Amps, watts and the one piece of arithmetic
Everything here comes from one relationship: amps equal watts divided by volts. A 1,500 watt coffee urn on a 120 volt circuit draws 12.5 amps. Two of them draw 25 amps, and a 20 amp breaker does not care that the label said 1,500. That is the entire mechanism behind almost every power failure at an event, and it is why the useful output is not a total but a per-circuit figure.
The second piece is that a circuit is not run to its breaker rating for a load that stays on. The rating is a trip threshold, not a working budget, and the working budget is a fraction of it. This calculator takes that fraction as an input with eighty percent in the box, because the correct figure depends on the equipment, the duty and the code that applies where you are, and it is a decision for whoever designs the distribution rather than a constant to assert. What matters for planning is that a 20 amp circuit gives you something like 1,900 usable watts, not 2,400.
Catering is the load, and everything else is rounding
Ask people to guess where the power goes at an event and they say the sound system and the lighting. They are wrong by an order of magnitude. Anything that makes heat draws heavily and anything that makes sound or LED light does not.
| Item | Typical watts | Amps at 120 V |
|---|---|---|
| Coffee urn, 100 cup | 1,500 | 12.5 |
| Portable convection oven | 1,500-1,800 | 12.5-15 |
| Warming cabinet | 1,200-1,600 | 10-13 |
| Chafing dish, electric | 400-800 | 3-7 |
| Refrigerated display | 400-700 | 3-6 |
| Powered speaker, mid size | 200-400 | 2-3 |
| LED uplight | 10-30 | under 0.3 |
| String of LED festoon | 40-80 | under 0.7 |
Twelve LED uplights draw less than two amps in total. One coffee urn draws twelve and a half. Look at that table and then look at where you assigned circuits, because the instinct to give sound and lighting their own generous supply while the caterer shares a single outlet is exactly backwards. Use the wattage on the actual plate of the actual equipment where you can; these are orientation figures and the spread between models is wide.
Splitting circuits is the fix
When a circuit comes out over the limit there is one correct response, which is to move load onto another circuit, and one incorrect response, which is to put a bigger breaker on the same wire. The breaker protects the conductor and changing it without changing everything behind it removes the protection rather than the problem. If a circuit is over and there is no other circuit to move to, the answer is more supply, which for a temporary installation means more distribution and possibly a generator.
Sequencing helps more than people expect. Much catering load is intermittent — urns cycle, ovens cycle, warming cabinets cycle — so the peak is when several happen to be heating at the same moment. Staggering when equipment switches on, and asking the caterer which items can wait, genuinely reduces the peak. It is also fragile, because it depends on people under pressure remembering the plan. Design so it survives everything being on.
The generator, and the part that is not arithmetic
The kilowatt figure at the bottom of this page is a starting point, not a specification. It handles steady load with headroom. It does not handle motor starting surge, and if your list includes refrigeration, a blower or a pump, the surge can be several times the running figure for a second or two and that second is what stalls a generator. The generator sizing calculator handles running against starting watts and is the right tool once motors are involved.
Everything past the arithmetic belongs to a qualified electrician. Temporary distribution across ground that guests walk on involves cord protection, ground fault protection, weather exposure, and grounding decisions that are licensed work in most places and are not guessed at from a web page. Generators bring their own set: they produce carbon monoxide continuously, they must never run indoors or inside a tent or near any opening or air intake, and the combination of fuel, hot exhaust and a crowd is a hazard that gets managed by placement and barriers decided on site. Plan the load here; hand the installation to someone who does it for a living. If you are running long cord runs from an existing building supply, the extension cord size calculator covers the voltage drop side of that.
Questions people ask
How many amps does a 20 amp circuit really give me?
Less than twenty for anything that stays on. The breaker rating is the point at which the circuit disconnects, not the load you design to, and a continuous load is planned at a fraction of the rating. Eighty percent is the figure most people use and it is the default here, which gives sixteen amps or about 1,900 watts at 120 volts. Take the actual derating from whoever is designing the distribution, because the applicable rule depends on the equipment, the duty cycle and the code in force where the event is.
Why did my breaker trip when the total was under the limit?
Almost always because the total was spread across circuits unevenly, and one circuit was over even though the sum was not. This is what the per-circuit output is for. The other common cause is a motor starting: a refrigeration compressor or a blower draws several times its running current for a moment when it kicks in, and if the circuit was already near the limit that surge is enough. A third possibility is that the outlets you thought were separate circuits were the same circuit behind the wall, which happens constantly in older buildings and halls.
What size generator do I need for a wedding?
It depends almost entirely on whether catering is electric. A marquee with LED lighting, a modest sound system and gas catering might sit under three kilowatts. The same event with electric ovens, urns, warming cabinets and refrigeration can be fifteen or twenty. List the actual equipment rather than looking for a per-guest figure, because no per-guest figure exists that survives the catering question. Then take the number to the generator sizing calculator to add motor starting surge, and to the supplier, who will ask about starting loads before they quote.
Can I run distribution cable across a walkway?
Not without proper cord protection, and the whole question is one for a qualified electrician rather than a planning calculator. Cable across a route people walk is both a trip hazard and a cable damage risk, and the way it is solved is with rated ramps and cord covers, with routes planned to avoid crossings where possible, and with ground fault protection appropriate to the installation. This site gives no wiring guidance and none should be inferred from the load numbers here.
Does the calculator handle 240 volt equipment?
Only if you run it separately. Set the voltage field to 240 and list the 240 volt items on their own to see their amps and their circuit loading, then treat the 120 volt list as a second calculation. Mixing both voltages in one list would produce a meaningless amp figure, because the amps for a given wattage halve at the higher voltage. The total watts across both lists is still the right input for generator sizing, so add those two totals together for that part.