Sizing circuit breaker and cables

Several questions:

Where are you located, the United States. If so, you probaby already have

220v without a transformer, and probably a lot of other places too.

Is this a residential or commerical location?

What kind of 220 volt tools do you want to run?

Are you certain you require a transformer?

Even most residences in the US, and other places, are serviced with to 120v circuits that measure 120 v from phase to ground, and if measured from phase to phase yeilds 220v. You simple wire both phases to your 220v requirements.

I wouldn't. I'd use a 30 A breaker - is that an off the shelf size? It's just above the amp rating of the transformer wire.

Don't be surprised if it kicks off trying to turn the transformer on the first time. Just reset and retry. Circuit breakers don't much like large transformers.

Maxwell is correct. All US electrical services have 220V available. All you need to do is install a 2 pole breaker to get 220V. If you really only have 110V available, then the KVA rating is what you should use. Remember that it is the load current that causes I^2R heating in the transformer. Also, breakers respond to current, not power. So 3000/110 = 27.3A is the prper rating to use. This is not a standard size. You probably are safe with a 30A breaker.

Just keep in mind that the NEC specifies you can only load a breaker to 80% of its rating. 30A * 80%= 24A Max.

The circuit breaker only knows about the current - it doesn't know about voltage, power, or power factor.

So, use a 30 Amp breaker - that is a standard size, and should trip on a fault before the transformer overheats.

As another poster mentioned, you should have 240V available already, using two 120V circuits on opposite phases.

--
Peter Bennett, VE7CEI  Vancouver BC, Canada
peterbb4 (at) interchange.ubc.ca  
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

Thanks to everyone for the help. I see now why all you need to know about is the current - I2R losses in the transformer

I hadn't thought of getting 240 from across a pair of phases. Thanks for the suggestion.

I'm still interested in the correct way of sizing breakers as well as thermal overload trips come to think of it. Take an induction motor for example. It is protected by a normal breaker as well as a thermal overload breaker that will trip if the motor is stalled by a heavy load. How do you calculate the size of the breaker that will handle the starting current but protect it during normal operation, and the rating of the thermal overload so it will trip when the motor is stalled. You need to know more than just the running current (say

10A). How do you know how much current it will draw when stalled so you can set the thermal overleoad trip properly. Is there a rule of thumb or do you need to get data from the manufacturer?

Now, say you a also have a heater rated at 10A. Will it need the same breaker as the motor since there is not start current spike?

Sometimes 208 V if it's a 3 phase service.

You size the breaker to protect the cables to the motor.

They are usually set by run amps or by horsepower.

That's a straight load - but you protect the cables.

Your breaker and conductor size should be selected based on the desired capacity of each of your circuits.

A single circuit can easily service several motors, depending on their collective current requirements. You need to list your motors including horsepower and amperage.

Overload protection for each motor, if desired or even needed, should selected for each motor and loctated at the motor.

If you have a conventional US type residential electric service, pitch the transformer idea.

NOT TRUE. Especially for High Magnetic Trip breakers. Also, the AIC Rating for the breaker is based on voltage.

Respectfully, it is SINGLE PHASE service. There are not 2 distinct phases, it is merely a center-tapped winding. But the intent is clear enough for the purposes here.

-mpm

When the circuit breaker is closed, it doesn't know about the circuit voltage. The voltage rating on a breaker (or fuse or switch) is the maximum voltage that should be across it when it is open/blown.

--
Peter Bennett, VE7CEI  Vancouver BC, Canada
peterbb4 (at) interchange.ubc.ca  
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

"Split phase" works well in describing it.

What would you call it?

Split service.

Sorry Homer, but my question was not directed to you, and that wasn't the question.

This isn't private email and my answer was correct.

I think the correct term would be single phase 240 VAC center tapped neutral. Split phase is used for motors.

Paul

>

Correct on both statements. But the question is: What is the proper term for refering to each 120 volt conductor if it's improper to refer to it as a phase? If it's not a phase, what is it?

I really don't know, that's why I asked the question.

I think it is proper to call them Phase A and Phase B. The two legs are out of phase with each other by 180 degrees (residential), with respect to the center tap. It is really no different then a unipolar stepper motor with a center tap. The industry standard is to call out one end of the coil A and the other end A/, and B, B/ for the other winding.