## User Tag List

#### Hybrid View

1. Junior Member
Join Date
Aug 2016
Location
Milford, Pennsylvania, USA
Posts
15
Mentioned
0 Post(s)
Tagged

## Effective Current calculation

I am reviewing plans for a floating marina project and need some clarification. The drawings provide a chart for the power pedestal branch circuit calculations that seems to use "effective current" to size overcurrent protection. To be brief, an example single phase 120/240v circuit has a load of 270 amps (9 x 30a receptacles). After the adjustments from NEC 555.12 (80% load factor & 90% meter factor) I arrive at 194.4 amps. The chart lists an effective current of 108 amps and a 125 amp circuit breaker. I am questioning the effective current. Is the value correct? And should that be used here?

Thank you,
JB78

2. Effective current concerns RMS, DC, and conversion formulas.

It really does not apply here.

I played with the numbers a bit and can get 108 from 270, none of which make sense NEC values correctly.

I looked at table in 555 and see the .8 adjustment and I think that is correct.

Where did you come up with this .9 for meters? Never heard of it before.

The 108 value seems to me that some fat fingered the calculator or doubled the adjustment factor. 108 is 40% of 270.

3. Senior Member
Join Date
Jan 2016
Location
Earth
Posts
6,049
Mentioned
0 Post(s)
Tagged
looks like 80% x 30 x 9 x 50%(duty cycle) = 108 A

4. Originally Posted by Ingenieur
looks like 80% x 30 x 9 x 50%(duty cycle) = 108 A
The arithmetic works. But where did the 50% duty cycle come from? It's not in 555.

5. Senior Member
Join Date
Jan 2016
Location
Earth
Posts
6,049
Mentioned
0 Post(s)
Tagged
Originally Posted by charlie b
The arithmetic works. But where did the 50% duty cycle come from? It's not in 555.

not saying it is correct but I've seen it down

6. Junior Member
Join Date
Aug 2016
Location
Milford, Pennsylvania, USA
Posts
15
Mentioned
0 Post(s)
Tagged
Originally Posted by Ingenieur
not saying it is correct but I've seen it down
Sorry, just getting back to this now. Thanks for all the input. I was trying to attach part of the chart that I was referring to but was having difficulty. Let me elaborate a bit. I stopped just short of calculating the "effective current" (at 194.4a) in my original question. My understanding is that the calculation for "effective current" is I/sqrt(2). If that's correct, eff. current would be 137.5 amps. The person creating this chart would be way off with the 108a. Also, proving the chart further, I find that the "meter factor" of 90% was only used sometimes. Actually, if you drop the meter factor in the example I provided and just use 270a x 0.8 = 216a, it looks like he divided the current by (2), arriving at 108a, and not the square root of (2). I don't know for sure but there are (14) circuits and the pattern is the same. I am just trying to understand this chart before pointing-out any discrepancies. Does this help at all?

Thanks,
JB78

7. Originally Posted by jumper
Where did you come up with this .9 for meters? Never heard of it before.
Note 2 following Table 555.12.

8. Originally Posted by Ingenieur
looks like 80% x 30 x 9 x 50%(duty cycle) = 108 A
Okay, did not see a duty cycle adjustment, but I skimmed quickly through the article.

Originally Posted by charlie b
Note 2 following Table 555.12.

Are you implying I should pay attention to the fine print stuff?
Durn details.....

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•