ampacities of branch ckts?

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steve66

Senior Member
Location
Illinois
Occupation
Engineer
haskindm said:
A thermostatically controlled baseboard heater is NOT a continuous load because it will NOT usually run all night. The thermostat will cycle the unit on and off. HOWEVER 424.3(B) requires that the conductors and overcurrent devices for electric space heating be sized at 125% of the total load. So same difference.

My 2005 book says:

"Fixed electric space heating equipment shall be considered continuous load."

I don't think even this group can argue that:grin:
 

haskindm

Senior Member
Location
Maryland
This was a change between 2002 and 2005 which, in my opinion, just muddies the water and makes it harder for people to understand. Earlier additions of the code said that baseboard heaters and some water heaters should be calculated at 125%. Now the code says that they shall be considered a continuous load. This makes the reader remember that a continuous load must be calculated at 125%. They "simplified" the rule by taking a one-step process and making it a two-step process!
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
haskindm said:
This was a change between 2002 and 2005 which, in my opinion, just muddies the water and makes it harder for people to understand. Earlier additions of the code said that baseboard heaters and some water heaters should be calculated at 125%. Now the code says that they shall be considered a continuous load. This makes the reader remember that a continuous load must be calculated at 125%. They "simplified" the rule by taking a one-step process and making it a two-step process!

What was confusing before was the reason for the 125% increase. If you thought they were continuous, should the 125% factor be applied twice (once for the unstated reason, and once for being continuous)? Perhaps they need to combine both -- state they are continuous and must be calculated at 125%...

I have the same issue with HVAC and motors. Is the 125% value for the largest motor to compensate for being continuous or some other reason? Would a swimming pool pump that runs 7 hours per day every day be hit with an additional 125% value?
 

tryinghard

Senior Member
Location
California
thetacon said:
I am trying to understand the ampacities of 15 and 20 amp ckts in 'romex'. I have had more than one boss tell me that i could not put more than 16 amps on a 20 amp circuit

If you are circuiting a corded appliance/equipment then your boss is correct because 210-23(A)(1) states: ?The rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating.? In other words your equipment tells your circuit what size to be, as example: 1-12A can be 15A circuited, 13-16A requires a 20A circuit. If you are supplying equipment from a receptacle with more than on receptacle in the circuit then your boss is correct again, see 210-21(B)(2) and it?s table.

You may be asking about residential general lighting circuitry, because you say ?ckts in romex?, if this is what you mean then table 220-3(A) applies which is 3VA per square foot example: 600 Sq Ft = 15A x 120V / 3VA & 800 Sq Ft = 20A x 120V / 3VA, these calculations include lighting and receptacles -- 220-3(B)(10) -- and 80% does not apply. In a dwelling unit if you have large lighting loads it?s only safe to calculate them separate from the receptacles but you can circuit them up to their Volt Ampere load: 1800VA = 15A and 2400VA = 20A and again the 80% does not apply. Residentially quantity does not matter whereas non-dwellings 180VA applies to every receptacle allowing up to 13 on a 20A circuit, 80% does not apply.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
tryinghard said:
. . . non-dwellings 180VA applies to every receptacle allowing up to 13 on a 20A circuit, 80% does not apply.
Are you saying that this applies to actual installations, or merely for load calcs?
 

iwire

Moderator
Staff member
Location
Massachusetts
tryinghard said:
. . . non-dwellings 180VA applies to every receptacle allowing up to 13 on a 20A circuit, 80% does not apply.

LarryFine said:
Are you saying that this applies to actual installations,

I don't know if he is saying that but I would and have said that. :)
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Pierre C Belarge said:
Dennis
That is a very interesting illustration. It would seem that parts of it are moving, until you focus your eye on one group of circles, then they stop moving while others seem to move...very good.

Where did you find that?

It is in a museum somewhere --- Painted by some artist who I can't remember as well--- Found it in an email of optical illusion sent to me and also saw it on AOL. It is pretty amazing since it isn't really moving.

You can copy and paste it for yourself.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
haskindm said:
This makes the reader remember that a continuous load must be calculated at 125%. They "simplified" the rule by taking a one-step process and making it a two-step process!
Isn't this a fairly well-known rule in the trade?

I can't remember percentages very well, but if a load is branded as continuous, that I can remember.

I guess different folks remember things different ways. :cool:
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Well, in my NEC (2002), 220.14 refers to motor loads, so I take it that you're referring to 220.3(B)(9):
Receptacle Outlets. Except as covered in 220.3(B)(10), receptacle outlets shall be computed at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke. A single piece of equipment consisting of a multiple receptacle comprised of four or more receptacles shall be computed at not less than 90 volt-amperes per receptacle.
Now, since this appears in Art. 220, I look at the scope, which says:
220.1 Scope. This article provides requirements for computing branch-circuit, feeder, and service loads.
This clearly says that Art. 220 covers computing loads, or load calcs, as we call them. This refers to allowing adequate service ampacity to be able to supply a given amount of power for receptacle loads.

Nowhere do I see a requirement of how to wire circuits for general-use receptacles, or to limit the quantity of receptacles per circuit. This is a capacity allowance, not a wiring instruction.

Look at it this way: Let's suppose we wire a building with the minimum requirements in mind. Let's also suppose we work it out so the receptacle circuits happen to coincide with the 13-on-a-20a-circuit theory. Let's say we have five circuits with 13 receptacles each, for a total of 65 receptacles.

According to the 13-on-a-20 theory, if for some reason, one circuit stops at 12 receptacles, we would have to run a sixth circuit just for that last remaining circuit. Personally, I would likely not even reach 13 receptacles on one circuit in a non-residential installation, but that's not the point.

Now, if there really was such a physical limitation, I would imaginge it would be clearly spelled out, either in 210.7 or Art. 406. How many times have we argued that the NEC is not a design manual, and for those who complain when it seems to cross that line, that it should not be?
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
iwire said:
Larry, in non-dwelling units we are indeed limited to 13 outlets per 20 amp circuit.
Would you mind showing me where, in reference to actual wiring and not load calcs?
 

iwire

Moderator
Staff member
Location
Massachusetts
LarryFine said:
Would you mind showing me where, in reference to actual wiring and not load calcs?

Article 220 Branch-Circuit, Feeder and service calculations.

Specifically 220.1

Are you suggesting that we have to follow 220 for just the service and not the branch circuits?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
iwire said:
Are you suggesting that we have to follow 220 for just the service and not the branch circuits?
No, I'm just trying to find a way to justify my position that there is a difference between calculations and actual implementation. :roll:

Of course, I have been known to fight losing battles before. :cool:
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
I don't recall the rationale at the moment, but Charlie Beck had a theory about this that was in line with Larry's opinion. I wrote down the older thread in the FAQ.

FWIW, I agree with Bob. I believe we are limited to 13 receptacles in a non-dwelling unit building.
 
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