What year did the requirement for 100amp main start?

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acrwc10

acrwc10

Master Code Professional
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Building inspector
Sec 230.79(C) requires the minimum size of a residential one family dwelling be 100 Amps. What year did that get put in the code?

On the same requirement, would anyone see a problem with reducing the main breaker on a service to 90 amps, to allow for a PV system, as long as the calculated load did not exceed 90 amps?
 
Dennis Alwon said:
How does reducing the main breaker help with pv. The larger the service the larger pv breaker you can use.
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Reducing the size of the incoming service breaker compared to the panel bus rating gives more room under the 120% rule for a load side PV connection

The ultimate limit is still the nominal service size for a supply side PV connection.
 
It was about 02 or 05
I know it was not 62 as the change happened after I starting teaching code classes in 96
 
acrwc10 said:
Sec 230.79(C) requires the minimum size of a residential one family dwelling be 100 Amps. What year did that get put in the code?

On the same requirement, would anyone see a problem with reducing the main breaker on a service to 90 amps, to allow for a PV system, as long as the calculated load did not exceed 90 amps?
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The 100A requirement is for the service disconnecting means, i.e. the equipment rating, not the OCPD rating.

The main thing if you use a 90A MCB is that the calculated load, continuous at 125%, does not exceed 90A.
 
tom baker said:
It was about 02 or 05
I know it was not 62 as the change happened after I starting teaching code classes in 96
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Somehow, I think we have two different distinctions of OP question.

In opening the writ I find that the 1959 NEC says:
1959 NEC 230-71. Rating of Service Equipment.

(a) The service disconnection means shall have a rating not less than the load to be carried determined in accordance with Article 220. In general the service disconnecting means shall have a rating of not less than 60 amperes where a switch is used, and not less than 50 amperes where a circuit breaker is used, except:

Exception No. 1. For single family residences with an initial load of 10 kw or more computed in accordance with Section 220-4, the service equipment shall have a rating of not less than 100 amperes.
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The 1956 NEC goes to an older Section numbering system, and at 2357 (Rating of Service Switch) there is no exception to the 60 A fuse, 50 A circuit breaker requirement.
 
The 1959 NEC 230-71 requirement, that I quote in Post #8, over the subsequent editions moved around and settled into Section 230-79(c).

In 1996, the NEC said:
1996 NEC 230-79(c). Rating of Disconnect.

The service disconnection means shall have a rating not less than the load to be carried, determined in accordance with Article 220. In no case shall the rating be lower than specified in (a), (b), (c), or (d) below.

(c) One-Family Dwelling. For a one-family dwelling, the service disconnection means shall have a rating of not less than 100 amperes, 3-wire under either of the following conditions: (1) where the initial computed load is 10 kVA or more, or (2) where the initial installation consists of six or more 2-wire branch circuits.
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In the 1999 NEC, 230-79(c) turned into the present wording by dropping the thresholds of 1996 NEC 230-79(c)(1) and (2). So, technically, the present wording was arrived at in 1999, but the actual "100 Amp" requirement was first introduced in 1959.

Think about that 10 kVA threshold a moment. That stated, that even though your calculated load would be served by a 14.4 kVA 60 Amp service, one was still required to make the service a 100 Amp.
 
al hildenbrand said:
The 1959 NEC 230-71 requirement, that I quote in Post #8, over the subsequent editions moved around and settled into Section 230-79(c).

In 1996, the NEC said:

In the 1999 NEC, 230-79(c) turned into the present wording by dropping the thresholds of 1996 NEC 230-79(c)(1) and (2). So, technically, the present wording was arrived at in 1999, but the actual "100 Amp" requirement was first introduced in 1959.

Think about that 10 kVA threshold a moment. That stated, that even though your calculated load would be served by a 14.4 kVA 60 Amp service, one was still required to make the service a 100 Amp.
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OK, resi is not my strong suit, but it looks like the service can had to be rated for 100 amps but the OCPD could be the 60 amp fuse or the 50 amp circuit breaker. So in the panel, you're looking at a 2-pole main CB with a "50" molded into the handle up to 1998, in theory.
 
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