Best Answer

== == == == Warning that must be heeded Firstly, you MUST have 4 wires run to your sub-panel. If the existing feed was 3 wire (two hots and ground) you cannot get 110 Volts off it safely. If this is in an outbuilding, you may be able to get away with it. There are many ifs to that (more than I can go into, or even know), so you must talk to a licensed electrician first. No matter what, if your existing circuit only has 3 wires STOP! You must pull new wire or at least talk with someone licensed. Also, this is a big job. You are adding a whole new breaker panel to your property. Are you comfortable doing this? If not, hire someone. If you botch neutral you can burn up anything plugged into it. If you botch ground you can zap yourself. Planning, planning, planning Now, first things first. What do you plan to power with this? What will you power with this in the future? You must sit down and answer these questions as they determine the size of box and current rating of the feed. Remember the golden rule, you want your expected load to be 80% of the breaker size. So if you plan to draw 24A or less you want a 30A feed. 32A or less on 40A, 40A or less on 50A, etc... Is the existing circuit big enough? If not, you need to run new wire. Also try to determine how many breakers you want to put in this. That will determine the physical size of the breaker box you buy. Installing something oversized now is easy. Upgrading an undersized sub-panel 5 years from now is not. How to install For the scope of this how-to you only need to buy a new breaker box and a clamp. I strongly recommend going with the same brand as your existing panel (if still available) so the breakers will be interchangeable. For a sub-panel you shouldn't run into current limitations, but check just to be safe. Home Depot and Lowes carry a line of 6 position Square D panels that are inexpensive and good for sub-panels. Also buy a clamp to secure the feed wire to your new panel. If this is in an outbuilding that does not have its own panel already, you will need a ground rod. Beyond that, it is all downhill from here. Steps to follow * Shut the outlet off. (If you needed me to tell you this, hire an electrician. :) ) * Remove the old outlet and box. * Mount the new panel and secure the wire to the box * Remove any bonds or screws that connect the neutral to ground. Neutral can only be bonded to ground in the main panel. * Connect the hot wires to the hot busses of your panel * Connect the neutral wire to the neutral bus. * Connect the ground wire to the ground bus. If this panel is not in the same building as the panel feeding it, drive in the ground rod and connect that to the ground bus as well. * Recheck the torque on the screws connecting the wires you just connected. They must be tight. That's it! Your panel is installed. Now it's just a matter of installing branch circuits, which is not within the scope of this how-to. Website How-to BCLEAR. Note BCLEAR's link seems to be gone. You can still find it in the way-back machine over at I found it after 11/6/2005 in their archive. Take a look, it's a good article.


As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

If you do this work yourself, always turn off the power

at the breaker box/fuse panel BEFORE you attempt to do any work AND

always use an electrician's test meter having metal-tipped probes

(not a simple proximity voltage indicator)

to insure the circuit is, in fact, de-energized.




User Avatar

Wiki User

โˆ™ 2009-08-18 18:00:24
This answer is:
User Avatar
Study guides

Create a Study Guide

Add your answer:

Earn +20 pts
Q: How do you wire an existing 240V circuit line as the feed to a new subpanel with 120V circuit breakers?
Write your answer...
Related questions

Can you tap into a 240V circuit to get 120V?

On a pure 240V circuit, no. There is no neutral. On a 240/120V circuit, yes. You have the needed neutral. Tapping 120volts off an existing 240volt branch circuit with a neutral is possible but is not a proper method. It does not comply with most electrical codes.

Can you convert a 240V circuit on your service panel to two 120V circuits?

A 240 volt breaker can be removed and replaced with 2 - 120 volt breakers. The existing 240 volt breaker should not be used to supply 2 separate circuits.

Can I use a 120v rated bulb in a 240v socket on a 120v circuit?


How can you split a 40 amp 240v circuit into 2 20 amp 110v circuits?

You don't, just replace the 40amp breaker with two 20amp breakers, now you should have a black 120v 20amp hot, and a red 120v 20amp hot. Do not use the 40amp double pole breaker. You can just tie your neutrals together for the wire that is going out from the existing wire and you have two 12/2 w ground wire that you can run anywhere.

Do you have 120 volts or 240 volts if you tie two 120 volt lines together?

Do you mean tying two breakers together or tying two 120v wire together. You can tie two breakers together with a breaker tie bar if they are each on seperate legs of the panel and yes you will then have 240. If you are talking about tying 2 120v WIRES together BE CAREFUL! If those two wires come from different 120v legs of the panel you will have a direct short and a pretty serious spark to put it mildly. You should only tie 2 120v wires together that are on the same circuit. And you'll still have 120 at that point

What is the resistance of a circuit with 120V and 10A?

12 ohms

If your air conditioner is 120V but the outlet is 240V can you convert from 240V to 120V and how?

