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The Service: Load Calcs, Grounding Electrodes & SDS
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The Service: Load Calcs, Grounding Electrodes & SDS
- Study each rule with its stated conditions.
- Use the linked current sources and check any marked point before relying on it.
Apply the rule and its conditions
- For a dwelling feeder or service calculation, the minimum general lighting and included receptacle unit load is 2 VA per square foot. Add 1500 VA per specified two-wire small-appliance branch circuit and at least 1500 VA per specified laundry branch circuit. When grouped with lighting, Table 120.45 takes the first 3000 VA at 100%, the next 117000 VA at 35%, and the remainder above 120000 VA at 25%. A 2000 sq ft example with two such small-appliance circuits and one laundry circuit gives 8500 VA connected and 4925 VA demand for this group, before other loads.
- Under 120.6, feeder or service calculations may use the largest noncoincident load when the installation makes simultaneous use unlikely. For a genuinely noncoincident 9000 VA cooling load and 10000 VA heating load, use 10000 VA. Do not assume every heating/cooling installation is noncoincident; branch-circuit treatment has its own interlocking condition.
Apply the rule and its conditions
- For one household electric range rated 12 kW, the permitted demand in Table 120.55, Column C is 8 kW. Choose the applicable table column and notes using the actual appliance type, quantity and ratings; the one-range example is not a universal demand for every cooking appliance.
- Table 250.66 sizes a grounding electrode conductor from the largest ungrounded supply conductor or equivalent area. A connection serving only rod, pipe or plate electrodes need not exceed 6 AWG copper; a portion serving only concrete-encased electrodes need not exceed 4 AWG copper. These are limits on the required minimum size for the qualifying portion, not bans on larger wire.
Source: NEC 2026 120.55; Table 120.55 and notes; 250.66; Table 250.66; 250.66(A)/(B)
Apply the rule and its conditions
- A separately derived system has no direct circuit-conductor connection to another electrical source, except grounding and bonding connections. For the ordinary grounded separately derived system under 250.30(A), the system bonding jumper is generally placed at one point from the source through the first system disconnect or overcurrent device. The grounding electrode conductor connects to the grounded conductor at that same point. Check special rules and exceptions; a generator’s actual connections determine whether it is separately derived.
- The default is one service disconnect; 230.71(B) permits two through six only in specified separated enclosure or compartment arrangements. Permitted multiple disconnects are grouped and identified under 230.72. Location also matters: one- and two-family dwellings ordinarily require the readily accessible outdoor location specified in 230.70(A)(1), subject to its emergency-disconnect exception; other buildings have the outside-or-nearest-entrance rule.
Conditions that change the answer
- Dwelling feeder/service: 2 VA per square foot; add the circuit allowances, then apply all three demand bands.
- Establish noncoincidence before omitting a load; do not automatically discard heating or cooling.
- One 12 kW household range: Column C gives 8 kW; other arrangements require the table notes.
- Use the table, then the electrode-specific limitation: qualifying rod portion 6 copper; concrete-encased portion 4 copper.
- One permitted system bonding point; connect the grounding electrode conductor at that same point.
- One is the default; two through six require permitted separation, grouping and the correct location.
Carry these checks into practice
- Dwelling feeder/service: 2 VA per square foot; add the circuit allowances, then apply all three demand bands.
- Establish noncoincidence before omitting a load; do not automatically discard heating or cooling.
- One 12 kW household range: Column C gives 8 kW; other arrangements require the table notes.
- Use the table, then the electrode-specific limitation: qualifying rod portion 6 copper; concrete-encased portion 4 copper.
- One permitted system bonding point; connect the grounding electrode conductor at that same point.
- One is the default; two through six require permitted separation, grouping and the correct location.
Check your recall
- How does the standard dwelling service load calculation start?
- Do you add heating and air-conditioning together in a load calc?
- How is a household electric range figured in the service load?
- How is the grounding electrode conductor sized, and where is the trap?
- Where is a separately derived system grounded and bonded?
- How many service disconnects are allowed?