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Old 01-01-2012, 12:32 PM   #1
whp
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Ampacity derating - generator > power inlet box > panel?


I'm running AWG 10 Romex (2 hot 1 neutral and unshielded ground) from Reliance PB30 30amp power inlet box to 200-amp service panel (Square D QO140M200), 30amp generator breaker in spaces #2 & #4 using QOCGK2 Interlock Kit. I'll be supplying power with a Honda EU6500is inverter generator (6500w max 30 minutes and 5500w continuous). My question:

What is the ampacity/voltage derating based on wire length? The generator puts out (120V/240V) 4-prong socket #4 (27.1 amps @ 6500w for 30minutes max and 22.9 amps continuous). Starting with 100' of wire, I've left about 25' of extra wire in the walls/ceilings for a total ~ 75 feet (walls/ceiling not closed yet).

Q: Should I remove the excess wire ~25+- feet to reduce the total run to about 50 feet or will the extra wire result in a negligible drop in ampacity/voltage supplied by the generator?

I've been told that a drop in ampacity is negligible unless you exceed 135'. I don't want to lose any capacity supplied by the generator. I've gone to several ampacity derating calculators, but I'm not familiar with all the variables that need to be entered.

Also, do you know of a derating calculator that I could use to understand ampacity/voltage derating?

Would appreciate informed responses.

Thanks in advance.

whp

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Old 01-01-2012, 12:44 PM   #2
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Ampacity derating - generator > power inlet box > panel?


If you mean voltage drop here is the equation

Vd=2*L*I*R/1000

Where

L= length of conductor in feet
I= current in the circuit
R= conductor resistance as shown in table 8, chapter 9 of code.

Btw for 10 AWG R will equal 1.24

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Old 01-01-2012, 04:04 PM   #3
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Ampacity derating - generator > power inlet box > panel?


Quote:
Originally Posted by Julius793 View Post
If you mean voltage drop here is the equation

Vd=2*L*I*R/1000

Where

L= length of conductor in feet
I= current in the circuit
R= conductor resistance as shown in table 8, chapter 9 of code.

Btw for 10 AWG R will equal 1.24
Julius793,

Thanks for your prompt response to my question. However, perhaps I am not posing the correct question when I enquire regarding ampacity derating.

My concern is as follows:

I don't want any loss (or as little as possible) in the power the generator is putting out when it arrives at my service panel, whether it be voltage or amperage. I was told not to worry about a drop in amperage unless the wire exceeds 135 feet, which in my case is less than 100 feet.

However, I have about 25 feet excess wire in the run (walls/ceiling) that I could shorten, if that would benefit on the power being delivered to the service panel. Sorry if I'm not using the correct terminology, but the bottom line is that I want to maximize the power delivered to my service panel output by the generator. If reducing the length of the wire would "significantly' do so, I will shorten the run.

I left the additional wire in the run from the power inlet box to the panel, since I pulled my own electrical permit and want the additional wire just in case the electrical inspector wants any modifications. I've carefully laid everything out and have installed all nail plates to protect wiring.

I would appreciate a clarification ASAP, because I want to call for an inspection tomorrow and my generator is due in later in the week.

Thanks again,

whp
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Old 01-01-2012, 04:31 PM   #4
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Ampacity derating - generator > power inlet box > panel?


Voltage drop for 100 ft #10 27 amps is

2.70%
6.48 Volts Line-to-Line
3.24 Volts line -to- N

For 23 amps

2.30%
5.52 Volts Line-to-Line
2.76 Volts Line-to-N
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Old 01-01-2012, 05:07 PM   #5
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Ampacity derating - generator > power inlet box > panel?


Most people would not worry too much about losses that small,
But if you are, go for AWG 8 gauge instead.
Standard practice, at least here in australia is to
derate the cable by 50% for long runs.
With cables the bigger the better,
If you are happy to pay the little bit extra.
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Old 01-01-2012, 05:36 PM   #6
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Ampacity derating - generator > power inlet box > panel?


When equipment is given a lower voltage than it wants, it may or may not draw more amperes as a result.

To figure out how many watts the generator or power supply is delivering to a system with voltage drop through long runs of wire, you measure the voltage at the source (as opposed to at the load) and multiply by amperes drawn.

You need to watch the total amperes or the total watts a generator is delivering so as prevent overload.

