Maximum wattage for American power outlets

[pianoparadise]

Is there a maximum power that can be drawn from electrical outlets in America?

Also, I was curious if this maximum power is exceeded, would it limit the charging of other devices in the same house but connected to other outlets.

 

[schip666!]

Normal household circuits are usually "fused" at 15 or 20 amps, so nominally 1800 to 2400 watts. If that power is exceeded the breaker (should) trip. Assuming that the wiring is good, drawing power from one circuit shouldn't affect other circuits -- very much -- you might see your lights dim when the refer goes on or something like that. Drawing a lot of power on one leg of a 220 house feed can make the voltage go UP on the other leg though.

 

[Topher925]

Drawing a lot of power on one leg of a 220 house feed can make the voltage go UP on the other leg though.

Schip (or anyone else), could you explain why this is?

 

[schip666!]

This is more of a hand wave than an explanation...but... Your 220v service is supplied by a center-tapped transformer. Putting a load on one leg of the transformer causes it to try to compensate which pushes the voltage up on the other leg.

 

[zgozvrm]

High or low voltage on a split phase 240/120 VAC system is generally due to a loose connection, most often in the neutral or ground wire. If everything is in good condition, there is no reason your voltage should rise or fall on one leg, due to the load on the other.

If everything is sized correctly and all the connections are good and tight, you will see little or no dimming of lights when a large load is introduced (like an air conditioner starting up). Even then, the dimming will be temporary; motors draw large amounts of power when they start, which causes the dip in voltage. Once motors get up to speed (generally within one second), the voltage recovers and the lights return to normal brightness.

By tapping the center of the secondary coil, you have (in effect) two secondary coils, each with half as many turns in their windings as the entire secondary coil. This is why the voltage measured from either "leg" to neutral is 120V (the primary to secondary turns ratio has been doubled). Loading one half of that split secondary will have no effect on the voltage of the other half (since the turns ratio hasn't changed).

Below, is a link to a PowerPoint presentation for an explanation of overvoltage due to loss of neutral in a split phase system. Note that when the text states that the neutral is disconnected, the picture shows the ground being disconnected, so imagine that the center tap of the transformer has been disconnected as well.

http://code-elec.com/userimages/Lost Neutral.ppt


It is possible (and common) for a loose neutral connection to exhibit no problems until a large load is introduced.

 

[mdjensen22]

One thing worth noting: It is up to local municipalities as to what version of the NEC code they choose to adopt. Some cities use versions far older than others. As such, never assume 20A - only assume 15A circuits for residential installations.

Also, per NEC code, a single device should only consume 80% of the circuit limits without having special installation instructions for agency certification. If you are designing something for commercial sale, stay under 12A and this will not be an issue.