Full size breakers vs. tandem (double) breakers?

road-runner wrote:

CJW8 wrote:
If I remember correctly, the total of all branch breakers shouldn't exceed 125 percent of the main breaker size. Someone more current with the Code may correct me.

I have two 200 amp panels in the house. Each one has 345 amps worth of branch breakers.

It's been a long time since I had to do it but there's a long, complicated equation for the number and size of breakers.
You can have a lot more than what they simply add up to!

Here's how it turned out. I didn't even have to buy a breaker - when the electrician installed the generator panel in the S&B, he left the old breakers in place so there wouldn't be a gap. I popped out the tandem 15/15 and popped in a single 15. It dropped right into the RV panel. Biggest problem was fishing the 14/2. "It's Miller Time"!

Joe, You are doing it correctly and safely. I got lost on some replies and now I wonder if in the last 63 years in the residential and traffic signal wiring I learned anything.

I don't have a 50amp service in my rig but I do have a 200amp in the S&B with a 50 amp sub panel. I really just balanced the two sides by possible loads, and other than the AC, I have no high amperage items.

Fun, Fun, Fun

CJW8 wrote:
If I remember correctly, the total of all branch breakers shouldn't exceed 125 percent of the main breaker size. Someone more current with the Code may correct me.

I have two 200 amp panels in the house. Each one has 345 amps worth of branch breakers.

mike-s wrote:

ScottG wrote:

mike-s wrote:

ScottG wrote:
A tandem breaker will work just fine for the OP. There is no need to run a space heater off each leg. It just doesnt matter.

It may matter. As he said in his original post - "better for when I'm running my generator." He may have a split-phase gen set.

And that's what I said in another post. If there's a generator or inverter involved then loading matters.

Don't expect us to keep track of all your inconsistencies.

I never changed anything I wrote, nor were there any "inconsistencies".
But I'll try to keep my responses more simple for you to understand.

westend wrote:
Joe,
Do exactly like you planned. You have been dutifully scolded and probably by users that don't have 50 amp service.

Woof. I have an F450 and a 15,000 lb fifth wheel trailer. You'd think by now I'd instantly recognize the difference between "tandem" and "dual".

Note to future self: Coffee first, then check forums.

joebedford wrote:

Stefonius wrote:
If you're running "one heater on each leg", you should use two single breakers rather than a tandem. You can easily make sure each breaker is on a different leg.

I only have one slot available so I can't add two breakers.

If you're using a tandem breaker with two potentially unbalanced 110v loads, you can cause problems.

I don't understand this - please explain.

Sorry... I thought you were talking about a 240v double breaker. You should have no trouble using a tandem breaker, but be aware that it will be pulling current from only one leg.

CJW8 wrote:
If I remember correctly, the total of all branch breakers shouldn't exceed 125 percent of the main breaker size. Someone more current with the Code may correct me.

In all my years in the biz, have never heard remotely like that. It's possible and very common to have the total of branch breaker ratings in a panel to far exceed it's mains rating and it works because of diversity - ie., the branch breakers are never all running at or near their ratings simultaneously and/or for long enough.

Paneboards in an RV would be covered by art. 551 in the NEC and I don't recall any rules governing total number of breaker spaces or ratings. I think converter/panel manufacturers probably provide a min. number of breaker spaces to comply with the various circuits that are specified under NEC 551 using 1" wide breakers.

For a 30 amp RV you have a 20 amp AC breaker and 4 or more 15 amp breakers totaling 90 amps and it's rare that a 30 amp main trips.

If I remember correctly, the total of all branch breakers shouldn't exceed 125 percent of the main breaker size. Someone more current with the Code may correct me.

Chris Bryant wrote:
The only problems I have with tandem breakers is where both sides have a large load on them- often the 30 amp service coaches will use a 30/20, with the 30 being the input, and the 20 being the air conditioner- these will get pretty hot.

Yes, this is correct. ^^^

Some electrical inspectors don't like tandem breakers but they are CSA/UL listed so there is NO code/standard problem using them. These are used everyday, everywhere, all the time without issue. The only potential downside is the heat from an adjacent breaker in a "tandem" breaker or in the panel. Circuit breakers are thermal/magnetic devices and heat from an adjacent breaker can possibly affect it's operation. If mutual heating from an adjacent breaker is a problem, you can always rearrange circuits. A typical tandem breaker is in the photo below. Each half is sometimes called a 1/2 slot size breaker. Note the rear of it only has one stab that goes onto the bus bar.

Square D used to make a 1" wide 2x15 amp QOT breaker that was called piggyback style because it had handles that were end to end instead of side by side. Not sure if it is still made or anyone else does.

The only way you can use a 2-pole breaker in an RV panel is if it is 120/240 volt as in a 50 amp panel and will take a 1" space. If you run out of breaker spaces in a panel you can typically replace a standard 1" wide breaker with a tandem breaker. The only exception would be if you would be adding too much load to the supply wire/cable (feeder) to a panel and it's breaker.

CJW8 wrote:
Just install your tandem breaker but don't put your two heater circuits on it. Move other loads around so that it will be more balanced. It will never be perfectly balanced. Your microwave and coffee maker only run for a brief period of time.

Agree. ^^^

Know that a circuit breaker does not trip the instant it reaches it's rating. It can run for a quite a while with a small overload and will allow for a coffee maker or MW to run fine. You may or may not benefit from trying to balance loads. You could easily check the ratings of your various appliances that could be running at the same time as the heaters and see how that compares to the 50 amp breaker rating by calculation alone. If you end up tripping the 50 amp breaker in your panel or a pedestal, you'll know for sure you need to juggle some circuits around - no harm done. Breakers are pretty cheap, you could also just balance the loads from the start. Low voltage can make quite a difference too. A typical time-current breaker graph is below.