Quick Hits

⚡ NEC Article 120 — Load Calcs Won't Kill You

⛵ Episode 43 — Marinas: Shore Power Gets Its Own Discount Curve

Article: 120 — Load Calculations
Part: Part VII — Marinas, Boatyards, Floating Buildings & Docking Facilities
Section: 120.120
Format: Quick Hit

🎯 What You'll Learn

Marinas, boatyards, floating buildings, and docking facilities have a special load-calculation rule for shore-power receptacles.

Section 120.120 provides a dedicated demand-factor table for receptacles supplying power to boats.

The more shore-power receptacles a facility has, the lower the percentage of the calculated receptacle load that must be included.

📖 The Quick Hit

⛵ 120.120 — Shore Power Receptacles

General lighting and other loads at marinas, boatyards, floating buildings, and docking facilities continue to be calculated using the applicable Part III methods.

But receptacles that provide shore power to boats get their own special treatment.

Instead of simply adding the full calculated load of every shore-power receptacle, Table 120.120 provides demand factors based on the number of receptacles.

⚠️ Important Neutral-Load Rule

There's an important limitation:

If Table 120.120 is used, you cannot also apply the neutral-load reduction permitted by 120.61(B).

In other words:

Table 120.120 demand factor

⬇️

Don't stack 120.61(B) neutral reduction on top of it.

That's an important detail to catch when working through the calculation.

📊 Table 120.120 — Shore Power Demand Factors

The table follows the familiar diversity pattern we've seen throughout Article 120.

🔌 1–4 Receptacles

➡️ 100%

The first tier receives no demand reduction.

🔌 9–14 Receptacles

➡️ 80%

The percentage drops as the number of shore-power receptacles increases.

🔌 15–30 Receptacles

➡️ 70%

This is the range that applies to the example in today's review question.

🔌 41–50 Receptacles

➡️ 50%

At this point, only half of the calculated receptacle load is included.

🔌 71 or More Receptacles

➡️ 30%

The demand factor reaches a much lower percentage for larger facilities.

📉 Why Does the Demand Factor Drop?

The idea is diversity.

A marina might have dozens of slips, but that doesn't necessarily mean every boat will simultaneously be connected and drawing its maximum calculated shore-power load.

So as the number of receptacles increases:

More receptacles

⬇️

More diversity

⬇️

Lower percentage of calculated load

This same basic concept appears repeatedly throughout Article 120.

📝 The Three Important Notes

Table 120.120 also includes several details that can change what gets counted.

1️⃣ Multiple Shore-Power Receptacles at One Slip

If a single slip has two shore-power receptacles with different voltage or ampere ratings:

➡️ Only the receptacle with the greater kW demand is included.

You don't simply add both receptacles together.

The larger kW demand controls.

2️⃣ Individual kWh Submeters

If the shore-power pedestal at each slip has an individual kWh submeter:

➡️ Multiply the final demand amperes by 0.9.

So after determining the applicable demand, the submeter provision provides another adjustment.

3️⃣ Boat Hoist + Shore Power

What happens when the same circuit supplies both:

⚓ A boat hoist

AND

🔌 Shore power?

Only the load with the greater kW demand is included.

Again, the Code isn't telling you to simply stack both loads together for this particular calculation.

🧮 Quick Example

Suppose a marina has:

20 shore-power receptacles

Looking at Table 120.120:

20 falls within the:

15–30 receptacle range

That gives us:

➡️ 70% demand factor

So if the calculated shore-power receptacle load before the demand factor were:

100 kVA

the table would result in:

100 kVA × 70% = 70 kVA

The key is identifying the correct receptacle-count range before applying the percentage.

🧠 Exam Takeaway

When you see 120.120, think:

⛵ Marinas / boatyards / floating buildings / docking facilities

🔌 Shore-power receptacles

📊 Table 120.120

📉 More receptacles = lower demand factor

⚠️ Don't also apply 120.61(B) neutral reduction

And remember these special notes:

Multiple receptacles at one slip → use the greater kW demand

Individual kWh submeter → final demand amps × 0.9

Boat hoist + shore power → use the greater kW demand

🎯 Test Yourself

Per Table 120.120, what demand factor applies to a marina with 20 shore-power receptacles?

A) 100%

B) 90%

C) 70%

D) 60%

✅ Answer

C) 70%

20 shore-power receptacles falls within the 15–30 receptacle range of Table 120.120.

Therefore:

➡️ 70% demand factor

📚 Keep Exploring

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View Sources & References:
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🔎 NEC References

Primary: Article 120, 120.120

Related: Table 120.120 — Shore-Power Receptacle Demand Factors

Related: 120.61(B) — Neutral Load Reduction

⚡ Series Progress

Article 120 — Load Calcs Won't Kill You

⛵ Part VII — Marinas, Boatyards, Floating Buildings & Docking Facilities

Episode 41: Hospital Receptacles Get Their Own Discount Table
Episode 42: Patient Care Circuits Get Their Own Rule Too
Episode 43: Marinas: Shore Power Gets Its Own Discount Curve

➡️ Next Up

Episode 44 — RV Parks: Every Site Gets a Starting Number

We're heading into Part VIII next, where RV parks get their own baseline load and demand-factor rules.

⚡ Quick Hit Summary

120.120 = Shore-power receptacles

Table 120.120 = Demand factor based on receptacle count

20 receptacles = 70%

And don't double-dip with 120.61(B).