If the 220V circuit is dedicated, is to derate the circuit to a dedicated 110V outlet. Replace the 220V breaker with a 110V breaker and install a 110V outlet in place of the 220V outlet. If the original circuit was 20A or greater go with a 20A breaker and a 20A outlet as Airconditioners are fairly large loads. Do not exceed the current rating of the old circuit as that is all the current the existing wiring can handle.

What is the voltage of a series circuit with four voltage drops of 30 volts?

there is 120V across the circuit.

What is the current in a 120v circuit if the resistance 20ohm?

6 amps

Can you convert two 120 volt outlets to one 220 volt?

Do not attempt this!If you need a 220V outlet in a location where there is an existing 120V outlet(s) hire an electrician to run entirely new 220V wires from the breaker panel. He may even have to add a new dual pole breaker for the new circuit (use of two standard single pole breakers on a 220V circuit is a code violation, which is what you would get if you did somehow connect two 120V lines and actually get 220V).

Is 150 a lot of power?

I'll assume you mean 150 Watts in which case it isn't that much power. It would depend on the circuit from which you are drawing this power to ultimately determine if it is "a lot of power". A typical house in the US has 120V circuits with 15-20A breakers which would allow the circuit to handle a load of 1,800-2,400 Watts.

In a 120V 24A circuit what amp fuse is needed to protect it?

30 amp

How many amps would a 150 watt bulb draw in a 120v circuit?

1 amp

What is the maximum current you can draw from a 120v AC outlet?

It's set by the circuit breaker that protects the circuit, and the setting on the circuit breaker is determined by the cable thickness.

Could 12 gauge wire is used on a 20 amp circuit at 120V?


How can you wire one half of a 20 amp 120 v duplex wall socket to run 240 volts over 2 legs 2 separate 120 v legs?

You can't. I'm not sure exactly what you're trying to do, but the way it is written is not possible. It seems you might want to use half of a duplex receptacle for 120 and half for 240. This would not be code compliant, nor would it make sense. A plug designed for 240v will not even fit into a 120v receptacle. You need a 240 volt receptacle rated for the amperage you will need. Also, an existing 120v receptacle has nothing to do with your 240v receptacle. For a 240 volt receptacle, you'll need to run 2 new 120v lines (in the same cable). The existing 120v circuit cannot be used here, even if you added another 120v circuit, because when a load uses 240v, both 120v circuits supplying the 240v must be controlled by a common disconnect (a 2 pole breaker designed for 240v circuit). My advice would be to show an electrician what you want done. I'm sure they can tell you how to make that happen.

Can you make 30 amp 240v breaker from two 120v breakers?

No. It would not be safe because it would interrupt only one phase.

Is the 120V outlet a dedicated circuit?

It depends on the way your house or business is wired. Usually not. Usually a circuit feeds several lights or outlets.

How many amps does 230 watts equal is you are using 120V ac circuit?

1.9 amps

How many watts can I plug into a 120 volt outlet handle?

Power [Watts] = Voltage [Volts] * Current [Amps] These equations assume purely resistive loads, all in the ideal format, anything other than ideal will vary based on your homes wiring, the types of breakers that you use, the type of device [load] that you put on the circuit, etc. For a 20A Circuit: 120V (RMS) * 20A = 2400W (Keep in mind that this is an ideal case) For a 15A Circuit: 120V (RMS) * 15A = 1800W (Keep in mind that this is an ideal case)

How many C9 lights can you put on one circuit?

C9 lights are 9watts each. On a unused 120v 15A circuit, you could use about 200 lights.

Can you feed a 208v 200amp panel from 480v 20amp circuit?

Not directly, you would need to transform 480v circuit to 120v with a transformer first.

I am wondering on 120v lines you have 1 Breakers and on 240v you have dual brakers my question is these breakers have a rating on each side say 20amps does that mean these are 40amp fuses?

No. 240V circuits using three wires normally have 120 volts on each of two legs and a common ground/return leg. Measuring across the two hot legs gives the 240 volts. Each of the hot legs has its own circuit breaker. Two breakers are protecting two separate hot legs. If each breaker is rated as 20 amps, then 20 amps is the maximum for EACH leg. It is not a 40 amp circuit. be careful if you start messing around with household electricity.... ;-)

There is a current of 0.83 A through a light bulb in a 120 V circuit What is the resistance of this light bulb?

V=IR 120V = 0.83A * R R= (120V)/(0.83A) R= 144.57 Ohms

How do you upgrade from 120V 15A to 120V 20A or from 240V 20A to 240V 30A?

You have to replace the wire (as you are increasing the current capacity), the outlet, and the breaker. Essentially you have to remove the old circuit and put in a new one. You can't reuse parts of the old circuit as you are increasing the current capacity and they would be underrated.