For many wires run next to each other it may be necessary to derate (and refrain from drawing as many amperes as the wires running singly allow). It is not necessary to draw less amperes than the wires allow simply because the wire is long. While there is always some heating up of the wires due to energy consumed within and voltage dropping taking place, the amount of heat per foot is the same for a given amperes load regardless of the length of the wire.
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Old 01-01-2012, 05:48 PM   #7
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Ampacity derating - generator > power inlet box > panel?


Quote:
Originally Posted by wareagle View Post
Voltage drop for 100 ft #10 27 amps is

2.70%
6.48 Volts Line-to-Line
3.24 Volts line -to- N

For 23 amps

2.30%
5.52 Volts Line-to-Line
2.76 Volts Line-to-N
wareagle,

So, given that my run with the extra wire is a total of about 62', then knocking off another 10' to 15' feet would probably not gain me that much in terms of delivered power (voltage and amps), correct?

If you were doing the installation for your own home, would you shorten the run given my situation? In addition, I didn't mention the length of the cable that will connect my generator to the power inlet box. It's will be 10' or 25', if I go with pre-made cables, but in no event shorter than 10'.

I don't want to make extra work for myself. However, I want to gain the most power from my generator.

Obviously, I'll have to make the final call, but I would appreciate your opinion.

Thanks again for a rapid response to my question.

whp
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Old 01-01-2012, 06:03 PM   #8
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Ampacity derating - generator > power inlet box > panel?


When equipment is given a lower voltage than it wants, it may or may not draw more amperes as a result.

To figure out how many watts the generator or power supply is delivering to a system with voltage drop through long runs of wire, you measure the voltage at the source (as opposed to at the load) and multiply by amperes drawn.

For many wires run next to each other it may be necessary to derate (and refrain from drawing as many amperes as the wires running singly allow). It is not necessary to draw less amperes than the wires allow simply because the wire is long. While there is always some heating up of the wires due to energy consumed within and voltage dropping taking place, the amount of heat per foot is the same for a given amperes load regardless of the length of the wire.
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Old 01-01-2012, 06:05 PM   #9
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Ampacity derating - generator > power inlet box > panel?


To all responders,

While I was composing my last post, I didn't notice the last to responses. I've all ready committed to the AWG 10, and it's installed in the ceiling and walls of my home. I suppose I could pull AWG 8, but my generator, the Honda EU6500is, only produces the power I noted in my first post above.

Based upon my situation, do you think a run of 62 feet (plus another 10' to 15' for a generator to inlet box cable) will have a significant negative effect on the power delivered to my panel by my generator?
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Old 01-01-2012, 06:14 PM   #10
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Ampacity derating - generator > power inlet box > panel?


Quote:
Originally Posted by whp View Post
To all responders,

While I was composing my last post, I didn't notice the last to responses. I've all ready committed to the AWG 10, and it's installed in the ceiling and walls of my home. I suppose I could pull AWG 8, but my generator, the Honda EU6500is, only produces the power I noted in my first post above.

Based upon my situation, do you think a run of 62 feet (plus another 10' to 15' for a generator to inlet box cable) will have a significant negative effect on the power delivered to my panel by my generator?

In most cases ,
NO ! not significant !
Only 2 to 3%.
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Old 01-01-2012, 06:19 PM   #11
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Ampacity derating - generator > power inlet box > panel?


You have chosen an excellent portable generator especially if your wanting to operate electronics in the event of a power loss.. Your worrying about nothing that results in a serious loss in voltage. Just the same there is no reason to leave 25 feet of excess wire in the run to the house panel. You will gain about 3 to 4 volts by shortening the wire if the generator is under full load. A good place to fool with these parameters is here ...

http://www.electrician2.com/calculators/elcal.html
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Old 01-01-2012, 06:32 PM   #12
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Ampacity derating - generator > power inlet box > panel?


To All Responders,

I want to express my sincere appreciation for your rapid responses to my requests for help. We here in New Jersey have been through quite a lot in the past several months with power outages caused by hurricanes and other bizarre weather - a Halloween storm that sent my family and a ninety-year-old mother to a hotel for a week. What a waste of money!

I can't get this generator installed soon enough. We just lost power again for two hours several night ago.

Again, thanks for you help.

whp

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