To question or not to question

[hardworkingstiff]

The EE that is preparing some drawings for a Marina I'm wiring is using 80% of the rating of the receptacle for the service calculation and then applying the derating allowances of (doing this from memory, I'm sitting in a hotel w/out a code book) 555.12 .

I asked about this and he said the receptacles are not supposed to be loaded more than 80%.

I'm not sure this is 100% correct, but I don't know if I should actually question his methodology. As an EE he has the privilege of designing this way (I think). As an EC, I'm not sure it is my responsibility to question (more firmly) his thinking. I'm more inclined to be the "dumb contractor" and let it slide.

What say y'all?

 

[ron]

I don't get involved with marinas, but .......

anyone, EE or EC can make the calculation more conservative than the minimum code.

 

[charlie b]

Question it. You are right, and even your recollection of the applicable article is right. The EE is wrong.

What is worse, the EE's method is not more conservative than the code requires; it is less conservative. By counting the receptacle at 80% of its rating, and then applying a demand factor (per Table 555.12), the results will be lower that you would have gotten, if you counted each receptacle at 100% of its rating. Thus, you will get a service that is too small for the application.

The second column of Table 555.12 says "Sum of the Ratings of the Receptacles (percent). That means take the ratings, not 80% of the ratings, then apply the demand factor.

 

[charlie b]

As an EE he has the privilege of designing this way (I think).

No he doesn't. The NEC does have the occasional opportunity for a licensed PE to perform a calculation, and to issue a design document on the basis of that calculation, even if the results are less conservative than you would get by just taking a value out of a table. There are even a couple places in which the only way to get an answer is to have a PE perform such a calculation.

But that does not give PEs the right to do anything we want. We still have to obey the code. If for example Article 220 of the code tells us to use 1500 VA for each of two small appliance circuits, a PE cannot perform a service calculation with only 1200 VA for these circuits, and claim that, "I'm a PE and I think that 1200 VA is enough for that circuit." The rule says 1500 VA, and we have to use 1500 VA in our calculations.

By the same token, 555.12 tells us to use the receptacle's rating, and that is what we have to use. It is true for an EC, and it is true for a PE.

 

[ron]

I misunderstood the OP question. I thought the receptacles was conservatively being counted as worth 100%, but loaded to only 80%, so effectively 125% toward the calculation.

 

[celtic]

..doing this from memory, I'm sitting in a hotel w/out a code book...

Give the brain a break:
http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7005SB
2005 NEC Online

 

[e57]

Depends on how many outlets...... ??

555.12 Load Calculations for Service and Feeder Conductors.
General lighting and other loads shall be calculated in accordance with Article 220, and, in addition, the load for each service and/or feeder circuit supplying receptacles that provide shore power for boats shall be calculated using the demand factors shown in Table 555.12. These calculations shall be permitted to be modified as indicated in notes (1) and (2).
Table 555.12 Demand Factors
Number of Receptacles Sum of the Rating of the Receptacles (percent)
1–4 - 100 %
5–8 - 90 %
9–14 - 80 %
15–30 - 70 %
31–40 - 60 %
41–50 - 50 %
51–70 - 40 %
71-plus - 30 %
Notes:
1. Where shore power accommodations provide two receptacles specifically for an individual boat slip and these receptacles have different voltages (for example, one 30 ampere, 125 volt and one 50 ampere, 125/250 volt), only the receptacle with the larger kilowatt demand shall be required to be calculated.
2. If the facility being installed includes individual kilowatt-hour submeters for each slip and is being calculated using the criteria listed in Table 555.12, the total demand amperes may be multiplied by 0.9 to achieve the final demand amperes.
FPN:These demand factors may be inadequate in areas of extreme hot or cold temperatures with loaded circuits for heating, air-conditioning, or refrigerating equipment.

But that is for the service calc'..... And should be done at 100% of branch OCP, (which may be the rating of the receptical, or at least limited to that.) less the demand factor from this table.

However he is, and isn't right on the rating of the receptical circuit vs alloable load on it. But this number would not be used for the calc' of the service.

210.20 Overcurrent Protection.

Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

They need to be rated for any combination of continuous and noncontinuous loads @ 125%, however a not so neat trick of math works out like this.... (And often leads to confusion.)
For instance:
30 X .80 = 24 (A 30A circuit, and its allowable load, 80% of the circuit.)
24 X 1.25 = 30 (The load allowed 24A on a 30A circuit @ 125%)

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So yeah, I would question that..............

 

[George Stolz]

(I'm sitting in a hotel w/out a code book)

What the heck are you going to do sitting in a hotel with $400 cash, three bottles of wine and no code book?!?

Somebody! Help this man!

 

[77401]

What the heck are you going to do sitting in a hotel with $400 cash, three bottles of wine and no code book?!?

Somebody! Help this man!

$400?
Two hookers!

Oh sorry ... answer to your question?
Don't question it Does it hurt anything?
Pick your battles.
Document it, as you may run into other issues during the job.

 

[boater bill]

Another thing to make sure is if there are multiple receptacle configurations, that the load is calculated using the highest rating and not the total rating. Some pedestals will have a 50 amp 240 volt plug and twin 30 amp plugs. The feeder to the pedestal is to be sized for the twin 30 amp plugs (60 amps), not for 90 amps or 50 amps or 30. And all of the conductor splices are to be above the electrcial datum plane as defined in 555.2.

A huge problem marinas will have is excessive voltage drop in the feeders. I have always specified conducters that have been calculated to include the reactive resistance because power facter can be poor in marinas. Mulitple tap transformers are a huge help in commisioning when the utility isn't as stiff as they claim to be also.

Hope this helps.