Electrical confusion!

ttrstudios · November 05, 2014, 07:20:00 PM

Hello again guys. Here with another question that I'm not finding a clear answer on.
I am at the point on my bus conversion to adding electrical(120v). I did research on all the appliances I plan on using as well as upgrades in the future to make sure I was going to have enough power.

So I had planed on putting a split system in. One 50 amp for house stuff and a dedicated 30 amp for hvac. Well after talking with an electrician, I don't need two systems. He says that a 50 amp service will carry more than 50 amps meaning that a 50 amp service is 220v and it will carry 50 amps per "leg". Now is this right? And if it is, are we sure that the wiring in a 50 amp camp site service box will carry the nearly 100 amp that one (not me) might put on it? Of course load balancing comes into play here but I thought that when you exceeded the amount of the breaker (50 amps), that was it. No more. Tripped breaker. I know that's right for 110v and thought it was the same for 220v. I understand that any appliance that is 220v used both legs at the same time and the breaker will trip if it exceeds the breaker amperage but in this situation how will one not create a fire by overloading one side I've the breaker? Any thoughts or answers?

Thanks, Craig
1980 Eagle

sparkplug188 · November 05, 2014, 07:51:49 PM

You are correct. 50 amp 240v service is two 50 amp 120v "legs". If you exceed 50 amps on either leg, the 50 amp breaker will trip and both legs will shut off. You can load both legs up to 50 amps at the same time for a total of 100 amps at 120v.

Sometimes it is easier to figure available power in watts.
50 amps at 240 volts is 12000 watts
100 amps at 120 volts is also 12000 watts

A 20 amp 240v load draws 20 amps from both legs.
A 20 amp 120v load draws 20 amps from only one leg.
If both of the above loads are on a 50 amp 240v RV pedistal, leg A as 20 amps 120v on it and leg B has 40 amps 120v on it.

Think of a 50 amp 240v breaker as two 50 amp 120v breakers. That is exactly what it is... breaker A and breaker B.

ttrstudios · November 05, 2014, 08:08:02 PM

Thank you. Well this changes everything. I do mean everything. I have been racking my brain trying to figure out how to keep the entire bus to ONLY 50 amps. This helps a lot.

sparkplug188 · November 05, 2014, 08:22:43 PM

You will be suprised at how little power 12000 watts is when you start adding everything up. This is my load calculation :

https://plus.google.com/117475453833540385589/posts/8SHfhNbX82X

Road side comp and curb side comp are banks of computers. That load calculation showed I needed to be able to switch between electric and propane heated water and I will have to get a propane range. I will transporting 6 to 8 people. It is very likely everything listed will be on at the same time.

Edit: There is a small typo on that load calculation. "Main 60" should be "Main 50". I got the 12000 watts right. That is what matters.

niles500 · November 05, 2014, 08:49:12 PM

Max rated Ampere draw is not the same as the nominal Amp draw. Add up the total breaker Amps in a standard load panel and it can sometimes be double the total Amps of the Main breaker - FWIW and HTH

Tony LEE · November 06, 2014, 02:03:29 AM

One thing to consider is that many campgrounds do not have a 50+50 service but only a single leg 30 amp. Normally use a dogbone adaptor to feed both 50 amp legs from the single 30 amp leg but of course you have to make sure you only switch on a maximum of 30 amps for the whole bus.

Then if you go to Mexico, you often have only a 15 amp service and need another adaptor and have to be even more careful what you switch on.

wg4t50 · November 06, 2014, 02:36:07 AM

Small item, Normally all electric ratings use the 80% rule, while a 20 amp breaker will carry 20 amps for a short time, it will heat and trip from the thermal heating, of course it will also trip from over loading. Why breakers are Thermal-Magnetic so will trip from either condition. 80% of 20 allows 16 amp load and if running 16 amp long enough it will also trip from the thermal heating. Also make sure your connections are tight or they can make heat also.
Dave M

sparkplug188 · November 06, 2014, 04:57:33 AM

Dave- That is all correct and good advice. In addition to what you said-- tight, but not too tight.

Story time: I had just bought my first RV and was upgrading the load center from 30 amps to 50 amps. Every guide I read said the connections had to be tight. If they aren't tight, they could heat up and start a fire. I really didn't want a fire, so I made sure the connections were REALLY TIGHT. I ate my Wheaties, put on my MAN HANDS and torqued the hell out of those 15 amp breakers. Later that day while I was running a circular saw, I started to smell burning plastic. To my surprise, one of the wires started to melt. Luckily I caught it and there wasn't a fire. I thought to myself, HOW could those connections not be tight enough!?! I unscrewed the wires from several breakers and found every 14 gauge wire was crushed thinner than a sheet of paper.

Thanks to that incident, I am now the proud owner of an electricians torque screwdriver. Every electrical connection has the recommend torque written somewhere on the device. That being said, a small amount of experience and reasonability are the only tools you need.

Oonrahnjay · November 06, 2014, 05:56:31 AM

Quote from: sparkplug188 on November 06, 2014, 04:57:33 AM
Dave- That is all correct and good advice. In addition to what you said-- tight, but not too tight.

Story time: I had just bought my first RV and was upgrading the load center from 30 amps to 50 amps. Every guide I read said the connections had to be tight. If they aren't tight, they could heat up and start a fire. I really didn't want a fire, so I made sure the connections were REALLY TIGHT. I ate my Wheaties, put on my MAN HANDS and torqued the hell out of those 15 amp breakers. Later that day while I was running a circular saw, I started to smell burning plastic. To my surprise, one of the wires started to melt. Luckily I caught it and there wasn't a fire. I thought to myself, HOW could those connections not be tight enough!?! I unscrewed the wires from several breakers and found every 14 gauge wire was crushed thinner than a sheet of paper.

Thanks to that incident, I am now the proud owner of an electricians torque screwdriver. Every electrical connection has the recommend torque written somewhere on the device. That being said, a small amount of experience and reasonability are the only tools you need.

Great info, Plug. Thanks.

TomC · November 06, 2014, 03:07:10 PM

My bus is wired straight 120vac. My 10kw Powertech generator is also wired straight 120vac-I don't have to worry about balancing the load. Only thing different is you have to run two 50amp 3 wire to the breaker box. While on generator, I have all the power I need. When on power pole, I only use one leg for 50amps. I can easily run two roof tops and have enough power to run my water heaters (one at a time). While I have tripped the power pole when it is only 30amp, I have yet to do that with 50amp. The only propane I have in the bus is the stove and furnace-all else is electric. I like the system so much, I'm repeating it in my truck.

ttrstudios · November 07, 2014, 06:12:10 PM

Thank you guys so much for the advice. Every bit of it is useful.

Jesse, I also did a load calculation and I should be fine as long as I don't crank up a hair dryer, microwave, heaters, and AC at the same time. Haha!

I appreciate all of you responding.
Thanks, Craig