is the most popular breaker type in North America.

what confuses me is I've seen so many panels overloaded when going to customers homes and wondered if I add up all the numbers they exceed the main breaker size how is that?

Oversubscribing panels is allowed because nobody maxes out all their circuits at the same time. Also, USA panels are actually 240V, and so we have two banks of 120V at full amperage.

There's an old joke... someone says to their banker "Overdrawn? How can my account be overdrawn? I still have checks left in my checkbook!" Number of spaces available in your panel is a lot like that. It has nothing to do with the actual electrical load.

Both these issues are resolved by a procedure called a Load Calculation. There is the official, NEC-approved way to do that. Some of the math gets tricky especially with ranges, but the good news is you don't need to tally up lighting or any of your plug-in 120V loads, as those are accounted for in "catch-all" figures.

When you say "Common" do you mean "common" off the timer labeling.... or do you mean "Neutral"? They are not the same thing. Neutral is not "common", common is a hot line actually.

I presume the house was heated previously, so it sounds like you are converting from another heating technology to electric resistive heating. This is a huge electrical load change, and absolutely requires a new Load Calculation. Really any new large appliance requires a new Load Calculation, but this most particularly.

Fully expect to find your electric service is inadequate and that a "heavy-up" would be called for. This would be a great time to address the full panel - a new main panel can be added, and the existing Challenger panel can be re-configured as a subpanel so you don't have to rewire.

Make sure you have a full understanding of your power company's local rate tariffs before installing electric resistance heating as a heat source. This is cheap to install but incredibly expensive to operate, unless you really know what you're doing re: power company tariffs. Resistive heating is cheap to install but prohibitive to operate. Better off with modern heat pump tech.

If your plan is to have the tenant pay the electric bill, then you can't DIY this work - all electrical work on rental units must be done by a pro electrician (who will do the permits, Load Calculation and possible heavy-up as part of the job). HVAC contractors have been known to do "light" electrical work related to their installation, but we see a lot of work that is just terrible.

Should I sub or replace with 150 or possibly 200 amp for future addition to the newly added addition the owner decided to do for an extra apt in the attic to extend the second floor apt.. once I know what to look for in the existing service line.

Nope, you doing electrical work for a third party is out of the question. To DIY electrical work, you must a) own the property, b) be the planned occupant of the unit in which you are working (and NO homeowner is going to install baseboard heat unless they have a favorable tariff), and c) pull permits and get inspections.

It sounds like a) this is for your customer and b) they are a landlord renting to tenants, which explains the cheap-to-build, expensive-to-run electric heating. Of course that particular slice of slumlord bastardry only works if there are individual electric meters to each unit! Which precludes the method you are proposing.

Then my other option is to have a licensed electrician install a new line, weather-head and socket, I can do the rest from there.
It wouldn't be my first time installing a new panel.. I'm very meticulous or anal about safety... something I have to be when it comes to stuff like this..

And yet, you are involved in this dubious scheme.

I've been running a micro business for years with plumbing, heating, electrical and now large kitchen and bath appliances with a very good success rate at not failing stuff I knew nothing about, but with reading here and elsewhere and by the grace of God things turned out well, all homes are intact years later..

Animals collect safety data only by what they witness for themselves. We have very bold pigeons in our local park. Catching one is easy, but if you do, only the pigeons who saw that happen become afraid of people. Whereas human beings are able to share experiences so all humans learn from the experience of others far away from them. So when you say "none of my buildings exploded (that I know of)", that is the animal way of analyzing data. The human way is what the Electrical Code is.

Professionals do real analysis of thousands of accidents. The result of that analysis is the Electrical Code. It's not New York lawyers dreaming up stupid requirements to inconvenience you or cost you money. Every rule is written in blood and ash.

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You've got three separate issues here:

  • Panel space

Sometimes that can be resolved by merging circuits and/or putting half-size breakers in place of full-size breakers. If that isn't enough (or isn't allowed - there are many constraints depending on panel type, circuit types, etc.) then you either have to replace the panel with a larger panel or install a subpanel.

  • Panel capacity

You currently have a 100A main breaker. In some cases (especially with newer panels, not as likely with an old Challenger) you can install a larger main breaker because the rest of the panel is designed to handle more current. But with a 100A main breaker on an older panel, it is quite likely to have a true maximum of either 100A or 125A. To be honest, adding 90A of load (which may all be used at once since the heaters are likely to all run at the same time on cold days) plus other loads (lighting, receptacles, appliances) is likely to get you way over 125A.

  • Service capacity

The service from the utility is out of your control. But your main breaker has to be matched (equal or less than) to the service capacity. If your utility is providing 100A service, you can't switch to a new panel with a 200A main breaker. On the other hand, if your utility is already providing 200A service then you can make that upgrade. The only people who can give you a definitive answer on that are at the utility company. My hunch is you have older service that was 100A maximum at the time, in which case (as in my house) the upgrade requires more than just a new panel but also a new meter and service entrance cable. But you need to find out before you add 90A of load.

In addition, you really should do a load calculation. This factors in heating, air conditioning (which, if different devices, don't run at the same time), cooking appliances (which have special rules), size of the house, type of appliances, etc. Once you have done that, with the new heating system included, you can determine what you need to do about your panel and your utility service.

As others have mentioned, electric resistance heat is cheap to install but expensive to run. Installing a heat pump will cost a lot more to install, but it might use just enough less power to avoid having to put in a new panel and upgrade the utility service - which might well be a significant net savings.

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