Holiday Lighting: Bulb Count and Amp Load

Excel Formulas › Holiday Lighting

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A lighting install has two numbers that matter: how many bulbs a run takes and whether the run will pop the breaker. Spacing gives the count; watts and volts give the amps — and one IF warns you before the customer's power does.

Quick formula: Bulbs from spacing, then flag the amp load:

=ROUNDUP(B2*12/C2,0)

120 feet of roofline at one bulb every 12 inches is 120 bulbs; at 7 watts each that is 7 amps — safely under a 15-amp circuit's 12-amp working limit.

Functions used (tap for the full reference guide):

ROUNDUP IF

The example

Three runs with their length, bulb spacing, and per-bulb wattage; the last column flags runs over the 12-amp working limit of a 15-amp circuit.

	A	B	C	D	E	F	G
1	Run	Feet	Space	Watts	Bulbs	Amps	Status
2	Roofline	120	12	7	120	7.0	OK
3	Mega display	300	12	7	300	17.5	Add circuit
4	Bushes	60	6	5	120	5.0	OK

The formula

Bulbs come from spacing; amps come from watts and volts; IF turns the amp draw into a go or no-go:

=ROUNDUP(B2*12/C2,0) // feet to inches, divided by spacing, rounded up to whole bulbs

How it works

Two calculations feed one decision:

B2*12 turns the run length into inches so it matches the spacing units.
/C2 divides by the spacing to get the bulb count, and ROUNDUP makes it whole.
=E2*D2/120 gives the amp draw: bulbs times watts, divided by 120 volts.
=IF(F2>12,"Add circuit","OK") compares the amps to the 12-amp working limit of a 15-amp circuit and flags the overloads.

The 12-amp figure is 80% of a 15-amp breaker — the continuous-load limit for holiday runs left on for hours. Drop it to 16 for a 20-amp circuit.

Try it: interactive demo

Interactive

Enter the run length, bulb spacing, and per-bulb wattage.

Run length (ft)

Spacing (in)

Watts per bulb

Variations

Bulbs across several runs

Total the bulb counts before ordering so you buy in whole boxes.

=SUM(E2:E10)

Runs a circuit can carry

How many identical runs fit on one 12-amp circuit.

=ROUNDDOWN(12/(E2*D2/120),0)

Pitfalls & errors

Spacing is in inches but run length is in feet — the *12 reconciles them. Drop it and you undercount bulbs by a factor of twelve.

LED bulbs draw a fraction of the watts, so an LED run almost never trips the amp check. The IF earns its keep on incandescent C7 and C9 sets.

Practice workbook

📊

Download the free Holiday Lighting: Bulb Count and Amp Load practice workbook

Edit the yellow feet, spacing, and watts cells; bulbs, amps, and the circuit check recalculate.

Frequently asked questions

Why 12 amps and not 15?

Code limits a continuous load to 80% of the breaker rating, and holiday lights run for hours. Eighty percent of 15 amps is 12, which is the number the check should use for a standard circuit.

How do I handle mixed bulb types on one run?

Split them into separate rows by wattage, calculate amps for each, and sum. Averaging the watts across a mixed run hides the peak the breaker actually sees.

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Function references: ROUNDUP IF