NEC Lighting, Appliance and HVAC Practice Questions

Rule 630.11(A): an arc-welder's supply conductor is sized from its rated primary current times a duty-cycle multiplier in Table 630.11(A).

630.11(A) — arc-welder duty cycle

Multiply the welder's rated primary current by the duty-cycle multiplier from Table 630.11(A). For a group of welders, the two largest count at 100 %, then apply 85 %, 70 %, and 60 % to the rest (630.11(B)).

1. Skipping the duty-cycle multiplier.
2. Reading the wrong duty-cycle factor in Table 630.11(A).
3. Sizing a group of welders with all of them at 100 %.

630.11(A) — duty-cycle factor

Multiply the welder's rated primary current by the duty-cycle multiplier from Table 630.11(A). For a group of welders, the two largest count at 100 %, then apply 85 %, 70 %, and 60 % to the rest (630.11(B)).

1. Skipping the duty-cycle multiplier.
2. Reading the wrong duty-cycle factor in Table 630.11(A).
3. Sizing a group of welders with all of them at 100 %.

630.11(B) — group of welders

Multiply the welder's rated primary current by the duty-cycle multiplier from Table 630.11(A). For a group of welders, the two largest count at 100 %, then apply 85 %, 70 %, and 60 % to the rest (630.11(B)).

1. Skipping the duty-cycle multiplier.
2. Reading the wrong duty-cycle factor in Table 630.11(A).
3. Sizing a group of welders with all of them at 100 %.

Use the nameplate values directly: size the conductor to the MCA (22.1 A → #10) and the breaker to the marked maximum (35 A).

Conductors = 125 % of the largest motor-compressor + 100 % of the other loads (440.33). The branch short-circuit / ground-fault device = up to 175 % of the compressor + the other loads, then the next standard size (440.22). When a nameplate gives MCA / MOCP, use those numbers directly.

1. Using the 125 % conductor factor for the protective device — it is up to 175 %.
2. Ignoring a nameplate MCA / MOCP when it is provided.
3. Leaving the fan load out of the total.

Rule 440.22: size the compressor short-circuit/ground-fault device at 175 % of the compressor current plus the other loads (the fan), then take the largest standard size that does NOT exceed that value.

440.22 — branch-circuit short-circuit/ground-fault

Conductors = 125 % of the largest motor-compressor + 100 % of the other loads (440.33). The branch short-circuit / ground-fault device = up to 175 % of the compressor + the other loads, then the next standard size (440.22). When a nameplate gives MCA / MOCP, use those numbers directly.

1. Using the 125 % conductor factor for the protective device — it is up to 175 %.
2. Ignoring a nameplate MCA / MOCP when it is provided.
3. Leaving the fan load out of the total.

440.33 — motor-compressor conductors

Conductors = 125 % of the largest motor-compressor + 100 % of the other loads (440.33). The branch short-circuit / ground-fault device = up to 175 % of the compressor + the other loads, then the next standard size (440.22). When a nameplate gives MCA / MOCP, use those numbers directly.

1. Using the 125 % conductor factor for the protective device — it is up to 175 %.
2. Ignoring a nameplate MCA / MOCP when it is provided.
3. Leaving the fan load out of the total.

41.55 A does not land on a standard size, and 440.22 does not allow the next size up here, so drop down to the next standard size below.

440.22 — short-circuit/ground-fault

Conductors = 125 % of the largest motor-compressor + 100 % of the other loads (440.33). The branch short-circuit / ground-fault device = up to 175 % of the compressor + the other loads, then the next standard size (440.22). When a nameplate gives MCA / MOCP, use those numbers directly.

1. Using the 125 % conductor factor for the protective device — it is up to 175 %.
2. Ignoring a nameplate MCA / MOCP when it is provided.
3. Leaving the fan load out of the total.

A welder's supply conductor is sized from its rated primary current × the duty-cycle multiplier in Table 630.11(A), then picked from the 60 °C column of Table 310.16.

Multiply the rated primary current by the Table 630.11(A) duty-cycle factor to get the required ampacity (62.4 A), then step down the 60 °C column until a conductor covers it: #6 is only 55 A, so you go up to #4 (70 A).

1. Skipping the duty-cycle multiplier.
2. Sizing from the 75 °C column — these welder branch conductors are taken at 60 °C, where #6 (55 A) won't cover 62.4 A.
3. Reading the wrong duty-cycle factor in Table 630.11(A).